Short Notes 05: Humidity

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Water Vapour (Importance)

• Most important gas for weather.
• Variable (0–4/5% by volume).
• Exists in all three states: vapour, liquid, ice.
• Responsible for clouds, fog, precipitation, latent heat release.
• Almost entirely confined to the troposphere.

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Relative Humidity (RH)

• Definition: Ratio of actual water vapour content to maximum possible (capacity) at the same temperature and pressure, expressed in %.
• Can be expressed as:
  • Actual vapour pressure / Saturation vapour pressure
  • Absolute humidity / Saturation content
  • HMR / SMR
• RH indicates degree of saturation, not total moisture content.

Behaviour

• RH = 100% → air saturated
• RH < 100% → air is dry (even 99% is dry, DGCA concept).
• RH changes by:
  • Changing temperature (most common).
  • Changing moisture content.

Temperature Effect (Moisture Constant)

• Temperature ↑ → capacity ↑ → RH ↓
• Temperature ↓ → capacity ↓ → RH ↑
• RH is inversely proportional to temperature (if moisture constant).

Diurnal Variation

• RH minimum: Afternoon (~1500 LMT, Tmax).
• RH maximum: Early morning (~30 min after sunrise, Tmin).

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Dew Point (DP)

• Definition: Temperature to which air must be cooled at constant pressure and constant moisture for saturation.
• At DP: RH = 100%.
• DP is a direct measure of actual moisture content.

Key Relationships

• DP ≤ T (can never exceed air temperature).
• T = DP → saturated air.
• T − DP (spread):
  • Small spread → high RH.
  • Zero spread → saturation, condensation begins.

Vertical Change

• Dew point lapse rate ≈ 0.5°C / 1000 ft (theoretical).

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Absolute Humidity (AH)

• Definition: Actual mass of water vapour per unit volume of air.
• Units: g/m³.
• Measures actual moisture, unlike RH (which is relative).

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Humidity Mixing Ratio (HMR)

• Definition: Mass of water vapour per unit mass of dry air (g/kg).
• In unsaturated adiabatic ascent:
  • HMR remains constant.
• Represents actual moisture content.

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Saturation Capacity

• Maximum water vapour air can hold depends mainly on temperature.
• Warm air → high capacity.
• Cold air → low capacity.
• Expressed by saturation vapour pressure or saturation content.

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Wet Bulb Temperature (WBT)

• Definition: Lowest temperature air can reach by evaporation.
• Measured using wet-bulb thermometer (psychrometer).
• Evaporation absorbs latent heat, causing cooling.

Relationships

• Unsaturated air: WBT < DBT.
• Saturated air:
  DBT = WBT = DP.
• Wet-bulb depression: DBT − WBT.
  • Large depression → dry air.
  • Zero depression → saturated air.

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Hygrometers

• Instruments used to measure humidity:
  • Psychrometer (wet & dry bulb).
  • Hair / electrical hygrometers.
• Psychrometer uses wet-bulb depression to find RH & DP.

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Phase Changes of Water

• Evaporation: Liquid → Gas (latent heat absorbed, cooling).
• Condensation: Gas → Liquid (latent heat released, warming).
• Sublimation: Ice → Vapour (latent heat absorbed).
• Deposition: Vapour → Ice (latent heat released).

Aviation Significance

• Latent heat release during condensation:
  • Slows cooling of saturated air.
  • Makes SALR < DALR.
  • Enhances convection and cloud growth.

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Saturation & Ice Clouds

• Below 0°C:
  • SVP over water > SVP over ice.
• Basis of Bergeron (Ice-Crystal) process:
  • Ice crystals grow at expense of supercooled droplets.

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Ascent & Descent Effects

Rising Unsaturated Air

• Cools at DALR.
• Moisture constant → RH increases.
• Saturation reached at condensation level.

Descending Air

• Warms adiabatically.
• Capacity increases → RH decreases.
• Air becomes drier.

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Key Exam One-Liners

• DP is the best indicator of actual moisture.
• DP remains constant unless moisture is added/removed.
• RH highest near sunrise, lowest in afternoon.
• Saturated air: T = DP = WBT.
• Evaporation cools; condensation warms.
• Wet runway + humid air → longer take-off & landing distance.

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