The relationship between an object’s temperature and the wavelength of its maximum emitted radiation is inversely proportional, a principle known as Wien’s law (or Wien’s displacement law). Therefore, as the temperature of an object increases, the wavelength at which it emits the maximum amount of radiation shifts toward shorter values.

Points to be Remembered:
• The relationship between temperature (T) and the wavelength of maximum emitted radiation (λmax) is governed by Wien’s law.
• Wien’s law states that λmax is inversely proportional to temperature.
• As temperature increases, the peak emission of radiation shifts toward shorter wavelengths.
• The formula is λmax ∝ 1 / T

Radiation from the sun, known as solar radiation, is predominantly of the short-wave type. Since the sun is extremely hot, it radiates the majority of its energy at these relatively short wavelengths, following Wien’s law. Most of the solar radiation reaching the Earth’s surface consists of wavelengths less than 2 microns.

Points to be Remembered:
• Radiation from the sun is of the short-wave type.
• Solar radiation is commonly referred to as shortwave radiation.
• Most solar radiation reaching the Earth’s surface has wavelengths less than 2 microns.
• The sun emits the majority of its energy at short wavelengths.
• Longwave radiation is typically used to describe the infrared energy emitted by the Earth and the atmosphere.

Points to be Remembered

• The Celsius and Fahrenheit temperature scales are equal at −40 degrees.
• The conversion formula from Celsius to Fahrenheit is:
F = (9/5 × C) + 32
• The Celsius and Kelvin scales never coincide.
• Absolute zero is −273°C or −459°F, which corresponds to 0 K.

High- level temperatures are most accurately and routinely obtained using a radiosonde, a balloon-borne instrument that transmits continuous readings of pressure, temperature, and humidity as it ascends through the atmosphere. These instruments provide a comprehensive vertical profile, or sounding, of the atmosphere. Conversely, aircraft temperature readings are susceptible to inaccuracies caused by aerodynamic heating (compressibility and lag).

Points to be Remembered:
• A radiosonde is a balloon-borne instrument that measures and transmits temperature, pressure, and humidity data continuously aloft.
• Radiosonde observations provide the atmospheric “sounding” (vertical profile).
• Aircraft readings of outside air temperature (often taken by a TAT Probe) are less accurate due to compressibility and lag effects.
• Tephigrams (or pseudo-adiabatic charts) are used to plot and analyze the data obtained from radiosondes, not to measure the temperature itself

The intensity, or rate, at which an object emits radiation is directly proportional to the fourth power of its absolute temperature (Stefan-Boltzmann law). Consequently, objects possessing high temperatures radiate energy at a significantly greater rate or intensity than objects at lower temperatures. For example, the hot sun emits radiation, primarily short-wave radiation, at a much higher intensity than the cooler Earth.

Points to be Remembered:
• All objects whose temperature is above absolute zero emit radiation.
• The intensity of radiation emitted is directly proportional to the object’s absolute temperature (Stefan-Boltzmann law).
• As an object’s temperature increases, the total rate of radiation emitted per unit area increases significantly.
• The sun, being a hot body, emits highly intense, predominantly short-wave radiation

The specific heat of land is lower than that of water. Specific heat is the quantity of heat required to raise the temperature of a substance by $1^\circ \text{C}$. Due to its lower specific heat, land surfaces warm up faster during the day and cool faster at night compared to water. Water requires a great deal more heat to raise its temperature by one degree than land. This unequal heating is crucial in driving thermal circulations and affecting local climates.

Points to be Remembered:
• Specific heat is the amount of heat energy required to raise the temperature of the surface by $1^\circ \text{C}$.
• Water has a high specific heat, while land (such as dry soil or bare rock) has a much lower specific heat.
• Substances with lower specific heat warm up faster and cool faster.
• Water heats more slowly and cools more slowly than land, causing oceans to act as huge heat reservoirs.
• Lower specific heat contributes to larger diurnal temperature variations over land.

Surface air temperature is measured using mercury thermometers housed inside a standardized instrument shelter, such as the Stevenson screen. To ensure the reading is representative of the air temperature and not unduly influenced by the heating or cooling of the ground surface, the shelter is held at a specific reference height. This universally accepted height standard is approximately $1.25 \text{ m}$ (or $4 \text{ ft}$) above the ground.

Points to be Remembered:

• Surface temperatures are measured in a Stevenson screen.
• The standard height for surface temperature measurement is $4 \text{ ft}$ or approximately $1.25 \text{ m}$ ($1.22 \text{ m}$).
• The shelter protects the thermometers from direct sunlight and precipitation.
• The shelter is held off the ground so the reading is not adversely affected by the ground temperature

Conduction is the process of heat transfer from one substance to another by molecular activity or direct contact. Heat is always transferred from the warmer body to the colder body. In the atmosphere, the air layer immediately in contact with the ground surface is heated or cooled by conduction. However, since air is a poor conductor of heat, this process is only effective within a shallow layer near the surface ( as mixing is confined to surface level). As warm air ascends( rising air ) the heat moves to the higher level. 

Points to be Remembered:
• Conduction transfers heat through molecular contact.
• Heat transfer occurs from the warmer body to the colder body.
• Air is an extremely poor conductor of heat.
• Conduction is one of the processes responsible for heating the atmosphere.
• Latent heat is the energy absorbed or released during a change in state without altering temperature.
• Convection is the transfer of heat by the mass movement (vertical motion) of a fluid.
• Radiation is the transfer of heat from a hot body to a cold body without physical contact

The lowest temperature of the day occurs approximately 30 minutes to one hour after sunrise. Although incoming solar radiation (insolation) commences at dawn, the surface and the air above it continue to cool until incoming solar energy and outgoing terrestrial energy reach equilibrium. This delay is due to the lag effect, as the low angular elevation of the sun during early morning hours means that the incoming radiation is weak and must pass through a substantial thickness of the atmosphere before effectively warming the surface.
Points to be Remembered:
• The lowest temperature occurs about a half an hour after sunrise.
• The period of cooling continues after sunrise because outgoing terrestrial radiation temporarily exceeds incoming solar radiation.
• The maximum temperature typically occurs around 1500 Local Mean Time (LMT) due to a similar lag effect. ( for India it is 1400 )

The sun is an extremely hot body, and according to the Stefan-Boltzmann law, the total energy emitted is proportional to the fourth power of the temperature, meaning the sun gives out a large amount of energy [k, m]. Following Wien’s law, this high temperature results in peak emission occurring at short wavelengths (shortwave radiation) [i, j, l]. The Earth, being significantly cooler, radiates a relatively small amount of energy [n, m], mainly at longer infrared wavelengths (longwave radiation) [l, n].

Points to be Remembered:
• Hot bodies emit radiation at a significantly greater rate or intensity than cooler bodies [k].
• The total intensity of radiation emitted is proportional to the fourth power of the object’s absolute temperature (Stefan-Boltzmann law) [k].
• The wavelength of maximum radiation emitted is inversely proportional to the object’s temperature (Wien’s law) [l].
• Solar radiation is short-wave radiation (wavelengths mostly less than $2 \mu\text{m}$) [j, l].
• The Earth radiates terrestrial (long-wave) radiation (wavelengths between $4$ and $80 \mu\text{m}$) [l, n].

Land surfaces heat up and cool off more rapidly than water surfaces because land has a lower specific heat. In winter (Northern Hemisphere), continental areas cool substantially, making oceanic areas relatively warmer. Conversely, in summer, continents warm significantly faster than oceans, resulting in land being relatively warmer than the sea. This process causes larger seasonal temperature variations over continents.

Points to be Remembered:
• Water has a higher specific heat than land, causing it to warm and cool more slowly.
• In winter (January, Northern Hemisphere), continents are colder than oceanic areas at corresponding latitudes.
• In summer (July, Northern Hemisphere), continents are warmer than oceanic areas at corresponding latitudes.
• Coastal areas assume the temperature characteristics of the land or water on their windward side.
• The climates of interior continental regions are generally more extreme, showing larger annual temperature ranges.

The troposphere is heated primarily indirectly from the Earth’s surface, which absorbs incoming solar (short-wave) radiation. This heat is transferred to the lowest air layers via conduction. Vertical air motion, known as convection, then distributes this heat upwards, warming the upper levels. Furthermore, when water vapor condenses into clouds, the release of latent heat (latent heat of condensation) warms the atmosphere, significantly contributing to the heating process and enhancing vertical motion. Overall, the lower atmosphere is mainly heated from the ground upward.
Points to be Remembered:
• Solar radiation (short-wave) passes through the lower atmosphere with little direct heating, but warms the Earth’s surface (insolation).
• The atmosphere is mainly heated from the ground upward.
• Conduction transfers heat from the warmed surface to the air in contact with it.
• Convection (vertical transport of air) helps heat the upper levels of the atmosphere.
• Latent heat of condensation is released when water vapor condenses, warming the atmosphere.
• The overall heat transferred to the atmosphere is comprised of sensible heat (conduction/convection) and latent heat (condensation/evaporation).
• Terrestrial radiation (long-wave) absorption by water vapor and $text{CO}_2$ is the main method by which the atmosphere is heated.

The primary method by which the Earth’s surface is heated is through the absorption of incoming solar radiation. Since the sun is extremely hot, this energy is classified as short-wave radiation. This short-wave radiation is largely transparent to the atmosphere, passing through with little absorption, and directly heating the ground, a process known as insolation. The atmosphere is subsequently heated indirectly from this warmed surface by conduction, convection, and long-wave terrestrial radiation.

Points to be Remembered:
• Solar radiation is short-wave radiation.
• The heating of the Earth’s surface by solar radiation is called insolation.
• Most solar radiation that reaches the Earth’s surface is of wavelengths less than 2 microns.
• Short-wave radiation does not significantly heat the atmosphere directly.
• The atmosphere is primarily heated from the ground upward via other processes (conduction, convection, terrestrial radiation)

The diurnal variation of temperature is maximized when conditions allow for the highest possible daytime temperature and the lowest possible nighttime temperature. Wind causes turbulent mixing, especially near the surface. During the day, wind mixes warm surface air with cooler air aloft, which reduces the maximum temperature reached. Conversely, at night, wind mixes the cold air near the ground (formed by radiational cooling) with warmer air above, which raises the minimum temperature. Therefore, the presence of wind, even light wind, reduces the overall diurnal range. The greatest diurnal variation occurs in the absence of wind, or “no wind” (calm) conditions.

Points to be Remembered:
• Diurnal temperature variation is greatest with clear skies and no wind.
• The effect of wind is to reduce the diurnal variation by lowering the maximum daytime temperature and raising the minimum nighttime temperature.
• Wind reduces maximum temperature by causing turbulent mixing of warm surface air with cooler air above.
• Wind reduces cooling (raises minimum temperature) at night by mixing cold surface air with warmer air aloft.
• The lowest temperatures usually occur about a half an hour after sunrise, contributing to the lowest point of the diurnal cycle.

The Kelvin scale is the absolute temperature scale used in meteorology, with intervals equal to those of the Celsius scale.
The relationship for conversion is: K=°C+273.
1. Boiling Point (Standard Pressure): Water boils at 100
∘
C.
2. Calculation: 100
∘
C+273=373 K.
Key Data to Remember:
Condition
Celsius (°C)
Kelvin (K)
Absolute Zero
−273
∘
C
0 K
Freezing Point
0
∘
C
273 K
Boiling Point
100
∘
C
373 K

The temperature to which a parcel of air must be cooled, at constant barometric pressure and constant water vapor content, for saturation to occur and for water vapor to condense into liquid water is the dew point. When unsaturated air is lifted, it cools adiabatically. The level at which the rising air cools to its dew point is termed the condensation level, where the air becomes saturated.
Points to be Remembered:
• Dew point is the temperature to which a parcel of air must be cooled for saturation to occur.
• When a rising air parcel cools to its dew point, the relative humidity reaches 100 percent, and the air is saturated.
• The level where rising air reaches the dew point is the condensation level.
• The dew point temperature is an indicator of the air’s actual water vapor content

Clouds significantly reduce the diurnal variation of temperature by influencing both daytime heating and nighttime cooling. During the day, clouds reflect solar radiation back to space, thus lowering the maximum temperature reached at the surface. During the night, clouds act as an insulating layer, absorbing the Earth’s outgoing terrestrial radiation and re-radiating heat back toward the ground, thereby inhibiting cooling and raising the minimum temperature. This dual action of suppressing the maximum temperature and elevating the minimum temperature results in the least overall diurnal temperature variation.
Points to be Remembered:
• Cloud cover, such as overcast conditions, reduces the diurnal variation of temperature.
• Cloud tops reflect incoming solar radiation, which reduces the maximum daytime temperature.
• Clouds trap outgoing terrestrial radiation, which raises the minimum nighttime temperature.
• The greatest diurnal variation is found over land, with clear skies and no wind.

In meteorology, a layer of the atmosphere where the temperature remains constant with increasing altitude is defined as an isothermal layer. This condition implies a zero lapse rate. Isothermal layers are highly stable and strongly resist vertical air motion. In contrast, an inversion is characterized by a temperature increase with height.
Points to be Remembered:
• An isothermal layer is characterized by a constant temperature profile with increasing altitude (zero lapse rate).
• An inversion is an increase in temperature with altitude.
• Isothermal layers represent stable atmospheric conditions.
• The average lapse rate in the troposphere is roughly $2^\circ\text{C}/1000 \text{ ft}$.

The diurnal variation of temperature is the difference between the maximum and minimum temperatures over a 24-hour period. This variation is highly dependent on the specific heat of the surface. Substances with a lower specific heat warm up faster and cool faster, resulting in a large diurnal temperature range. Bare rock and concrete have a low specific heat compared to water or vegetation (which contains water). Rock or concrete surfaces attain a higher temperature by insolation and lose their heat quickly, maximizing the diurnal temperature fluctuation.

Points to be Remembered:
• Diurnal variation is the change in temperature from day to night.
• It is inversely proportional to the specific heat of the surface material.
• Bare rock and concrete have low specific heat, leading to rapid heating and cooling and a high diurnal variation.
• Water has a high specific heat, leading to minimal diurnal variation.
• Snow surfaces also have a minimal diurnal variation due to high reflectivity and latent heat required for melting.
• Vegetation (grasslands) experiences a smaller diurnal range than barren surfaces because energy is used to evaporate water rather than heat the air.

This principle is governed by Wien’s law, which states that the wavelength of maximum emitted radiation ($\lambda_{max}$) is inversely proportional to the object’s absolute temperature ($T$). Consequently, as the temperature of an object increases, the wavelength of its most intense radiation shifts toward shorter values. For example, the extremely hot Sun emits most of its energy as shortwave radiation (shorter wavelengths), while the cooler Earth emits longwave (terrestrial) radiation (longer wavelengths).

Key Data to be Remembered:
• The relationship is defined by Wien’s law.
• The wavelength of maximum emitted radiation ($\lambda_{max}$) is inversely proportional to the object’s absolute temperature ($T$).
• Hotter bodies emit radiation at shorter wavelengths.
• The Sun’s peak emission occurs near $0.5 \mu\text{m}$ (visible light

The standard boiling point of pure water is based on standard atmospheric pressure (1013.25 hPa or 29.92 in. Hg).
• On the Fahrenheit scale, the boiling point of water at standard pressure is defined as 212
∘
F.
• For reference in aviation meteorology, which primarily uses the Celsius scale, this corresponds to 100
∘
C.
Key Data to Remember:
Condition
Fahrenheit (°F)
Celsius (°C)
Kelvin (K)
Boiling Point
212
100
373
Freezing Point
32
0
273

Clouds are effective absorbers and emitters of long-wave terrestrial radiation. During the night, clouds absorb the infrared energy radiated by the Earth’s surface and re-radiate a significant portion of this heat back toward the ground. This greenhouse-like effect inhibits radiational cooling, thereby raising the minimum temperature reached near the surface. Consequently, calm, cloudy nights are typically warmer than calm, clear nights.

Points to be Remembered:
• Cloud cover increases the minimum nighttime temperature ($T_{min}$).
• The overall effect of clouds is to reduce the diurnal temperature variation

The total energy radiated by a black body is proportional to its temperature (T)
T$^4$ (*)
Explanation
The relationship between the total radiant energy emitted by a body ($E$) and its absolute temperature ($T$) is defined by the Stefan-Boltzmann Law. A blackbody is an idealized physical body that is a perfect absorber and emitter of all incident electromagnetic radiation. The law states that the total radiant energy emitted is proportional to the fourth power of the absolute temperature.
Key Data to Remember
• Law: Stefan-Boltzmann Law.
• Formula: $E \propto T^4$ (or $E = \sigma T^4$).
• Temperature Unit: $T$ is the absolute temperature in Kelvins (K

The reflectivity of a surface, known as albedo, determines the amount of solar radiation reflected back to the atmosphere. Snow is a highly reflective surface, reflecting approximately 80% of solar radiation. This high reflectivity minimizes the energy absorbed by the surface, which is a major factor contributing to very cold surface air temperatures.

Key Data to Remember
• Reflectivity: A snow surface reflects approximately 80% of solar radiation.
• Albedo Range: Snow reflectivity ranges from 75% up to 95% for fresh snow.
• Effect: High reflectivity results in minimal surface heating.

On a clear day, the atmosphere is largely transparent to incoming solar radiation (shortwave radiation), allowing a high percentage of energy to reach the Earth’s surface. This heating of the surface is referred to as insolation.
Key Data to Remember
• Fraction/Percentage: Approximately 85% of the sun’s radiation reaches the Earth’s surface on a clear day. This is equivalent to approximately $5/6^{\text{th}}$ of the total solar radiation.
• Condition: This high transmission occurs when there is no cloud present

Albedo is the measure of the reflecting power of a surface. Specifically, it is the percentage of solar radiation returning from a given surface compared to the amount of radiation initially striking that surface.
Key Data to Remember
• Definition: The ratio of outgoing to incoming solar radiation, usually expressed as a percentage.
• Earth/Atmosphere Average: The earth and its atmosphere (including clouds) have a combined average albedo of 30%.
• Highly Reflective Surfaces: Fresh snow has a very high albedo, reflecting between 75% to 95% of solar energy.
• Effect: High albedo minimizes the solar energy absorbed by the surface

The Celsius ($\text{C}$) and Fahrenheit ($\text{F}$) temperature scales coincide at a single point. This point occurs at minus forty degrees, where $-40^{\circ}\text{C}$ is equal to $-40^{\circ}\text{F}$.
Key Data to Remember
• Coincidence Point: The Celsius and Fahrenheit scales coincide at $-40$ degrees.
• Conversion Formula (C to F): $\text{F} = (\text{C} \times \frac{9}{5}) + 32$.
• Conversion Check: When $-40^{\circ}\text{C}$ is applied to the conversion formula, the result is $-40^{\circ}\text{F}$.

Minimum Thermometer

A minimum thermometer uses alcohol as the working liquid. Alcohol is suitable because it has a very low freezing point of −130°C (−202°F), which is much lower than the freezing point of mercury at −39°C (−38°F). This property allows the minimum thermometer to accurately measure and record very low temperatures.

Key Data to Remember

• Liquid used: Alcohol
• Reason: Alcohol freezes at −130°C (−202°F), unlike mercury which freezes at −39°C (−38°F)
• Function: Records the lowest temperature reached during a given period
• Alternative: Mercury thermometers are commonly used for surface temperature measurements in a Stevenson screen

An isothermal layer is defined as an atmospheric layer where the temperature remains constant with increasing altitude. This condition represents a zero lapse rate.
Key Data to Remember
• Definition: Temperature remains constant with height.
• ISA Conditions: In the International Standard Atmosphere (ISA), the temperature remains constant at $-56.5^{circ}text{C}$ from $11text{ km}$ ($36,090text{ ft}$) up to $20text{ km}$ ($65,617text{ ft}$).
• Boundary: The bottom of this zone is the tropopause, marking the transition from the troposphere to the stratosphere

At coastal aerodromes, the wind direction is the most significant factor determining the diurnal variation of temperature. This is primarily because of the alternating land and sea breezes.
During the day, the sea breeze blows onshore, carrying cool, stable maritime air that moderates the maximum surface temperature. At night, the land breeze blows offshore, carrying warmer (or colder, depending on the season and inland distance) continental air. The diurnal range is minimized when the prevailing wind is onshore (from the cooler water).
Key Data to Remember
• Coastal Control: The high heat capacity of water compared to land causes the differential heating necessary for land/sea breezes.
• Effect of Wind Direction: The change in wind direction (onshore/offshore flow) transports air masses with different thermal properties, directly modulating the temperature range.
• Impact: In a calm wind diurnal variation is greater, while in a fast wind diurnal variation is less

The diurnal variation of temperature over the sea is minimal. This low variability occurs because water has a high specific heat compared to land, meaning it absorbs and releases heat slowly. Consequently, the temperature change between day and night over large water bodies is small.
Key Data to Remember
• Range: The diurnal temperature variation over the sea is generally less than $1^{\circ}\text{C}$.
• Physical Principle: Water has a higher Specific Heat than land, absorbing heat over a long period and losing it slowly.
• Operational Context: Minimal diurnal variation over water surfaces is why radiation fog rarely forms over the sea

Conversion of Temperature from Fahrenheit (°F) to Kelvin (K)

To convert temperature from degrees Fahrenheit (°F) to Kelvin (K), the conversion is done in two steps: first from Fahrenheit to degrees Celsius (°C), and then from Celsius to Kelvin (K).

Step 1: Fahrenheit to Celsius
The formula used is:
C = (F − 32) × 5/9

For a temperature of 68°F:
C = (68 − 32) × 5/9
C = 36 × 5/9
C = 20°C

Step 2: Celsius to Kelvin
The formula used is:
K = C + 273

K = 20 + 273
K = 293 K

Key Data to Remember
• Conversion formulas:
– C = (F − 32) × 5/9
– K = C + 273

• Absolute zero:
0 K = −273°C

The Stevenson screen is a louvred wooden box designed to house surface temperature instruments, primarily mercury thermometers. Its main meteorological function is to ensure accurate ambient air temperature readings by protecting the thermometers from direct sunlight (solar radiation) and precipitation, while allowing adequate air circulation through its louvers.
Key Data to Remember
• Function: Protects thermometers from direct solar radiation and wind.
• Location/Height: Held $4text{ ft}$ ($1.22text{ m}$) above the ground.
• Instruments Housed: Includes mercury thermometers and wet-bulb thermometers

Solar radiation is primarily shortwave electromagnetic radiation emitted by the sun. The energy is distributed across the electromagnetic spectrum, with nearly half of the total radiant energy being contained within the Infrared (IR) region.
Key Data to Remember
The components of solar radiation are approximately:
• Infrared (IR) radiation: 46% to 49%.
• Visible light: 44% to 45%.
• Ultraviolet (UV) radiation: 7% to 9%

Surface air temperatures are typically measured using thermometers housed within a Stevenson screen. The Stevenson screen is a louvred wooden box designed to protect the thermometers from direct sunlight and precipitation while allowing air circulation.
Key Data to Remember
• Instrument Shelter: Stevenson screen.
• Height: The screen is held $4\text{ ft}$ ($1.22\text{ m}$) above the ground.
• Purpose of Height: This height prevents the reading from being adversely affected by the direct ground temperature

The Fahrenheit temperature scale is based on fixed reference points. The freezing point of pure water at standard atmospheric pressure (sea level) is defined as $32^{\circ}\text{F}$.
Key Data to Remember
• Fixed Points (Fahrenheit Scale): Freezing point of water is $32^{\circ}\text{F}$; boiling point of water is $212^{\circ}\text{F}$.
• Scale Interval: The difference between freezing and boiling points is $180^{\circ}\text{F}$.
• Conversion: To convert Celsius ($\text{C}$) to Fahrenheit ($\text{F}$), use the formula $\text{F} = \frac{9}{5}\text{C} + 32$.
• Coincidence: The Celsius and Fahrenheit scales are numerically equal at $-40$ degrees

*Incoming solar radiation is reflected back to space and outgoing terrestrial radiation is re-radiated from the cloud layer back to the surface.
Reasoning: Clouds modulate temperature extremes by affecting both incoming shortwave solar radiation during the day and outgoing longwave terrestrial radiation at night. During daytime, clouds reflect solar radiation, thus reducing the maximum temperature reached at the surface. At night, clouds absorb terrestrial radiation emitted by the Earth and re-radiate this heat back downwards, preventing excessive cooling of the surface. This combined effect of lowering the maximum temperature and raising the minimum temperature reduces the overall diurnal temperature variation.
Points to be Remembered:
• Cloud cover reduces the diurnal variation of temperature.
• During the day, clouds reflect solar radiation back by the cloud tops, reducing the maximum temperature.
• At night, clouds absorb outgoing terrestrial radiation and re-radiate heat back toward the ground.
• The effect of clouds is to keep nighttime temperatures higher and daytime temperatures lower, resulting in a small daily temperature range.

Air is scientifically classified as a poor conductor of heat. In the context of atmospheric dynamics, a rising or sinking air parcel undergoes temperature changes—cooling upon expansion or warming upon compression—with very little exchange of heat with its surroundings. This condition, fundamental to vertical motion and cloud formation, is known as an adiabatic process, meaning the parcel behaves as if it is effectively insulated.
Key Data to Remember
• Air Property: Air is a poor conductor of heat.
• Process: The thermodynamic process where an air parcel changes temperature without exchanging heat with the environment is adiabatic.
• Significance: This principle is vital for determining atmospheric stability

Wind causes turbulent mixing of the air near the surface with air located slightly higher. During the day, this mixing transports cooler air downward, lowering the maximum temperature ($\text{T}{\text{max}}$). During the night, wind mixes cold surface air with warmer air aloft, raising the minimum temperature ($\text{T}{\text{min}}$). Since the wind simultaneously decreases the maximum temperature and increases the minimum temperature, the overall effect is to reduce (decrease) the diurnal variation.
Key Data to Remember
• Effect: Wind reduces the diurnal range of temperature.
• Maximum DV: The greatest diurnal variation occurs under clear skies and light (calm) winds

The Diurnal Variation (DV) of temperature is the difference between the daily maximum and minimum temperatures. DV is maximal over land surfaces because land has a lower specific heat capacity compared to water. Since land heats up and cools down much faster than water, continental areas experience significantly greater temperature fluctuations between day and night.
Key Data to Remember
• Maximum DV Location: Land surfaces.
• Controlling Factor: The Specific Heat of the surface. Land surfaces heat and cool quickly.
• Minimum DV Location: Over the sea, where DV is typically minimal (generally less than $1^{\circ}\text{C}$).
• Conditions: Greatest diurnal variation occurs under clear skies and light (calm) winds

Diurnal variation (DV) is the difference between the maximum and minimum daily temperatures. DV is maximum over arid landmasses (deserts) because land has a low specific heat capacity, meaning it heats up quickly during the day due to insolation and cools down quickly at night due to terrestrial radiation loss. Additionally, dry air and typically clear skies found over deserts allow maximum solar radiation to reach the surface by day and maximize radiational cooling at night, leading to the largest daily temperature ranges.
Key Data to Remember
• Maximum DV: Occurs over land. The largest diurnal range occurs on high deserts.
• Minimum DV: Occurs over water/sea (high specific heat). Typically less than $1^{\circ}\text{C}$ over the ocean.
• Conditions: DV is greatest with clear skies and light winds.
• Physical Cause: Low specific heat of dry soil/rock results in rapid surface temperature change.

During the day, solar radiation is primarily absorbed by the Earth’s surface (insolation), causing the ground to heat up significantly. The atmosphere (ambient air) is then heated primarily from the ground upward. For heat to transfer from the ground to the air by conduction—which occurs when two bodies are touching—the warmer body must transfer heat to the colder body. Therefore, the ambient air temperature in the layer immediately above the surface is lower than the ground temperature, enabling the upward heat flow.
Key Data to Remember
• Heating Mechanism: The atmosphere is heated from the ground upward by terrestrial radiation, conduction, and convection.
• Conduction Principle: Heat flows from the warmer ground surface to the cooler air in contact with it.
• Result: This process causes warm air parcels (thermals) to rise, distributing heat upward

Heat is defined as the total energy stored in all the molecules of a substance (internal energy) . It is energy in the process of being transferred from one object to another due to a temperature difference between them .
In contrast, temperature is a measure of the average kinetic energy (average speed) of the atoms and molecules .
Key Data to Remember
• Heat: The total energy stored in all molecules (sum total/internal energy).
• Temperature: A measure of the average kinetic energy of molecules .
• Transfer: Heat transfer occurs because of temperature differences .

The Kelvin temperature scale (Absolute scale) is directly related to the Celsius scale, where 0 K corresponds to -273°C (absolute zero). The freezing point of pure water is 0°C. The conversion formula from Celsius to Kelvin is K = °C + 273. Applying this conversion, the freezing point of water is 0 + 273 = 273 K.

Key Data to Remember:
• Freezing Point: 273 K or 0°C.
• Boiling Point: 373 K or 100°C (at standard pressure).
• Conversion: K = °C + 273.
• Absolute Zero: 0 K = -273°C

Converting temperature from degrees Fahrenheit (F) to Kelvins (K) requires a two-step process: first, converting Fahrenheit to degrees Celsius (C), and then converting Celsius to Kelvin.

1. Fahrenheit (F) to Celsius (C) Conversion:
C = (F – 32) × 5/9
C = (82 – 32) × 5/9 = 50 × 5/9 ≈ 27.8°C

2. Celsius (C) to Kelvin (K) Conversion:
K = C + 273
K = 27.8 + 273 = 300.8 K

The result, 300.8 K, is approximated as 300 K based on the options provided.

Key Data to Remember:
– Fahrenheit to Celsius: C = (F – 32) × 5/9
– Celsius to Kelvin: K = °C + 273
– Absolute Zero: 0 K is equal to -273°C
– Freezing Point: 0°C is equal to 273 K

Land surfaces have a lower specific heat capacity compared to water surfaces (sea), meaning land heats up and cools down much more rapidly.
During winter, continental landmasses cool rapidly due to terrestrial radiation, making them significantly colder than adjacent large water bodies. Conversely, the high specific heat of water causes the oceans to retain heat longer, resulting in oceanic areas being relatively warmer than continental areas during winter. This temperature difference is the mechanism driving wind flow such as the land breeze (cold air from land flowing over warmer water).
Key Data to Remember
• Winter Contrast: Continents are colder than oceans at corresponding latitudes.
• Summer Contrast: Continents are warmer than oceans at corresponding latitudes.
• Physical Principle: Land has a low specific heat and changes temperature rapidly; water has a high specific heat and changes temperature slowly

Clouds act like a blanket at night by trapping outgoing infrared radiation emitted by the Earth’s surface and radiating part of it back downward, thus reducing cooling and keeping the surface warmer. On clear nights, more longwave radiation from the Earth’s surface escapes directly into space, causing it to cool more rapidly and be colder compared to cloudy nights.

The clouds partly radiate the infrared energy back to Earth, preventing full heat loss by nocturnal radiation and resulting in warmer temperatures during cloudy nights.

Thus, clear nights have more effective radiation cooling of the ground, making them cooler than cloudy nights, where clouds reduce this radiation cooling effect by reflecting and trapping radiation.​

Point To remember
nocturnal radiation are partly radiated back by clouds to earth,
cloud prevent nocturnal radiation to escape fully (are partly prevent),
radiation escape is reduced (not full escape).

Cloud cover, both by day and night, significantly impacts the diurnal variation of temperature.
• During the day: Clouds prevent some solar radiation from reaching the Earth’s surface, which reduces the maximum air temperature reached near the surface (Tmax).
• During the night: Clouds absorb outgoing terrestrial (longwave) radiation and re-radiate a portion of this heat back toward the surface, thereby raising the minimum temperature that the air drops to (Tmin).
• Overall effect: Since clouds lower the maximum daytime temperature and raise the minimum nighttime temperature, the net effect is to reduce the diurnal range (or diurnal variation) of temperature compared to clear-sky conditions.
Key Data to Remember
• The overall effect of cloud cover is to reduce the diurnal temperature variation.
• The greatest diurnal variation is typically found over land with clear skies and no wind.
• The least diurnal variation is typically found over the sea and ice caps when skies are cloudy and windy

Insolation is the process by which incoming shortwave solar radiation reaches and heats the Earth’s surface. Most solar radiation passes through the atmosphere to warm the ground, which then heats the atmosphere from below via conduction and convection.
Key Data to Remember
• Insolation: Solar radiation absorbed by the Earth’s surface.
• Heating: The troposphere is mainly heated from the ground upward through this process.
• Controls: The amount of insolation received depends on factors such as angular elevation of the sun, latitude, season, and time of day

Water vapor is described as a selective absorber of radiation [*]. This means it allows some wavelengths of radiation to pass through while absorbing others. Water vapor and carbon dioxide are transparent to short wavelength solar radiation but are less permeable to long wavelengths (terrestrial radiation). Water vapor absorbs a portion of the longwave terrestrial radiation and radiates some of it back (re-radiates) towards the Earth’s surface. This process is known as the greenhouse effect, which helps maintain a suitable temperature necessary for life

On a clear, calm night, the ground cools rapidly by emitting terrestrial (longwave) radiation (radiational cooling) []. The air in contact with this cool surface is cooled by conduction []. Since air is a poor conductor of heat, this cooling primarily affects a shallow layer near the surface []. This results in a radiation inversion where the temperature increases with height immediately above the ground (a stable condition) []. Above this inversion layer, the temperature typically begins to decrease with height again, approximating the average Environmental Lapse Rate (ELR) of 1.98
∘
C per 1000ft (often simplified to 2
∘
C per 1000ft) [*].
Key Data to Remember
• Mechanism: Radiational cooling of the surface creates the inversion [*].
• Result: The coldest air is found next to the ground.
• Stability: The atmosphere is generally most stable near sunrise, when the inversion is strongest.
• Lapse Rate Above: Above the inversion, the temperature decreases, often near the average ELR (2
∘
C/1000ft)

The rise in the temperature of a surface (ΔT) is inversely or indirectly proportional to its Specific Heat (SH) [*]. Specific Heat is defined as the amount of heat required to raise the temperature of the surface by 1
∘
C (or 1K) [i]. Substances with a higher specific heat warm up slowly and cool down slowly, resulting in a smaller temperature change compared to those with lower specific heat [i]. This relationship can be expressed generally as ΔT∝
Specific Heat
1
​
[i, j].
Key Data to Remember
• Relationship: Temperature rise is inversely proportional to specific heat [j].
• Water vs. Land: Water has a much higher specific heat (1 cal/gm/K) than dry soil (0.2 cal/gm/K) [i].
• Effect: The diurnal temperature variation over the sea is minimal (approximately 1
∘
C) compared to over land (approximately 20
∘
C) because water has a high specific heat [j, k].
Supporting Data The specific heat of various substances varies, with water having a specific heat of 1 cal/gm/K and dry soil having a specific heat of 0.2 cal/gm/K [i].
• If the specific heat of water is unity, compared to other substances whose specific heat is much less, the diurnal temperature variation over the sea is small, generally less than 1
∘
C [k].
• The rise in temperature is inversely proportional to the specific heat [k].
The specific heat of some examples of surfaces is listed in order of increasing specific heat, showing the relative difference [l]:
1. Bare rock/stone
2. Concrete
3. Dry soil
4. Wet soil
5. Oceans
6. Snow surfaces

The primary function of the Stevenson screen is to house meteorological instruments (such as thermometers) and protect them from direct solar radiation and terrestrial radiation while allowing air to circulate [i, j]. If the door were opened facing the sun, solar radiation would enter and directly strike the thermometers, causing an inaccurate reading that would be higher than the true ambient air temperature [k]. Therefore, the screen is typically designed so the door opens away from the sun (often facing North in the Northern Hemisphere) to ensure the observer’s body shields the interior during readings, maintaining measurement integrity [l].
Key Data to Remember
• Purpose: To shield thermometers from direct sunlight [i, j].
• Structure: Louvered wooden box [i, j].
• Height: Mounted 4 feet (1.25 m) above the ground [i, j, m].
• Result: Prevents temperature readings from being adversely affected by ground heating or direct solar radiation

A temperature inversion is a meteorological condition where the temperature of the air increases with height, reversing the normal atmospheric lapse rate [*]. Inversions signify an extremely stable layer in the atmosphere. Common types include the radiation (ground) inversion, which forms near the surface on clear, calm nights due to radiational cooling, and the subsidence inversion, resulting from the compressional warming of sinking air within high-pressure systems. Inversions act as a lid, inhibiting vertical mixing and trapping pollutants or low clouds below the inversion base.
Key Data to Remember
• Definition: Temperature increases with height [*].
• Stability: Represents absolute stability.
• Opposite: The condition where temperature remains constant with height is an isothermal layer.
• Lapse Rate: The rate of temperature change is negative (increases with height)

Heat is transferred from the Earth’s surface to the atmosphere through a combination of latent heat and sensible heat []. Latent heat transfer, primarily involving evaporation, condensation, and sublimation, accounts for approximately of the heat flow from the surface []. When water evaporates from the surface, it absorbs latent heat, storing this energy in the water vapor, which is then released back into the atmosphere upon condensation (cloud formation), contributing significantly to the heating of the troposphere [*].
Key Data to Remember
• Dominant Flow: Latent Heat constitutes approximately 77% of the heat transferred from the Earth’s surface to the atmosphere [*].
• Secondary Flow: Sensible Heat (through conduction, convection, and terrestrial radiation) accounts for approximately 23% of the heat transferred

The Earth continuously loses heat by radiating longwave infrared energy (terrestrial radiation) []. This process is known as radiational cooling []. The rate at which an object radiates energy depends primarily on its temperature; the higher the object’s temperature, the greater the rate or intensity of emitted radiation (Stefan-Boltzmann law) [k, l, m]. Since heat flows from warmer to colder regions, the hotter the ground is, the more intense the nocturnal radiation will be [k, n].
Key Data to Remember
• Radiation: All objects with temperatures above absolute zero emit radiation [k].
• Intensity: Higher temperature objects emit radiation at a greater rate (intensity) [k].
• Process: The rate of outgoing terrestrial radiation is proportional to the fourth power of the absolute temperature of the surface (Stefan-Boltzmann law) [k, l].
• Effect: On a clear, calm night, the ground cools rapidly by emitting terrestrial longwave radiation [*].
Supporting Evidence The Stefan-Boltzmann law states that objects that have a high temperature emit radiation at a greater rate or intensity than objects with a lower temperature [k, l]. Therefore, the hotter the surface, the greater the emission of infrared energy [n]. The earth’s surface radiates heat at all times as longwave radiation [n]. This longwave radiation is the main method by which the atmosphere is heated [n].
Additional Context On a clear night, if the surface air is relatively dry and cloud-free, the ground cools rapidly by emitting infrared radiation