Semi-diurnal pressure changes are most pronounced in:
Tropics
Sub-tropics

A trough is defined as an elongated area of relatively low atmospheric pressure, often characterized by maximum cyclonic curvature of the wind flow.
1. Cyclonic Flow: In the Northern Hemisphere (NH), circulation around a low-pressure system (cyclone or trough) is counter-clockwise.
2. Backing Definition: A change in wind direction in an anti-clockwise direction is termed backing. The cyclonic flow associated with a trough involves this counter-clockwise motion, hence the wind “backs” relative to movement around the system center.
3. Surface Wind Deflection: Near the surface, friction causes the wind to deflect across the isobars toward lower pressure (the trough axis). In the NH, this deflection is defined as a back.
(Note: While option 2, “Trough has frontal characteristics,” is sometimes true—as troughs can be frontal or non-frontal—the statement that winds back in the NH hemisphere describes the fundamental cyclonic nature of the pressure system itself.)
⭐️ ⭐️ Key Data to Remember:
• Trough Type: Extension of a low-pressure system.
• NH Low Pressure Circulation: Counter-clockwise (Cyclonic).
• Back/Veer: Back = Anti-clockwise change; Veer = Clockwise change.
• Surface Wind Friction Effect (NH Low): Wind speed reduces, Coriolis force weakens, Pressure Gradient Force dominates, causing wind to deflect toward the low (Backing).

The correct answer is 5,000 ft.
Explanation: In the International Standard Atmosphere (ISA), pressure decreases with altitude. The 850 hPa pressure level is a standard reference and corresponds to an approximate altitude of 5,000 feet above Mean Sea Level (MSL).

When the pressure altimeter subscale is set to the local QNH (QFE reduced to Mean Sea Level (MSL) using ISA conditions), the altimeter is designed to indicate the vertical distance above MSL.
Therefore, when the aircraft is on the surface of the aerodrome (at the datum level) with QNH set, the altimeter displays the aerodrome’s elevation.
Crucially, the altimeter will read the correct elevation of the airfield when the aircraft is on the ground with QNH set, regardless of the actual atmospheric temperature.
⭐️ ⭐️ Key Data to Remember (ICAO/FAA Altimeter Settings):
• QNH set on the ground: Reads Aerodrome Elevation.
• QFE set on the ground: Reads Zero feet.
• Temperature Effect: The QNH reading on the ground (elevation) is correct regardless of non-standard temperature.

A high pressure area, or anticyclone, is typically characterized by widely spaced isobars. The spacing of isobars determines the magnitude of the pressure gradient force (PGF). Since the isobars are generally widely spaced in a high, this indicates a gentle or weak pressure gradient, resulting in light winds.
⭐️ ⭐️ Key Data to Remember:
• Isobars: Widely spaced.
• Pressure Gradient: Weak/Gentle.
• Wind Speed: Light/Weak.
• Flow Mechanism: Air flows out (divergence) from the center of the high at the surface, and air descends (subsidence).

In the International Standard Atmosphere (ISA), the 200 hPa constant pressure surface corresponds closely to an altitude of 40,000 feet above Mean Sea Level (MSL).
• Specifically, the ISA defines the 200 hPa pressure level as occurring at a height of 38,662 feet (11.78 km).
• Standard upper wind and temperature charts are routinely available for the 200 hPa level, which is equivalent to Flight Level 390 (FL390).
⭐️ ⭐️ Key Data to Remember (ISA Approximations):
• 300 hPa: ≈30,000 ft.
• 200 hPa: ≈40,000 ft.
• 100 hPa: ≈53,000 ft.

An isobar is fundamentally defined as a line drawn on a chart connecting points of equal barometric pressure at the same level and at the same time. Isobars commonly depict Mean Sea Level (MSL) pressure (QFF) on surface analysis charts.
⭐️ ⭐️ Key Data to Remember (Definitions):
• Isobar: A line joining places of the same atmospheric pressure.
• Contour Line: A line that connects points of equal elevation above a reference level on a constant pressure chart (isobaric map).
• Isallobar: A line joining places having the same pressure tendency (rate of pressure change).
• Isotherm: A line connecting points of equal temperature.

Atmospheric pressure is defined by the weight of the air column above a specific level. Since air is highly compressible, the majority of the air molecules (mass) are concentrated near the Earth’s surface.
Consequently, as altitude increases:
1. Pressure always decreases.
2. The rate at which the pressure decreases is most rapid near the surface and progressively slows down (decreases) as you ascend. The barometric lapse rate reduces as altitude increases. This means pressure decreases rapidly at first, then more slowly at higher altitudes.
⭐️ ⭐️ Key Data to Remember:
• Rate of Change: Pressure decreases fastest at low altitudes and slower at high altitudes.
• Near Surface (Troposphere): Pressure decreases roughly 1 inch of mercury (approximately 34 hPa) for each 1,000 feet increase in altitude in the lower levels.
• Standard Pressure: At 5.5 km (≈18,000 ft) in the International Standard Atmosphere (ISA), the pressure is approximately 500 hPa, meaning half of the atmosphere’s mass is below that level.

A cyclone (or low-pressure system) is characterized by a central region of relatively low atmospheric pressure. The movement of air associated with a surface low is defined by two key actions:
1. Convergence (Horizontal Movement): Air flows horizontally from surrounding higher pressure areas inward toward the center of the low. This process is called convergence.
2. Ascending (Vertical Movement): Because the converging air cannot flow downward into the ground, it is forced to rise slowly in the center of the low. This rising motion (convection) leads to expansional cooling, condensation, and the formation of clouds and precipitation.
This widespread ascent caused by surface convergence distinguishes a low-pressure system from a high-pressure system, which is characterized by descending (subsiding) and diverging air.
⭐️ ⭐️ Key Data to Remember:
• Low/Cyclone: Surface convergence leads to vertical ascent.
• Weather: Ascent causes adiabatic cooling, cloud formation, and precipitation.

A barograph is a type of aneroid barometer equipped with a mechanism, typically a pen attached to an indicating arm, that draws a continuous record of atmospheric pressure on chart paper wrapped around a rotating drum. It is specifically designed to provide a continuous record of pressure changes over time. The instrument used for continuous recording of atmospheric pressure is therefore defined as a barograph. The chart produced is useful for measuring pressure tendency (the rate of pressure change).
⭐️ ⭐️ Key Data to Remember:
• Function: Provides a continuous record of atmospheric pressure.
• Mechanism: A recording aneroid barometer.
• Output: Used to measure pressure tendency, which is important for forecasting.
• Distinction: A standard barometer (mercurial or aneroid) measures pressure instantaneously, whereas the barograph records it over time.

A subsidence inversion is the type of inversion that forms in a high-pressure area (anticyclone).
In a high-pressure system:
1. Air flows horizontally outward near the surface (divergence).
2. To compensate for the diverging surface air, air aloft slowly sinks or descends (subsides).
3. As air subsides, it is compressed and warms adiabatically.
4. This adiabatic warming of the descending air creates a stable layer where warmer air lies above cooler air, resulting in a subsidence inversion. This inversion acts as a lid on vertical air motion and pollution below.
⭐️ ⭐️ Key Data to Remember:
• Cause: Sinking/descending air (subsidence) associated with high pressure/anticyclones.
• Mechanism: Adiabatic warming due to compression during descent.
• Effect: Creates an extremely stable layer that caps vertical motion.

The 200 hPa pressure level is commonly used for upper air charting in aviation meteorology. In the International Standard Atmosphere (ISA), the 200 hPa pressure surface is found at an altitude of approximately 38,662 feet (11.78 km), or typically approximated as 40,000 feet above Mean Sea Level (AMSL).
When vertical position is referenced using the Standard Pressure Setting (SPS) of 1013.25 hPa, the resulting figure is divided by 100 to yield the Flight Level (FL).
Therefore, the 200 hPa constant pressure surface is applicable to Flight Level 390 (FL390).
⭐️ ⭐️ Key Data to Remember (ISA Approximations):
• Pressure Level: 200 hPa.
• ISA Altitude: ≈ 38,662 ft.
• Flight Level: FL390.
• Chart Usage: 200 hPa charts are routinely produced for this flight level.

In the International Standard Atmosphere (ISA), the pressure level of 500 hPa corresponds closely to an altitude of 18,000 feet above Mean Sea Level (AMSL). This altitude is also referred to as Flight Level 180 (FL180).
The 500 hPa pressure surface is significant because, in ISA, it represents the level above which approximately half of the total mass of the atmosphere lies.
⭐️ ⭐️ Key Data to Remember (ISA Approximations):
• Pressure Level: 500 hPa.
• ISA Altitude: Approximately 18,000 ft AMSL or 18,289 ft.
• Significance: Half the mass of the atmosphere is below this level.

In the Northern Hemisphere (NH), if an aircraft experiences a starboard (right) drift, the wind component is coming from the left (port side) of the aircraft’s track. When flying along a constant indicated altitude (which is a constant pressure surface), the relationship between the wind and pressure/temperature fields is crucial for determining true altitude changes.
1. Pressure/Temperature Relationship (NH): Due to the Coriolis effect, the wind blows with lower pressure and typically colder air masses located generally to the left of the wind direction (Buys Ballot’s Law). When flying with the wind from the left, you are generally flying parallel to, or traversing towards, regions where the constant pressure surface height is lower, or towards regions of colder air.
2. Altimeter Error Principle: Altimeters are calibrated for the International Standard Atmosphere (ISA). Flying into regions of lower pressure or colder air masses causes the altimeter to over-read; meaning the Indicated Altitude (IA) is higher than the True Altitude (TA).
◦ This is summarized by the memory aid: “High to low – beware below!”. Since the drift indicates a constant relationship with the pressure/temperature gradient, the aircraft is flying relative to the pressure gradient in a manner that results in an over-reading altimeter.
Therefore, the actual altitude (True Altitude) will be lower than the indicated altitude.
⭐️ ⭐️ Key Data to Remember:
• NH Right Drift ⟹ Wind from Left.
• NH Wind from Left ⟹ Lower Pressure/Colder Air to Left.
• Flying into Colder/Lower Pressure ⟹ Altimeter Over-reads (IA > TA).

The QNH (Altimeter setting for Mean Sea Level using ISA) is the barometric pressure at the aerodrome (QFE) reduced to Mean Sea Level.
Since the aerodrome is Above Mean Sea Level (AMSL) (160 m), the actual pressure at the aerodrome (QFE) must be lower than the pressure converted to sea level (QNH).
1. Calculate the pressure difference (P_diff): Using the approximation that 1 hPa corresponds to 8 m:
P
diff
​
=
Height per hPa
Elevation
​
=
8 m/hPa
160 m
​
=20 hPa
2. Calculate QFE: Subtract the pressure difference from the QNH:
QFE=QNH−P
diff
​
=1005 hPa−20 hPa=985 hPa
⭐️ ⭐️ Key Data to Remember:
• QNH > QFE when the aerodrome is AMSL.
• The relationship 1 hPa≈27 ft (or ≈8 m) is typically used for these calculations near the surface.

A ridge is defined as an elongated area of relatively high atmospheric pressure, often characterized by maximum anticyclonic curvature. Ridge weather and associated air movement are very similar to those found in an anticyclone (High pressure system).
In a high pressure system:
1. Air aloft converges and slowly descends (this sinking motion is called subsidence).
2. At the surface, the air flows outward from the center (this horizontal spreading is called divergence).
Therefore, the net vertical movement of air relating to a ridge is descending and diverging. The sinking air warms adiabatically, leading to increased stability, dissipation of clouds, and generally good weather conditions.
⭐️ ⭐️ Key Data to Remember:
• Ridge Type: Extension of a High Pressure system (Anticyclone).
• Vertical Motion: Subsidence (Descending air).
• Horizontal Motion (Surface): Divergence (Outflow).
• Associated Weather: Typically periods of good weather.

QNH is the QFE (aerodrome pressure) reduced to Mean Sea Level (MSL) using International Standard Atmosphere (ISA) conditions. QFF is the QFE reduced to MSL using the actual outside air temperature.
When an aerodrome is located exactly at MSL:
• The aerodrome pressure (QFE) is measured directly at MSL.
• Since the elevation is zero, the required pressure reduction calculation to convert QFE to MSL is zero for both QNH (using ISA conditions) and QFF (using actual temperature conditions).
Therefore, at an aerodrome located at MSL, the QNH and QFF values are always equal to the QFE, and thus equal to each other, regardless of the temperature deviation from ISA.
⭐️ ⭐️ Key Data to Remember:
• Aerodrome at MSL: QNH=QFF=QFE.
• Temperature Influence: The relationship holds true regardless of the ambient temperature.

Atmospheric pressure is fundamentally defined as the force per unit area exerted by the weight of the column of air above a specific surface.
Since air molecules are held near the Earth by gravity, the atmosphere is compressible, leading to the majority of air mass being concentrated near the surface.
Air density is the measure of mass per unit volume. Because pressure is directly proportional to density (according to the gas law, assuming constant temperature), high density air columns exert greater pressure. Therefore, the decrease of pressure with height is linked directly to the decrease in air density with height.
In short, atmospheric pressure is a direct result of the mass (density) distribution of the atmosphere, influenced by gravity.
⭐️ ⭐️ Key Data to Remember:
• Definition: Pressure is the weight of the overlying air column.
• Relationship: Density decreases rapidly with height, causing pressure to decrease rapidly with height.
• Gas Law: Pressure (P) is proportional to Density (ρ) and Temperature (T): P∼T×ρ.

The rate at which atmospheric pressure decreases with increasing height (the pressure lapse rate) is determined by the density of the air column.
1. Cold Air: Cold air is denser than warm air. Because the air molecules are packed closer together in a column of cold air, the weight (pressure) decreases more rapidly over a given vertical distance compared to a standard atmosphere. Therefore, pressure falls more rapidly with height in cold air.
2. Warm Air: Conversely, warm air is less dense. In a column of warm air, the decrease in pressure with height is slower than standard.
This phenomenon is critical in altimetry, as flying into colder air results in the pressure surface being lower than indicated (altimeter over-reads).
⭐️ ⭐️ Key Data to Remember:
• Density Relationship: Pressure decreases more rapidly in cold (denser) air.
• Rate of Change: Cold air increases the pressure lapse rate; warm air decreases it.
• Altimetry Principle: High pressure is found at a lower height over cold air.

In the International Standard Atmosphere (ISA), the 400 hPa constant pressure surface corresponds to an altitude of approximately 24,000 feet above Mean Sea Level (AMSL).
Constant pressure charts are routinely available for the 400 hPa level, which is equivalent to Flight Level 240 (FL240).
⭐️ ⭐️ Key Data to Remember (ISA Approximations):
• Pressure Level: 400 hPa.
• Equivalent Flight Level: FL240.
• Nearby Levels: 500 hPa is approximately 18,000 ft, and 300 hPa is approximately 30,000 ft.

When an aircraft maintains a constant indicated altitude, it is flying along a constant pressure surface (isobaric surface).
If the aircraft is gaining true altitude while maintaining this constant indicated altitude, it means the pressure surface itself is rising.
• Pressure Relationship: Pressure levels (isobaric surfaces) rise in regions of relatively higher pressure (such as a High Pressure system or ridge) and are found at lower heights over regions of low pressure (troughs or depressions).
• Result: Flying toward an area of higher pressure causes the constant pressure surface (which the altimeter is following) to physically rise, resulting in an increase in the aircraft’s True Altitude.
• Altimeter Reading: In this scenario, the altimeter is indicating an altitude lower than the actual True Altitude (it is “under-reading”).
This situation is generally considered safe, contrasting with the principle: “High to low – beware below!”.

The troposphere is characterized by decreasing temperature with height. The tropopause marks the level where the decrease in temperature ceases.
1. Tropospheric Lapse: Since FL300 (300 hPa) is below the tropopause (FL330), the air mass is still cooling adiabatically between FL300 and FL330. The vertical distance is 3,000 ft.
2. Applying Standard Lapse Rate: Using the practical International Standard Atmosphere (ISA) lapse rate for calculation, which is 2
∘
C per 1,000 ft:
Temperature Drop=3×2
∘
C=6
∘
C
3. Temperature at Tropopause (FL330): The temperature at FL330 is therefore:
−54
∘
C−6
∘
C=−60
∘
C
4. Stratospheric Condition: In the lower stratosphere, just above the tropopause, the temperature typically remains constant (isothermal) before beginning to increase. Since FL350 is only 2,000 ft above the tropopause (FL330), the most likely temperature at FL350 is assumed to be the temperature established at the tropopause.
The most likely temperature at FL350 is −60
∘
C.
⭐️ ⭐️ Key Data to Remember:
• The temperature usually ceases to decrease at the tropopause.
• The layer just above the tropopause (lower stratosphere) is isothermal (constant temperature).
• Use 2
∘
C/1000 ft lapse rate for calculation in the troposphere.

A contour line on a constant pressure chart joins points of equal elevation above Mean Sea Level (MSL) at which that specific pressure occurs.
In the International Standard Atmosphere (ISA), the pressure level corresponding to a height of approximately 9,160 meters (or 30,000 feet) is 300 hPa.
⭐️ ⭐️ Key Data to Remember (ISA Approximations):
• Pressure Level: 300 hPa.
• ISA Height (Metric): 9,160 m (or 9,144 m).
• ISA Height (Imperial): 30,000 ft or 30,065 ft.
• Equivalent Flight Level: FL300.
• Chart Usage: 300 hPa charts are routinely used for upper wind and temperature information at this flight level.

A steep (or strong) pressure gradient is defined by a rapid change in pressure over a short horizontal distance.
• Isobars: When isobars (lines connecting points of equal pressure) are drawn close together on a synoptic chart, they indicate a steep pressure gradient.
• Pressure Gradient Force (PGF): The magnitude of the PGF, which initiates air movement, is directly proportional to the pressure gradient. Closely spaced isobars correspond to a large PGF.
• Wind Speed: A strong PGF results in high or strengthened winds.
Therefore, a steep pressure gradient is characterized by closely spaced isobars and resulting strong winds.
⭐️ ⭐️ Key Data to Remember:
• Steep Gradient: Isobars close together ⟹ Strong PGF ⟹ Strong Winds.
• Weak Gradient: Isobars widely spaced ⟹ Weak PGF ⟹ Light Winds.
• Application: Steep pressure gradients are often found around the centers of depressions (Lows).

QFE is defined as the atmospheric pressure measured at the aerodrome reference point. When the QFE is set on the altimeter subscale, the instrument is specifically configured to read zero feet when the aircraft is located at the aerodrome datum level. When the aircraft is airborne with QFE set, the altimeter reading corresponds to the aircraft’s height above the aerodrome reference point.
⭐️ ⭐️ Key Data to Remember (QFE):
• Definition: Pressure at the aerodrome datum level.
• Altimeter Indication (on ground): Reads zero feet.
• Altimeter Indication (in air): Reads height above aerodrome level.

An isobar is defined as a line drawn on a meteorological chart (such as a synoptic chart) connecting points of equal barometric pressure at the same level and time. On surface analysis charts, isobars are drawn based on the Mean Sea Level pressure (QFF).
⭐️ ⭐️ Key Data to Remember (Definitions):
• Isobar: A line connecting places of equal atmospheric pressure.
• Isallobar: A line connecting places having the same pressure tendency (rate of pressure change).
• Isotherm: A line connecting points of equal temperature.

A high-pressure system, or anticyclone, is fundamentally characterized by two coupled atmospheric movements: horizontal outflow and vertical settling of air.
1. Divergence (Horizontal Movement): Near the Earth’s surface, air flows outward away from the center of the high toward areas of lower pressure. This spreading out of air is called divergence.
2. Sinking/Subsidence (Vertical Movement): To replace the air diverging at the surface, air from aloft slowly descends or sinks (subsidence) within the center of the high. This descending air warms adiabatically, leading to increased atmospheric stability, dry air, and usually fair weather conditions.
⭐️ ⭐️ Key Data to Remember:
• High Pressure (Anticyclone): Surface divergence, vertical subsidence (sinking).
• Weather Association: Descending air favors the dissipation of cloudiness (good weather).
• Opposite (Low/Cyclone): Surface convergence, vertical ascent (climbing).

A constant pressure chart (contour chart) is used to depict meteorological information at a specific pressure level, which corresponds to an approximate Flight Level (pressure altitude) in the International Standard Atmosphere (ISA).
While major standard upper air charts are often issued for 850 hPa (Flight Level 050 or 5,000 ft) and 700 hPa (Flight Level 100 or 10,000 ft), Flight Level 060 (6,000 ft) sits between these levels.
Using the relationship that pressure decreases with height, and interpolating between the known ISA altitudes:
• 850 hPa≈5,000 ft.
• 700 hPa≈10,000 ft.
Flight Level 060 (6,000 ft) is only 1,000 ft above the 850 hPa level. An approximate calculation shows the pressure at 6,000 ft would be close to 820 hPa. Among the choices provided, the 800 hPa chart is the closest and most representative standard pressure level for analyzing conditions at or near FL060.
⭐️ ⭐️ Key Data to Remember:
• FL/hPa Relationship: 850 hPa=FL050. 700 hPa=FL100.
• FL060: This level is slightly above the 850 hPa surface, making 800 hPa the most appropriate chart level listed.

A trough is an elongated area of relatively low atmospheric pressure. The air movement associated with a low-pressure system (depression or trough) involves a process where air flows horizontally inward toward the center, known as convergence.
Since this converging air cannot flow downward into the ground, it is forced to move upward (ascent). This widespread lifting cools the air adiabatically, which is conducive to cloudiness and precipitation.
⭐️ ⭐️ Key Data to Remember:
• Horizontal Motion: Convergence (inward flow) at the surface.
• Vertical Motion: Ascent (rising air).
• Weather: Rising air leads to cooling, saturation, cloud formation (often cumulus or cumulonimbus), and unstable weather.
• Contrast: This movement is the opposite of a ridge (High pressure), which features divergence and sinking air.

QNH (Quoted Nautical Height) is the atmospheric pressure at the aerodrome reference point (QFE) reduced to Mean Sea Level (MSL) using International Standard Atmosphere (ISA) conditions.
When the QNH is set on the altimeter subscale:
1. The altimeter is calibrated to indicate the aircraft’s altitude, which is its vertical distance above MSL.
2. On the ground at the aerodrome datum level, the altimeter will indicate the aerodrome elevation (or field elevation).
In contrast, QFE (Quoted Field Elevation) is the pressure measured at the aerodrome datum level; when QFE is set, the altimeter reads zero feet when the aircraft is on the ground.
⭐️ ⭐️ Key Data to Remember:
• QNH set (on ground): Altimeter reads Aerodrome Elevation (Altitude above MSL).
• QFE set (on ground): Altimeter reads Zero.
• QFF: Used for meteorological charts (QFF is the QFE reduced to MSL using actual temperature) but must never be used for altimetry.

The altimeter functions as an aneroid barometer, measuring atmospheric pressure and translating this measurement into a height or altitude reading. The height indication is always relative to the specific pressure value manually set on the altimeter sub-scale (baroscale). Depending on this pilot-selected datum:
1. QNH set: Indicated altitude above Mean Sea Level (MSL).
2. QFE set: Indicated height above the aerodrome datum level.
3. SPS (1013 hPa) set: Indicated Pressure Altitude (above the standard datum).
⭐️ ⭐️ Key Data to Remember:
• The altimeter measures pressure, not geometric height.
• The reading is fundamentally determined by the pressure datum input on the sub-scale.
• Altitude and height are relative to the selected datum pressure.

When an aircraft maintains a constant indicated altitude, it is flying along a specific constant pressure surface. If the aircraft flies from an area of relatively Low pressure to an area of High pressure, the pressure surface itself rises.
If the altimeter sub-scale setting is not adjusted, the instrument will indicate an altitude lower than the actual True Altitude (TA), a phenomenon known as under-reading.
• Flying towards Higher Pressure: True Altitude > Indicated Altitude.
• The memory aid for the inverse situation is: “High to low – beware below!”.

In the center of a high-pressure system (anticyclone) during summer in temperate latitudes, atmospheric stability dominates due to subsiding air.
1. Winds: Isobars are generally widely spaced in an anticyclone, indicating a weak pressure gradient force, resulting in light or calm winds.
2. Cloud/Precipitation: The sinking air (subsidence) warms adiabatically, leading to increased stability and the dissipation of clouds and precipitation.
3. Visibility: This subsidence often creates a subsidence inversion below which air pollution, moisture, and haze are trapped, resulting in generally moderate or poor visibility.
Therefore, the typical conditions are generally fair weather with light winds and possible haze.
⭐️ ⭐️ Key Data to Remember:
• High Pressure ⟹ Subsidence ⟹ Stability.
• Summer Highs ⟹ Light Winds, No Precipitation, Haze/Poor Visibility.

QNH is defined as the aerodrome barometric pressure (QFE) converted or reduced to Mean Sea Level (MSL) using the established International Standard Atmosphere (ISA) conditions.
The use of ISA conditions ensures a standard conversion, meaning the QNH value does not account for any actual temperature deviation away from ISA. This standardized pressure setting is used by pilots to ensure the altimeter indicates altitude (height above MSL) when set on the ground, the altimeter reads the aerodrome elevation.
In contrast, QFF (Quoted Fictitious Forecast) is the QFE reduced to MSL using the actual outside air temperature and assuming isothermal conditions exist, making QFF the true mean sea level pressure (used primarily for meteorological charts, not altimetry).
⭐️ ⭐️ Key Data to Remember (QNH):
• Basis: QFE reduced to MSL.
• Conditions: Uses ISA temperature and pressure lapse rate.
• Operational Use: When set, the altimeter indicates altitude above MSL.
• On Ground: Reads aerodrome elevation.

Poor visibility, often caused by haze, smoke, or fog, is typically associated with high-pressure systems (anticyclones).
This relationship is due to the inherent nature of a high-pressure system:
1. Subsidence and Stability: Highs are characterized by widespread sinking air (subsidence) and horizontal divergence at the surface. Subsidence creates a stable atmosphere, often forming a subsidence inversion.
2. Trapping Mechanisms: Below this inversion, the air is stagnant or has light winds. This stable layer traps moisture, smoke, and atmospheric pollutants near the surface, leading to a significant drop in visibility at low levels.
3. Conditions: In anticyclones, hazy conditions occur in summer, and foggy conditions (such as radiation fog) occur in winter.
4. Col Association: A Col (the neutral area between two Highs and two Lows) is also a region of stagnant air and weak pressure variation, and is likewise prone to poor visibility and fog, particularly in autumn and winter.
⭐️ ⭐️ Key Data to Remember:
• High Pressure: Sinking air ⟹ Stability ⟹ Poor visibility (Haze, Fog, Smoke).
• Low Pressure: Rising air ⟹ Mixing ⟹ Generally good visibility (except in precipitation).

A region of low atmospheric pressure, often called a low, cyclone, or depression, is primarily associated with unsettled or “bad” weather.
The characteristic atmospheric circulation within a low-pressure system is:
1. Convergence: Air flows horizontally inward toward the center of the low.
2. Ascent: Since the converging air cannot go downward, it is forced upward (ascent or convection).
3. Weather: Rising air cools adiabatically, leading to condensation, cloud formation, and associated precipitation. Lows are also associated with generally strong winds, especially if the isobars are closely spaced.
Conversely, high-pressure systems (anticyclones) are associated with descending air (subsidence), which prevents cloud formation and results in generally good, fair weather. Low pressure systems quite often are regions of poor flying weather.
⭐️ ⭐️ Key Data to Remember:
• Low Pressure/Cyclone: Convergence and rising air (Ascent/Convection).
• Associated Weather: Cloudiness, precipitation, and strong winds (Bad Weather).
• High Pressure/Anticyclone: Divergence and sinking air (Subsidence) (Good Weather).

The 850 hPa constant pressure chart is the standard meteorological chart used to depict conditions (such as wind and temperature) at or near Flight Level 050 (FL50).
In the International Standard Atmosphere (ISA):
• The 850 hPa pressure level is located at an altitude of 1.46 km, which is precisely 4,781 ft above Mean Sea Level (MSL).
• Upper air charts are constructed using constant pressure surfaces because aircraft operating at high altitude typically fly at Flight Levels (pressure altitudes), which are surfaces of constant pressure.
⭐️ ⭐️ Key Data to Remember:
• Pressure Level: 850 hPa.
• Equivalent Altitude (ISA): 4,781 ft.
• Equivalent Flight Level (ICAO): FL050.

QNH is defined as the aerodrome barometric pressure (QFE) converted or reduced to Mean Sea Level (MSL) using the conditions and lapse rate prescribed by the International Standard Atmosphere (ISA).
This reduction uses the ISA pressure lapse rate and does not account for any actual deviation of ambient temperature from ISA. When QNH is set on the altimeter sub-scale, the instrument indicates the aircraft’s altitude, which is its vertical distance above MSL.
⭐️ ⭐️ Key Data to Remember:
• Source: QFE.
• Reference Level: Mean Sea Level (MSL).
• Method: Reduced using ISA conditions.
• Result: When set, the altimeter reads aerodrome elevation on the ground and altitude (height above MSL) when airborne.

The atmospheric pressure is measured using instruments called barometers. The barometer can be either mercurial or aneroid.
However, the specific instrument designed to make a continuous printed record of atmospheric pressure changes over a period of time (known as pressure tendency) is the Barograph. The barograph is essentially a recording aneroid barometer.
⭐️ ⭐️ Key Data to Remember:
• Barometer: An instrument that measures atmospheric pressure.
• Barograph: A specific type of barometer (aneroid) used for continuous recording of pressure changes (pressure tendency).
• Anemograph: An instrument that records wind speed and direction.
• Hygrometer: An instrument used for measuring the water vapor content (humidity) of the air.

The 700 hPa constant pressure level corresponds to an International Standard Atmosphere (ISA) Pressure Altitude of approximately 10,000 feet (FL100).
1. ISA Temperature Calculation: Using the ISA lapse rate of 2
∘
C per 1,000 feet:
T
ISA
​
at FL100=15
∘
C−(10×2
∘
C)=−5
∘
C
.
2. ISA Deviation: The observed temperature of −15
∘
C is 10
∘
C colder than the ISA temperature of −5
∘
C.
3. Characterization: Since the temperature is significantly colder than the standard atmosphere at that level, it is characterized as “Low”.
⭐️ ⭐️ Key Data to Remember:
• 700 hPa Level: Corresponds to FL100 (approx. 10,000 ft).
• ISA Temp at FL100: −5
∘
C.
• Observed Deviation: −10
∘
C (colder than ISA).
• Operational Context: Flying in air colder than ISA means the True Altitude is lower than the Indicated Altitude, presenting a potential terrain clearance hazard if navigating near minimum altitudes (“Warm to cold – don’t be bold!”).

The aircraft altimeter is essentially an aneroid barometer. The specific reading it displays is determined by the pressure value set on its subscale.
QFE (Quoted Field Elevation) is defined as the atmospheric pressure measured at the aerodrome reference point.
When the pilot sets the QFE value on the altimeter subscale, the altimeter is calibrated to indicate zero feet when the aircraft is located at the aerodrome datum level. When airborne with QFE set, the instrument indicates the aircraft’s height above the aerodrome level.
⭐️ ⭐️ Key Data to Remember:
• QFE set (on ground): Reads zero feet.
• QNH set (on ground): Reads aerodrome elevation (altitude above MSL).
• QFF: Must never be used for altimetry.

A constant pressure surface, such as the 500 hPa level, represents a specific barometric pressure found at a certain elevation above Mean Sea Level (MSL).
1. Upward Bulge (High Height/Contour): When a constant pressure surface “bulges upwards,” it means that the pressure level is found at a relatively greater height (high contour values). This feature on an upper-air chart is typically referred to as a ridge or high.
2. Temperature/Density Relationship: Pressure decreases more slowly with height in warm, less-dense air compared to cold, more-dense air.
3. Conclusion: A high altitude for a specific pressure level indicates that the column of air below it is relatively warm. Therefore, an upward bulging pressure surface corresponds to a Warm High (or high pressure area associated with warm air aloft).

A steep (or strong) pressure gradient is defined by a rapid change in atmospheric pressure over a short horizontal distance.
• Isobar Spacing: When isobars (lines connecting points of equal pressure) are closely spaced or tightly packed on a weather chart, they indicate a steep pressure gradient.
• Wind Speed: The magnitude of the Pressure Gradient Force (PGF)—the force that initiates air movement—is directly proportional to the pressure gradient. Therefore, closely spaced isobars create a strong PGF, which results in high or strong winds.
⭐️ ⭐️ Key Data to Remember:
• Steep Gradient: Isobars close together ⟹ Strong PGF ⟹ Strong Winds.
• Weak Gradient: Isobars widely spaced ⟹ Weak PGF ⟹ Light Winds.
• PGF Direction: Always acts perpendicular to the isobars, directed from high pressure toward low pressure.

The International Standard Atmosphere (ISA) defines specific pressure levels that correspond to standard pressure altitudes. The 300 hPa constant pressure level is a standard upper air chart reference.
In ISA conditions:
• The 300 hPa pressure level is approximately located at 30,000 feet above Mean Sea Level (AMSL).
• This pressure altitude is designated as Flight Level 300 (FL300).
⭐️ ⭐️ Key Data to Remember (Standard Pressure Levels):
• 850 hPa≈5,000 ft (FL050).
• 700 hPa≈10,000 ft (FL100).
• 500 hPa≈18,000 ft (FL180).
• 300 hPa≈30,000 ft (FL300).

An isobar is defined as a line connecting places of equal or constant atmospheric pressure at the same level and time. On typical surface weather charts, isobars depict the Mean Sea Level (MSL) pressure, known as QFF.
⭐️ ⭐️ Key Data to Remember (Related Terms):
• Isobar: Equal Pressure.
• Isotherm: Equal Temperature.
• Isallobar: Equal Pressure Tendency (rate of pressure change).

The altimeter is an instrument designed to measure altitude or height by utilizing the fundamental principle that atmospheric pressure decreases with increasing altitude.
The altimeter functions as an aneroid barometer that measures local pressure and translates this into an altitude reading in feet, rather than units of pressure (hPa). The instrument is calibrated in accordance with the International Civil Aviation Organization (ICAO) International Standard Atmosphere (ISA) to ensure a standardized pressure-height relationship for accuracy and common datum.

A constant pressure chart is utilized because flights at higher altitudes are conducted at Flight Levels, which are surfaces of constant pressure.
Flight Level 170 (FL170) corresponds to a pressure altitude of 17,000 feet. The standard constant pressure chart for analyzing conditions near this altitude is the 500 hPa chart.
In the International Standard Atmosphere (ISA), the 500 hPa level is located at approximately 18,000 feet (FL180). Since 17,000 feet is only 1,000 feet below 18,000 feet, the 500 hPa chart is the most relevant standard chart available to assess meteorological conditions at FL170.
⭐️ ⭐️ Key Data to Remember:
• Target FL: FL170 (17,000 ft).
• Closest Standard Chart: 500 hPa.
• 500 hPa ISA Altitude: 18,000 ft (FL180).

The 500 hPa constant pressure level is a standard upper air chart used to depict conditions at or near Flight Level 180 (FL180). The height of a constant pressure surface is determined based on International Standard Atmosphere (ISA) conditions, which approximate average conditions in temperate/moderate latitudes.
In the ISA:
1. The 500 hPa pressure level is located at approximately 18,000 feet above Mean Sea Level (AMSL).
2. Converting the ISA altitude to kilometers, 18,289 feet is equivalent to 5.51 km.
Therefore, 5.5 km (or 5,500 meters) is the closest expected average height for the 500 hPa level in moderate latitudes.
⭐️ ⭐️ Key Data to Remember:
• Pressure Level: 500 hPa.
• Equivalent Altitude (ISA): 18,000 ft (FL180).
• Equivalent Altitude (km): ≈5.5 km.

The altimeter is fundamentally an aneroid barometer. It utilizes a sealed, partially evacuated capsule (aneroid cell) that expands and contracts based on changes in ambient atmospheric pressure. This mechanism translates the measured pressure into a displayed height or altitude reading, typically calibrated in feet according to the International Standard Atmosphere (ISA) pressure-height relationship.
⭐️ ⭐️ Key Data to Remember:
• Type: Aneroid barometer.
• Mechanism: Partially evacuated metal capsule.
• Function: Measures pressure but displays altitude/height.
• Calibration: Calibrated to the ICAO International Standard Atmosphere (ISA).

A constant pressure chart, such as the 500 hPa chart, utilizes contour lines (isohypses) to connect points that have the same elevation above Mean Sea Level (MSL) for that specific pressure value. These lines provide essential information regarding how the pressure level height changes horizontally.
The heights on upper air charts are often expressed in decametres (10s of metres), which are the practical units for the geopotential height, commonly referenced as gpm in this context. The closer the contour lines, the stronger the wind. The 500 hPa level corresponds to approximately 18,000 feet (FL180) in the International Standard Atmosphere (ISA).
⭐️ ⭐️ Key Data to Remember:
• Chart Type: Constant Pressure Chart (500 hPa).
• Lines: Contour lines (isohypses) joining places of equal height AMSL.
• Contour Units: Heights are often expressed in decametres (gpm or 10 m).
• ISA Altitude: FL180.

Subsidence is the meteorological term for a vertical down flow of air over a broad area. This sinking motion is characteristic of high-pressure systems (anticyclones).
As air subsides (sinks), it enters regions of higher atmospheric pressure, causing it to be compressed and consequently warmed adiabatically. This adiabatic warming leads to increased atmospheric stability, which inhibits cloud formation and precipitation. Subsidence is checked above the ground, often forming a subsidence inversion.
⭐️ ⭐️ Key Data to Remember:
• Definition: Vertical downward movement of air.
• Pressure System: Associated with Highs (Anticyclones).
• Effect: Causes adiabatic warming, increased stability, cloud dissipation, and often leads to the formation of a subsidence inversion.
• Surface Circulation: Associated with divergence at low levels.

Atmospheric pressure is fundamentally defined as the force per unit area exerted by the atmosphere on any surface in contact with it.
If atmospheric pressure is considered as the weight of a column of air of unit cross-sectional area above a surface, the definition holds true that it is the force exerted over a specific surface area.
⭐️ ⭐️ Key Data to Remember (Definitions):
• Atmospheric Pressure: The force per unit area exerted by the weight of the atmosphere.
• Weight Context: It is the weight of a column of air of unit cross-sectional area above a surface.
• Units (Standard MSL): 1013.25 hPa (mb), 29.92 in Hg, or 14.7 lb/in
2
.

This scenario involves comparing QNH and QFF for an airfield that is Above Mean Sea Level (AMSL) (100 m) where the actual temperature is significantly Colder than International Standard Atmosphere (ISA) conditions.
1. QNH Calculation Basis (ISA): QNH is calculated by reducing the aerodrome pressure (QFE) to Mean Sea Level (MSL) using the ISA temperature lapse rate.
2. QFF Calculation Basis (Actual): QFF is calculated by reducing QFE to MSL using the actual prevailing temperature (−15
∘
C), assuming isothermal conditions.
3. Temperature Effect: In cold air (Colder than ISA), pressure decreases more rapidly with height than in ISA conditions.
4. Resulting Relationship: When an aerodrome is AMSL and the air is colder than ISA, the pressure correction required to reach MSL using the actual cold air (QFF) is greater than the pressure correction using the standard warmer air (QNH). Therefore, QFF will be higher than QNH.
Since QFF=1030 hPa and QFF>QNH, the value of QNH must be less than 1030 hPa.
⭐️ ⭐️ Key Data to Remember (AMSL, Colder than ISA):
• Conditions: Aerodrome AMSL and temperature <ISA.
• Relationship: QFF>QNH.
• Reason: Cold air is denser, leading to a steeper pressure lapse rate and thus a larger correction needed for QFF.

The Quoted Fictitious Forecast (QFF) is defined as the atmospheric pressure measured at the aerodrome reference point (QFE) converted or reduced to Mean Sea Level (MSL) using the actual outside air temperature prevailing at the time of observation. This calculation often assumes isothermal conditions exist between the aerodrome and MSL.
⭐️ ⭐️ Key Data to Remember (QFF):
• Purpose: QFF provides the true mean sea level pressure, which forecasters use to construct accurate surface analysis charts.
• Condition Used: Actual outside air temperature.
• Comparison to QNH: QNH uses the International Standard Atmosphere (ISA) conditions for reduction to MSL, regardless of the actual temperature.
• Operational Context: QFF must never be used for altimetry or set on the altimeter subscale. Isobars on surface weather charts normally depict QFF values.

For the True Altitude (TA) to be greater than the Indicated Altitude (IA) when flying at a constant IA, the pressure levels must be physically higher than the altimeter is calibrated to expect. This occurs under conditions of high temperature and high pressure relative to the reference settings:
1. Temperature Effect (Hot/Warm Air): The altimeter is calibrated based on the International Standard Atmosphere (ISA) temperature profile. If the air column is warmer (hotter) than ISA, the air is less dense and expands upward. The pressure surfaces (isobars) are therefore found at a higher vertical distance (TA>IA).
◦ If the air is warmer than ISA, the true altitude is higher than the indicated altitude.
2. Pressure Effect (High Pressure): When flying from an area where the reference pressure (QNH set) is low toward an area of true higher pressure, the pressure surfaces rise, and the true altitude increases relative to the set datum.
The combination of Hot (Warmer than ISA) and High Pressure yields the greatest positive error (TA>IA).
Conversely, the hazardous combination where TA decreases relative to IA (TA < IA) is Cold/Low (Cold air and/or Low pressure), giving rise to the memory aid: “High to low – beware below!” and “Warm to cold – don’t be bold!”.
⭐️ ⭐️ Key Data to Remember:
• Warm/Hot air: Less dense ⟹ Pressure surfaces higher ⟹ TA > IA.
• High Pressure (relative to setting): Datum higher ⟹ TA > IA.
• Condition for Maximum True Altitude: Hot/High.

On constant pressure altitude charts (isobaric maps), such as the 500 hPa chart, the lines drawn are contour lines (isohypses) connecting points of equal height above Mean Sea Level (MSL) at which that specific pressure occurs,.
1. High Contour Value: A high contour value indicates that the constant pressure surface is physically located at a greater height or elevation above MSL,.
2. Pressure Relationship: Regions where a constant pressure surface is found at a high altitude correspond to areas of relative High Pressure aloft, often depicted as a ridge or high,. Areas of high pressure aloft are typically associated with warmer air below that level,.
3. Level Flight: Since “level flight” means maintaining a constant pressure altitude, the aircraft is flying along this pressure surface. Moving toward a higher contour value means the aircraft is ascending in terms of True Altitude and entering a region of higher pressure aloft (a High or Ridge),.
⭐️ ⭐️ Key Data to Remember:
• High Contour: Higher True Altitude ⟹ High Pressure (Ridge) aloft ⟹ Warmer air,.
• Low Contour: Lower True Altitude ⟹ Low Pressure (Trough) aloft ⟹ Colder air,.

Subsidence is the meteorological term defining the vertical movement of air over a broad area where the motion is downward or sinking. It is characterized as a descending motion of air, usually associated with high-pressure areas (anticyclones) and upper-level divergence.
As air subsides (sinks), it is compressed, leading to adiabatic warming. This warming decreases the relative humidity and stabilizes the atmosphere, often resulting in cloud dissipation and generally clear skies above the friction layer.
⭐️ ⭐️ Key Data to Remember:
• Definition: Vertical downward movement/flow of air.
• Mechanism: Associated with divergence at the surface and convergence aloft in high-pressure systems (Anticyclones).
• Result: Causes adiabatic warming and increases atmospheric stability.
• Inversion: Leads to the formation of a subsidence inversion.

An isobar is a line drawn on a meteorological chart (specifically a synoptic or surface analysis chart) connecting places that have the same atmospheric pressure.
The pressure value used for surface charts is typically the Quoted Fictitious Forecast (QFF). QFF is the atmospheric pressure at a place, corrected or reduced to Mean Sea Level (MSL) considering the prevailing temperature conditions.
⭐️ ⭐️ Key Data to Remember:
• Isobar: Line of equal pressure.
• Surface Charts: Isobars depict Mean Sea Level pressure (QFF).
• QFF vs. QNH: QFF uses actual temperature for reduction to MSL, while QNH uses International Standard Atmosphere (ISA) conditions.
• Operational Context: QFF is vital for weather forecasting but should never be used for altimetry or set on the altimeter subscale.

The tropopause is the boundary layer between the troposphere and the stratosphere, where the temperature ceases to decrease with height.
The height of the tropopause is controlled by the temperature of the air near the surface:
1. Equator (Warm Air): The tropopause is highest over the Equator (tropical regions), where the air is warmest, typically ranging from 52,000 ft to 65,000 ft (approximately 16 km to 20 km).
2. Poles (Cold Air): The tropopause is lowest over the poles, where the air is coldest, typically ranging from 20,000 ft to 26,000 ft (approximately 8 km).
This variation means the tropopause slopes downward from the Equator toward the poles.
⭐️ ⭐️ Key Data to Remember:
• Equator: High tropopause (≈16 km to 20 km).
• Poles: Low tropopause (≈8 km).
• Mid-Latitudes (≈45
∘
): Average height is 36,090 ft (≈11 km).

Constant pressure charts, or contour charts, utilize contour lines (isohypses) that connect points of equal elevation above Mean Sea Level (MSL) at which the specific constant pressure (300 hPa) is observed. These heights are often expressed in decametres (gpm), where 1 decametre=10 metres.
The interval at which these contour lines are drawn is generally standardized for operational upper air charts. While lower level charts (like 500 hPa) might use smaller intervals, standard practice for high-altitude charts, such as the 300 hPa level (which corresponds to approximately 30,000 ft or FL300 in ISA), uses a 80 geopotential decametre (80 gpm) interval. The closer the contour lines, the stronger the wind.
⭐️ ⭐️ Key Data to Remember:
• 300 hPa Level: Approximate ISA altitude is 30,000 ft (FL300).
• Line Type: Isohypse (contour line) connects points of equal height AMSL.
• Contour Interval: The height interval increases for higher altitude charts.

A Col is defined as a neutral region of very little pressure variation located between two high-pressure systems and two low-pressure systems.
The Col area is also geometrically described as the intersection of a trough (elongated low pressure area) and a ridge (elongated high pressure area).
⭐️ ⭐️ Key Data to Remember:
• Pressure: Characterized by almost level or very little pressure variation.
• Winds: Winds are typically very light in a Col due to the weak pressure gradient.
• Weather: This region is often an area of stagnation, which can lead to fog or low stratus in winter, or instability cloud and thunderstorms in summer.

The aircraft are flying along a constant pressure surface (a Flight Level, since the altimeters are set to the Standard Pressure Setting, 1013.2 hPa). The altimeter is calibrated to the International Standard Atmosphere (ISA).
1. Warm Air Mass: Warm air is less dense. Atmospheric pressure decreases more slowly with height in warm air. Consequently, the constant pressure surface (Flight Level) is physically located at a higher True Altitude.
◦ Flying over air warmer than ISA results in True Altitude (TA) being higher than Indicated Altitude (IA).
2. Cold Air Mass: Cold air is more dense. Atmospheric pressure decreases more rapidly with height in cold air. Consequently, the constant pressure surface dips, and the True Altitude is lower.
◦ Flying over air colder than ISA results in True Altitude (TA) being lower than Indicated Altitude (IA).
Therefore, the aircraft flying over the warm air mass has the greater True Altitude.
⭐️ ⭐️ Key Data to Remember (Altimeter Temperature Error):
• Warm ⟹ High: Warmer air results in the True Altitude being higher than the indicated altitude (TA>IA).
• Cold ⟹ Low: Colder air results in the True Altitude being lower than the indicated altitude (TA<IA).

On a meteorological chart, isobars are lines connecting points of equal atmospheric pressure. Specifically, on surface analysis or synoptic charts, these lines depict the Mean Sea Level (MSL) pressure corrected for prevailing temperature conditions, which is known as QFF. Forecasters use QFF to construct accurate analysis charts.
⭐️ ⭐️ Key Data to Remember:
• QFF Definition: QFE (aerodrome pressure) reduced to MSL using the actual temperature.
• Chart Use: Isobars on synoptic/surface charts represent QFF.
• QNH vs. QFF: QNH is QFE reduced to MSL using the International Standard Atmosphere (ISA), whereas QFF uses the actual ambient temperature.
• Operational Context: QFF is for meteorological analysis and must not be set on the altimeter.

The rate of fall of atmospheric pressure with increasing height (the pressure lapse rate) is determined by the density and temperature of the air column.
1. Warm Air: Warm air is less dense than cold air. Because the air molecules are farther apart (the air has expanded), the vertical column of warm air is taller than a standard or cold column containing the same total mass of air. Therefore, pressure decreases less quickly or less rapidly with height in a warm air mass.
2. Cold Air: Cold air is denser. The air column is compacted or shorter. Consequently, pressure decreases more quickly or more rapidly with height in a cold air mass.
Relative to a cold air mass, the rate of pressure fall in a warm air mass will be less.

The pressure of 1013 hPa (or 1013.25 hPa) is the standard atmospheric pressure defined at Mean Sea Level (MSL) in the ICAO International Standard Atmosphere (ISA). In aviation, this specific fixed value is known as the Standard Pressure Setting (SPS).
When the altimeter subscale is set to 1013 hPa, the altimeter indicates pressure altitude or, when divided by 100, a Flight Level (FL).
⭐️ ⭐️ Key Data to Remember:
• Standard Pressure Setting (SPS): 1013 hPa or 1013.25 hPa.
• Operational Use: Used to fly Flight Levels (FL) above the Transition Altitude.
• Standard MSL Pressure Equivalents: 1013.25 hPa=29.92 in Hg.

The relationship between QNH (using International Standard Atmosphere (ISA) conditions for pressure reduction) and QFF (using actual prevailing temperature conditions for pressure reduction) depends on the aerodrome elevation relative to Mean Sea Level (MSL) and the actual air temperature relative to ISA.
1. Conditions: The airfield is AMSL (200 m), and the air temperature is colder than ISA.
2. Pressure Lapse Rate: Cold air is denser than standard air. In cold air, pressure decreases more rapidly with height (steeper pressure lapse rate).
3. Calculation Comparison: Since the airfield is AMSL, QNH and QFF are calculated by adding a pressure correction to the aerodrome pressure (QFE). Because the actual cold air is denser, the correction required to reduce the QFE down to MSL using actual temperature (QFF) is greater than the correction used under warmer ISA conditions (QNH).
4. Conclusion: When an aerodrome is AMSL and the air is colder than ISA, the QFF will be greater than the QNH.
Therefore, QFF must be more than 1009 hPa.
⭐️ ⭐️ Key Data to Remember:
• AMSL, Colder than ISA: QFF>QNH.
• Reason: Cold air is denser, leading to a steeper pressure lapse rate and thus a larger correction value needed for the QFF reduction.

The atmospheric pressure decrease associated with an increase in height is dependent on the International Standard Atmosphere (ISA) relationship near Mean Sea Level (MSL).
1. Approximate Lapse Rate: In the lower few thousand feet of the troposphere, pressure decreases roughly 1 inch of mercury (in Hg) for each 1,000 feet increase in altitude.
2. Conversion: The standard conversion factor relates 1 in Hg to hectopascals (hPa):
◦ 29.92 in Hg=1013.25 hPa.
◦ Therefore, 1 in Hg is approximately equal to 33.86 hPa.
Thus, a 1000 ft increase at MSL results in a pressure decrease of approximately 33.86 hPa, making 33 hPa the correct option.
(Note for ICAO calculations): The precise ISA conversion used for altimetry calculations near the surface is 27 feet per 1 hPa. Using this precise value, 1000 ft/27 ft/hPa≈37 hPa. However, 33 hPa is the closest provided option based on standard meteorological approximations.
⭐️ ⭐️ Key Data to Remember:
• Near Surface Rate (Approximation): 1 in Hg per 1000 ft.
• Conversion: 1 in Hg≈33.86 hPa.
• Result (Approximate hPa drop per 1000 ft): ≈34 hPa.

The relationship between QNH and QFF depends on the aerodrome elevation relative to Mean Sea Level (MSL) and the actual air temperature relative to the International Standard Atmosphere (ISA) at that elevation.
1. Conditions Analysis: The airfield is 200 m Above Mean Sea Level (AMSL). The actual surface temperature ( 5
∘
C) is significantly colder than the ISA temperature expected at that low elevation (ISA at MSL is 15
∘
C). Therefore, the conditions are AMSL and Colder than ISA.
2. Pressure Reduction Principle:
◦ QNH is calculated by reducing QFE (aerodrome pressure) to MSL using the ISA pressure lapse rate.
◦ QFF is calculated by reducing QFE to MSL using the actual (colder) temperature.
◦ Cold air is denser, meaning pressure decreases more rapidly with height (steeper lapse rate) compared to ISA air.
◦ Since the airfield is AMSL, pressure must be added to the measured QFE to reach MSL. Because the cold air results in a steeper pressure gradient, the correction factor used for the QFF calculation is greater than the standard ISA correction used for QNH.
3. Conclusion: When the aerodrome is AMSL and the temperature is colder than ISA, the QFF value is greater than the QNH value (QFF>QNH).
Since QFF is 1030 hPa, the QNH value must be Less than 1030 hPa.
⭐️ ⭐️ Key Data to Remember:
• AMSL Colder than ISA: QFF>QNH.
• Reason: Cold air is dense, leading to a larger pressure correction when reducing QFE to MSL (thus QFF is higher).

This scenario describes an airfield located Below Mean Sea Level (BMSL) flying in air that is Colder than International Standard Atmosphere (ISA).
1. QNH vs. QFF Definition: QNH is the aerodrome pressure (QFE) reduced to Mean Sea Level (MSL) using ISA conditions, while QFF is the QFE reduced to MSL using the actual prevailing temperature (Colder than ISA).
2. Pressure Reduction (BMSL): For an aerodrome BMSL, the pressure correction requires subtracting a value from the QFE to find the sea level pressure.
3. Temperature Effect: Cold air is denser than ISA air, causing the pressure to decrease or increase more rapidly with height (steeper pressure lapse rate).
4. Resulting Relationship: Because the pressure changes more rapidly in the actual cold air, the amount that must be subtracted from the QFE to reach MSL (which yields QFF) is greater than the standard amount subtracted for QNH. A greater subtraction results in a lower final number.
Therefore, when the aerodrome is BMSL and the air is colder than ISA, QNH will be greater than QFF (QNH>QFF). Since QFF=1030 hPa, the QNH must be More than 1030 hPa.
⭐️ ⭐️ Key Data to Remember (BMSL, Colder than ISA):
• Conditions: Aerodrome BMSL and temperature <ISA.
• Relationship: QNH>QFF.
• Correction: Reduction factor for QFF is greater than for QNH.

A Low-Pressure Area (LPA), also referred to as a Depression or Cyclone, is characterized by air converging at the surface and ascending in the center.
The direction of wind circulation around a low pressure system is determined by the pressure gradient force (PGF) and the Coriolis force. In the Southern Hemisphere (SH), the Coriolis force deflects moving air to the left of its intended path.
When air is accelerated inward toward the low pressure center by the PGF, the Coriolis force deflects this inward movement to the left. The result is a clockwise (cyclonic) circulation around the Low in the SH. At the surface, friction causes the wind to cross the isobars slightly inward toward the center of the low.
⭐️ ⭐️ Key Data to Remember:
• Northern Hemisphere (NH) Low: Counterclockwise circulation.
• Southern Hemisphere (SH) Low: Clockwise circulation.
• Vertical Motion (Low): Convergence at the surface and ascending air.
• Buys Ballot’s Law (SH): If you stand with your back to the wind, the low pressure is on your right.

The relationship between altitude and pressure in the International Standard Atmosphere (ISA) defines that the 850 hPa constant pressure level occurs at approximately 5,000 feet Above Mean Sea Level (AMSL).
• 5,000 feet is equivalent to 1524 metres AMSL.
• 1620 metres (5,315 ft) is slightly above the 850 hPa level.
Since 850 hPa is the closest standard pressure level to the given height, it represents the average pressure expected at that altitude in mid-latitudes.
⭐️ ⭐️ Key Data to Remember (ISA Pressure Levels):
• 850 hPa: 5,000 ft AMSL.
• 700 hPa: 10,000 ft AMSL.
• 500 hPa: 18,000 ft AMSL.

In the International Standard Atmosphere (ISA), the 700 hPa constant pressure level corresponds to an approximate height of 10,000 feet Above Mean Sea Level (AMSL). Constant pressure charts, such as the 700 hPa chart, provide weather information for this approximate flight level (FL100). The precise ISA height for 700 hPa is often cited as 9882 ft or 3.01 km.
⭐️ ⭐️ Key Data to Remember (ISA Pressure/Height):
• Pressure Level: 700 hPa.
• ISA Altitude: 10,000 ft AMSL (FL100).
• Operational Context: Used for constant pressure charts in upper air analysis.

QNH (pressure reduced to Mean Sea Level (MSL) using International Standard Atmosphere (ISA) conditions) is always greater than QFE (pressure at the aerodrome datum) when the airfield is Above Mean Sea Level (AMSL). The relationship is given by the vertical pressure gradient.
1. Determine the Pressure Correction (ΔP): The height difference (200 m) must be converted into a pressure difference using the assumption that 1 hPa=8 m.
ΔP=
Lapse Rate(m/hPa)
Elevation(m)
​
=
8 m/hPa
200 m
​
=25 hPa
2. Calculate QFE: Since QNH=QFE+ΔP for an aerodrome AMSL, we calculate QFE:
QFE=QNH−ΔP

QFE=1015 hPa−25 hPa=990 hPa
⭐️ ⭐️ Key Data to Remember:
• Relationship (AMSL): QFE<QNH.
• Conversion (Assumption): 1 hPa change per 8 m height change.
• Operational Context: QFE indicates the height above the aerodrome datum.

A Ridge is an elongated area of relatively high atmospheric pressure. The highest pressure is found along the central line of the ridge.
Consequently, moving perpendicular to this centerline in either direction (away from the axis of the ridge) involves moving toward regions of lower pressure, resulting in the pressure falling.
Conversely, moving perpendicularly away from a Low or Trough (elongated low pressure area) would result in the pressure rising.
⭐️ ⭐️ Key Data to Remember:
• Ridge: Elongated High Pressure. Pressure decreases (falls) away from the center.
• Trough: Elongated Low Pressure. Pressure increases (rises) away from the center.
• Contour Charts: Ridges correspond to areas of high contour heights (warmer air aloft), while troughs correspond to low contour heights (colder air aloft).

This scenario describes a crucial relationship necessary for the development or intensification of a low-pressure system (Depression) at the surface.
1. Pressure Change: When air diverges (spreads out) at upper levels, more air is removed from the vertical column than may be flowing in at the surface (convergence). This removal of mass causes the total weight of the air column to decrease, leading to a fall in pressure at the surface.
2. Vertical Motion and Clouds: A fall in surface pressure indicates the formation or deepening of a surface Low (Cyclone). In a surface low, air flows inward (converges) and is forced to rise. Rising air cools adiabatically, and if the air is sufficiently moist, this cooling leads to condensation and cloud formation.
3. Overall System: Upper-level divergence is typically found directly above a developing surface Low, allowing the converging air below to ascend, resulting in inclement weather.
⭐️ ⭐️ Key Data to Remember:
• Upper Divergence: Removal of mass ⟹ Surface pressure falls.
• Resulting Surface System: Low pressure (Depression).
• Vertical Motion: Rising/Ascending air.
• Weather: Cooling ⟹ Cloudiness and precipitation.
• Conversely: Upper-level convergence causes surface pressure to rise and leads to sinking air and dissipating clouds (High Pressure/Anticyclone).

Pressure always decreases with increasing height (altitude) due to the reduction in the weight of the overlying air column. The rate of pressure decrease is rapid near the surface and slows down at higher altitudes.
Based on the International Standard Atmosphere (ISA) definitions used for aviation reference:
1. At 10,000 ft AMSL: The approximate pressure is 700 hPa.
2. At 30,000 ft AMSL: The approximate pressure is 300 hPa.
⭐️ ⭐️ Key Data to Remember (ISA Pressure Levels): | Pressure (hPa) | Approximate Height (ft) | | :—: | :—: | | 700 | 10,000 AMSL | | 300 | 30,000 AMSL | | 1013.25 | MSL (0 ft) | | 850 | 5,000 AMSL | | 500 | 18,000 AMSL | | 200 | 40,000 AMSL |

A Low-Pressure Area, also known as a Depression or Cyclone, is characterized by air flowing horizontally inward toward the center at the surface. This horizontal inflow is defined as convergence.
Because this air cannot go downward into the ground, surface convergence forces the air to rise vertically (ascending motion/convection) in the center of the low. Regions of low-level convergent winds are generally favorable for cloud formation and precipitation.
⭐️ ⭐️ Key Data to Remember (Low Pressure System):
• Surface Winds: Converge (flow inward) across the isobars toward the center.
• Vertical Motion: Ascending or Rising.
• Upper Levels: Divergence (outflow).
• Weather: Typically associated with cloud formation and precipitation.

The barometric pressure measured at the aerodrome reference point (datum level) is known as QFE. This pressure value is measured at the highest usable point on the aerodrome, or corrected to that point, taking prevailing temperature into account.
When a pilot sets the altimeter subscale to the QFE value, the altimeter will indicate zero feet when the aircraft is positioned at the aerodrome datum level. When airborne, QFE provides the height above the aerodrome level.
⭐️ ⭐️ Key Data to Remember (ICAO/FAA Terms):
• QFE Definition: Atmospheric pressure measured at the aerodrome reference point.
• Altimeter Indication: Reads zero feet on the ground at the aerodrome datum.
• Use: Provides height above the aerodrome (AGL).

A Low-Pressure Area (LPA), or Cyclone/Depression, is characterized by horizontal convergence of air at the surface followed by general ascending motion.
1. Bad Weather: The ascending air cools adiabatically, leading to condensation, extensive cloud formation, and associated precipitation, which constitutes “bad weather”.
2. Better Visibility: Low pressure systems, due to the lifting and mixing of air, generally maintain better overall visibility than high pressure systems or Cols, especially outside of precipitation. In contrast, Highs and Cols are characterized by descending or stagnant air (subsidence), which traps moisture, haze, and smoke near the surface, leading to poor or restricted visibility and fog, particularly in stable conditions.
⭐️ ⭐️ Key Data to Remember (Low Pressure):
• Winds: Converge (inward flow).
• Vertical Motion: Ascending/Lifting air.
• Weather: Cloudiness and Precipitation (“Bad Weather”).
• Visibility: Good, except inside precipitation.

The 200 hPa constant pressure level corresponds to an approximate height of 40,000 feet Above Mean Sea Level (AMSL) in the International Standard Atmosphere (ISA). This pressure altitude level is commonly referred to as FL390 or FL400. Since pressure decreases rapidly with height, high-altitude constant pressure charts, such as the 200 hPa chart, are crucial for representing weather phenomena at typical jet cruising altitudes.
⭐️ ⭐️ Key Data to Remember:
• Pressure Level: 200 hPa.
• ISA Altitude: Approximately 40,000 ft AMSL.
• Operational Context: Used for high-altitude contour charts (FL400).
• Density: At this altitude, air density is approximately 25% of the surface value.

QFE is defined as the atmospheric pressure measured at the aerodrome reference point. It is the pressure at the datum level of an aerodrome. The term QFE refers to this specific barometric pressure value itself.
⭐️ ⭐️ Key Data to Remember (Operational Context):
• Definition: The pressure measured at the aerodrome reference point (datum level).
• Altimeter Indication: When QFE is set on the altimeter subscale, the altimeter reads zero feet when the aircraft is on the aerodrome surface.
• Use: When airborne, flying on QFE provides the approximate height above the aerodrome level (AGL).
• Rounding: QFE is always rounded down to the nearest whole hectopascal.

For an aerodrome situated exactly at Mean Sea Level (MSL), the definitions of QFE (aerodrome pressure), QNH (QFE reduced to MSL using ISA conditions), and QFF (QFE reduced to MSL using actual temperature) converge.
Since the elevation is zero (at MSL), no vertical correction is required to reduce the measured barometric pressure (QFE) to MSL. Consequently, the QFE, QNH, and QFF values are all numerically equal. This equality holds regardless of the actual air temperature or the specific pressure reading.
Therefore, if the QNH is 1014.0 hPa, the QFF must also be 1014.0 hPa.
⭐️ ⭐️ Key Data to Remember:
• Aerodrome at MSL: QNH=QFF=QFE.
• Context: This relationship holds regardless of the actual temperature deviation from ISA.

A Trough is defined as an elongated area of relatively low atmospheric pressure. The lowest pressure runs along the center line of the trough.
Therefore, moving perpendicular to the center line of a Trough in either direction means moving toward regions of higher pressure, causing the pressure to rise.
Conversely, moving perpendicularly away from a Ridge (an elongated area of high pressure) results in pressures falling.
⭐️ ⭐️ Key Data to Remember:
• Trough: Elongated Low Pressure. Pressure increases (rises) away from the center.
• Ridge: Elongated High Pressure. Pressure decreases (falls) away from the center.