Atmospheric Moisture, Humidity & Dew Point — Summary Notes

1. Water Vapour in the Atmosphere

Water vapour (H₂O) is the most important atmospheric gas from a weather and meteorological standpoint.

• It is highly variable, ranging from almost 0% to about 4–5% by volume.
• It is the only atmospheric substance that naturally occurs in all three states:
  • Solid: Ice
  • Liquid: Water
  • Gas: Water vapour
• Water vapour is essential for:
  • Cloud formation
  • Fog and mist formation
  • Dew formation
  • Precipitation
  • Atmospheric energy transfer
• Changes of state involve latent heat, which is an important source of atmospheric energy.
• Latent heat release is particularly important in:
  • Convection
  • Thunderstorms
  • Hurricanes
  • Cloud development
  • The difference between the Dry Adiabatic Lapse Rate (DALR) and Moist/Saturated Adiabatic Lapse Rate (MALR/SALR).

Key Point

Water vapour → moisture, clouds, precipitation and latent-heat energy.

2. Relative Humidity (RH)

Relative Humidity is the ratio of the actual water vapour content of air to the maximum amount of water vapour the air can hold at its current temperature and pressure, expressed as a percentage.

Formula

RH = (Actual Water Vapour Content / Maximum Water Vapour Capacity) × 100%

RH can also be expressed as:

RH = (HMR / SMR) × 100%

or:

RH = (Actual Vapour Pressure / Saturation Vapour Pressure) × 100%

Where:

• HMR = Humidity Mixing Ratio
• SMR = Saturation Mixing Ratio

What RH Tells Us

RH indicates how close the air is to saturation.

It does not directly indicate the total amount of water vapour present.

Saturation

• RH = 100% → air is saturated.
• RH < 100% → air is unsaturated/dry.
• Even 99% RH is technically classified as unsaturated or dry air.

3. Factors Affecting Relative Humidity

RH can change in two principal ways:

1. By changing the actual water vapour content
2. By changing the air temperature

If Temperature Remains Constant

Adding water vapour:

Moisture ↑ → RH ↑

Removing water vapour:

Moisture ↓ → RH ↓

If Moisture Content Remains Constant

Warming the air:

Temperature ↑ → Capacity ↑ → RH ↓

Cooling the air:

Temperature ↓ → Capacity ↓ → RH ↑

Important Relationship

With constant moisture content, RH is inversely related to temperature.

4. Saturation Capacity of Air

The maximum amount of water vapour that air can contain is called its saturation capacity.

It is primarily controlled by temperature.

Relationship

Temperature ↑ → Saturation capacity ↑

Temperature ↓ → Saturation capacity ↓

Warmer air can hold more water vapour before reaching saturation.

Saturation capacity can be represented by:

• Saturation Content
• Saturation Vapour Pressure

5. Absolute Humidity

Absolute Humidity (AH) is the actual mass of water vapour contained in a unit volume of air.

Formula Concept

AH = Mass of water vapour / Volume of air

Unit

Usually:

g/m³

Important Distinction

AH represents the physical concentration of water vapour, whereas RH represents how close the air is to saturation.

6. Humidity Mixing Ratio (HMR)

Humidity Mixing Ratio is the mass of water vapour contained in a unit mass of dry air.

Formula

HMR = Mass of water vapour / Mass of dry air

Unit

Usually:

g/kg

During Unsaturated Adiabatic Ascent

When unsaturated air rises:

• Pressure decreases.
• Air expands.
• Temperature decreases.
• No moisture is exchanged with the surroundings.
• Mass of water vapour remains constant.
• Mass of dry air remains constant.
• Therefore, HMR remains constant.

7. Specific Humidity

Specific humidity represents the mass of water vapour relative to the total mass of air.

During an unsaturated adiabatic ascent, there is no exchange of mass with the surroundings, so:

Specific Humidity remains constant.

Compare

• HMR: water vapour / dry air
• Specific Humidity: water vapour / total air

8. Dew Point Temperature

Dew Point (DP) is the temperature to which air must be cooled, at constant pressure and constant water vapour content, to become saturated.

At the dew point:

RH = 100%

and:

Air Temperature = Dew Point

Relationship

T ≥ DP

Therefore:

• Saturated air → T = DP
• Unsaturated air → T > DP
• Dew point can never exceed air temperature.

Importance of Dew Point

Dew point is a direct indicator of the actual moisture content of the air.

Higher DP → greater actual moisture content.

9. Temperature–Dew Point Spread

The difference between air temperature and dew point is called the temperature–dew point spread.

Formula

Spread = T − DP

Most Important Rule

T = DP → RH = 100% → Saturation

A decreasing spread indicates that the air is approaching saturation and therefore increases the possibility of:

• Fog
• Mist
• Dew
• Cloud formation

10. Example: Calculating Cooling Required for Saturation

Given:

• Air temperature = +12°C
• Dew point = +5°C

Required cooling:

12 − 5 = 7°C

Therefore, the air must cool by 7°C to reach saturation.

At +5°C:

• T = DP
• RH = 100%
• Air is saturated.

Further cooling can result in condensation.

11. Dew Point and Moisture Content

Dew point is primarily controlled by the actual water vapour content.

If air is simply heated or cooled without adding or removing moisture:

• Actual moisture content remains unchanged.
• Dew point remains constant.
• RH changes because the saturation capacity changes with temperature.

Example: Clear Night

During a clear night:

• Temperature decreases.
• Air capacity to hold water vapour decreases.
• RH increases.
• Dew point does not change, provided no moisture is added or removed.

12. Dew Point During Ascent

The source gives a theoretical dew-point lapse rate of approximately:

0.5°C per 1,000 ft

Therefore, as air rises, the dew point generally decreases at approximately:

0.5°C / 1,000 ft

This is distinct from the temperature lapse rate of the air parcel.

13. Unsaturated Air Rising Adiabatically

When an unsaturated air parcel rises:

1. Atmospheric pressure decreases.
2. The parcel expands.
3. The parcel cools at the Dry Adiabatic Lapse Rate (DALR).
4. No moisture is exchanged with the surroundings.
5. HMR remains constant.
6. Specific humidity remains constant.
7. Air’s moisture-holding capacity decreases.
8. RH increases.
9. The parcel eventually reaches RH = 100% at the condensation level.

Absolute Humidity

As the parcel rises:

• Volume increases because of expansion.
• Water vapour mass remains constant.
• Therefore, absolute humidity decreases.

14. Descending Air

When air descends:

1. Ambient pressure increases.
2. Air is compressed.
3. Compression causes adiabatic warming.
4. Actual moisture content remains constant, assuming no moisture is added or removed.
5. Warmer air has a greater moisture-holding capacity.
6. RH therefore decreases.
7. The air moves farther away from saturation.

Memory Rule

Descending air → warming → capacity increases → RH decreases.

15. Saturated vs Unsaturated Air

Important Examination Point

Air with 95% or 99% RH is still unsaturated/dry air because it has not reached 100% saturation.

Unsaturated air cools at the:

Dry Adiabatic Lapse Rate (DALR)

16. Condensation

Condensation is the change of state:

Water vapour → Liquid water

It is an exothermic process, meaning it releases latent heat.

Effects

The released latent heat:

• Warms the surrounding air.
• Adds energy to the atmosphere.
• Slows the cooling of rising saturated air.
• Reduces the lapse rate compared with dry adiabatic cooling.
• Supports buoyancy and cloud development.

Aviation Significance

Condensation contributes to the formation of:

• Clouds
• Fog
• Mist
• Dew

17. Latent Heat and Saturated Adiabatic Lapse Rate

During condensation:

Latent heat is released.

This heat partially offsets the cooling of rising saturated air.

Therefore:

SALR/MALR < DALR

In other words:

Saturated air cools more slowly with height than dry air.

The amount of latent heat released depends on the amount of water vapour that condenses.

More condensation → more latent heat released.

18. Evaporation

Evaporation is the change of state:

Liquid water → Water vapour

Evaporation requires energy.

Therefore:

Latent heat is absorbed.

The energy is taken from the surrounding environment, producing a cooling effect.

Memory Rule

Evaporation → absorbs heat → cooling

Condensation → releases heat → warming

19. Sublimation and Deposition

Sublimation

Direct change:

Ice → Water vapour

• Liquid state is bypassed.
• Latent heat is absorbed.

Deposition

Direct change:

Water vapour → Ice

• Liquid state is bypassed.
• Latent heat is released.

The source also refers to the gas-to-solid process as sublimation or deposition.

Aviation Relevance

Deposition contributes to hoarfrost/frost formation when temperature and dew/frost point are below freezing.

20. Saturation Vapour Pressure Over Water and Ice

At the same temperature below 0°C:

SVP over liquid water > SVP over ice

Reason

Water molecules escape more easily from a liquid water surface than from an ice surface.

Therefore, a greater vapour pressure is required to saturate air over liquid water than over ice at the same sub-freezing temperature.

Importance

This difference forms the basis of the Ice-Crystal / Bergeron Process, where ice crystals can grow at the expense of surrounding supercooled water droplets and contribute to precipitation formation in cold clouds.

21. Diurnal Variation of Relative Humidity

The daily variation of RH is mainly controlled by the daily temperature cycle, assuming actual moisture content remains approximately constant.

Daytime

Solar heating causes:

Temperature ↑ → Moisture-holding capacity ↑ → RH ↓

• RH decreases as temperature rises.
• Lowest RH generally occurs around 1500 LMT, when temperature is near its daily maximum.

Nighttime / Early Morning

Cooling causes:

Temperature ↓ → Capacity ↓ → RH ↑

• RH increases during the night.
• Highest RH occurs around the coolest part of the day.
• The source specifies approximately 30 minutes after sunrise, when temperature is minimum.

Operational Significance

High RH during the night and early morning increases the possibility of:

• Fog
• Mist
• Dew
• Other condensation phenomena

22. Relative Humidity and Temperature — Quick Table

23. Wet-Bulb Temperature

Wet-Bulb Temperature (Tᵂ) is the lowest temperature to which air can be cooled by evaporation of water.

It is measured using a wet-bulb thermometer, which forms part of a:

Psychrometer / Wet-and-Dry-Bulb Hygrometer

How It Works

• The thermometer bulb is covered by a wet muslin wick.
• Water evaporates from the wick when the air is unsaturated.
• Evaporation absorbs latent heat.
• Heat is removed from the thermometer bulb.
• The wet-bulb temperature falls below the dry-bulb temperature.

24. Wet-Bulb Depression

Wet-Bulb Depression is the difference between:

Dry-Bulb Temperature − Wet-Bulb Temperature

In Unsaturated Air

• Evaporation occurs.
• Latent heat is absorbed.
• Wet-bulb temperature is lower than dry-bulb temperature.

In Saturated Air

When:

RH = 100%

• No net evaporation occurs.
• No additional latent heat is absorbed.
• Wet-bulb temperature equals dry-bulb temperature.

At saturation:

Dry Bulb Temperature = Wet Bulb Temperature = Dew Point

25. Wet Bulb vs Dew Point

The two temperatures should not be confused.

Important

Wet-bulb temperature is generally not equal to dew point.

They become equal only at saturation.

26. Psychrometer / Hygrometer

A hygrometer is an instrument used to measure atmospheric moisture/humidity.

Examples include:

• Hair hygrometer
• Electrical hygrometer
• Psychrometer

A psychrometer consists of:

• Dry-bulb thermometer
• Wet-bulb thermometer

The difference between the dry- and wet-bulb temperatures, known as wet-bulb depression, is used to determine:

• Relative Humidity
• Dew Point
• Humidity Mixing Ratio

Saturation Check

If:

RH = 100%

then:

Dry Bulb = Wet Bulb = Dew Point

27. Why Does the Wet Bulb Cool?

In unsaturated air:

1. Water evaporates from the wet muslin wick.
2. Evaporation requires latent heat.
3. Latent heat is absorbed from the surrounding environment and thermometer bulb.
4. The bulb loses sensible heat.
5. Its temperature falls.
6. The lowest temperature reached is the wet-bulb temperature.

Key Rule

Greater evaporation → greater wet-bulb depression.

At saturation, evaporation stops and:

Wet-bulb depression = 0

28. Troposphere and Atmospheric Moisture

The troposphere contains virtually all of the atmosphere’s water vapour.

Troposphere

• Extends from the surface to the tropopause.
• Average tropopause height is approximately 11 km.
• Contains almost all atmospheric moisture.
• Temperature generally decreases with altitude.
• Almost all normal weather occurs in this layer.

Stratosphere

• Located above the tropopause.
• Contains very little moisture.
• Generally lacks clouds, apart from rare cloud types such as nacreous clouds.

Tropopause

The boundary between:

Troposphere ↔ Stratosphere

It is associated with an abrupt change in the temperature lapse-rate characteristics.

29. Rain and Runway Performance

On a rainy day, required runway length can increase for both takeoff and landing.

Two main factors are involved.

Takeoff

Rain is often associated with high humidity.

Higher moisture content can contribute to lower air density because water vapour is less dense than dry air.

Lower density results in:

• Reduced aerodynamic lift
• Reduced engine performance/thrust
• Higher density altitude
• Longer takeoff distance

Landing

Rain produces a wet runway.

A wet runway can cause:

• Reduced tyre/runway friction
• Reduced braking effectiveness
• Aquaplaning/hydroplaning risk
• Increased landing roll

Overall Result

Low-density air + wet runway → greater required runway length.

30. High-Value Relationships

Relative Humidity

RH ↑ when temperature ↓, if moisture content is constant.

Moisture Addition

Moisture ↑ → RH ↑, if temperature is constant.

Moisture Removal

Moisture ↓ → RH ↓, if temperature is constant.

Saturation

T = DP → RH = 100%

Unsaturated Air

T > DP → RH < 100%

Dew Point

Higher DP → Higher actual moisture content

Temperature–Dew Point Spread

Large spread → Low RH

Small spread → High RH

Zero spread → RH 100%

Rising Unsaturated Air

Rising → cooling → RH ↑

Descending Air

Descending → warming → RH ↓

Phase Changes

Evaporation → latent heat absorbed → cooling

Condensation → latent heat released → warming

Ice → vapour → latent heat absorbed

Vapour → ice → latent heat released

31. Important Numerical Values

32. Exam Quick Revision Table

33. One-Minute Memory Sheet

RH = Actual moisture / Capacity × 100

RH 100% = Saturated

T = DP = 100% RH

DP can never be higher than T

High DP = High moisture

Large T–DP spread = Low RH

Small T–DP spread = High RH

Cooling → RH increases

Warming → RH decreases
(when moisture content is constant)

Adding moisture → RH increases

Rising unsaturated air → cools → RH increases

Descending air → warms → RH decreases

HMR remains constant during unsaturated adiabatic ascent

Condensation → releases latent heat

Evaporation → absorbs latent heat

Saturated air cools more slowly than dry air

DBT = WBT = DP at saturation

Water vapour → liquid = condensation

Liquid water → vapour = evaporation

Vapour → ice = deposition

Ice → vapour = sublimation

Below 0°C: SVP over water > SVP over ice

Troposphere contains virtually all atmospheric water vapour