L4 Humidity and Stability

Humidity

• Humidity refers to the amount of water vapor in the atmosphere.
• Absolute humidity: The total amount of water vapor in the atmosphere.
• Water vapor is an invisible gas, not mist or fog.
• The higher the proportion of water vapor, the higher the humidity.

Water Vapor Capacity

• A parcel of air can only hold a certain amount of water vapor.
• The proportion of water vapor air can hold depends on temperature.
• Warm air can hold more water vapor than cold air at the same pressure.
• Example:
  • At 0°C, 1 kg of air can hold up to 4 g of water vapor.
  • At 15°C, 1 kg of air can hold up to 10 g of water vapor.
  • At 30°C, 1 kg of air can hold up to 27 g of water vapor.

Saturation and Condensation

• When air contains the maximum amount of water vapor it can hold at a given temperature, it is saturated.
• If air is cooled further, water vapor condenses into liquid droplets, forming clouds, mist, or fog.
• Example:
  • A 1 kg parcel of air at 25°C holds 17 g of water.
  • As it rises and cools to 21°C, it reaches its maximum capacity of 17 g and becomes saturated.
  • Further cooling results in condensation.

Dew Point Temperature

• The temperature at which air becomes saturated is known as the dew point.

Significance of Dew Point

• Dew point is measured in meteorological observations and reported in weather reports.
• The relationship between temperature and dew point indicates the amount of water vapor in the air.
  • Large gap: dry air, high cloud base or no cloud.
  • Small gap: moist air, low cloud or fog.

Examples

• Wide temperature/dew point spread:
  • Gatwick (EGKK) METAR: 25°C temperature, 3°C dew point, no cloud.
• Narrow temperature/dew point spread:
  • Manchester (EGCC) METAR: 7°C temperature, 6°C dew point, drizzle, cloud down to 100 ft.
    +Image - Manchester Airport control tower (134 ft AGL just in the cloud base).

Relative Humidity (RH)

• Relative humidity is the percentage of water vapor a parcel of air is holding compared to its maximum capacity before saturation.
• If air contains half the water vapor it could hold, RH is 50%.
• Saturated air has 100% RH.

Temperature and Relative Humidity

• Warm air with 90% RH contains more moisture than cold air with 90% RH.
• Pilots do not receive RH information, but it is important for meteorologists in forecasting.
• Pressure, temperature, and humidity are the most important measurements in computer forecasting models.
• Higher RH means the air is closer to saturation, potentially leading to cloud, mist, or fog.

Humidity Mixing Ratio

• Meteorologists measure water vapor content using the Humidity Mixing Ratio: grams of water vapor per kilogram of dry air.
• Saturation Mixing Ratio: The value corresponding to saturation.
• Mixing ratio remains constant with temperature changes, but relative humidity and saturation mixing ratio change.

Absolute and Specific Humidity

• Absolute humidity: Mass of water vapor per unit volume of air (grams per cubic meter).
• Specific humidity: Same as mixing ratio, mass of water vapor per unit mass of air (grams per kilogram).
• These values do not change with temperature.

Density and Humidity

• Water vapor is less dense than dry air.
• Moist air (high humidity) is less dense than dry air (low humidity).

Change of State

• Water exists in three states: solid (ice), liquid (water droplets), and gas (water vapor).
• Changes of state involve energy transfer:
  • Solid to liquid to gas: absorbs energy from surrounding air, cooling the air.
  • Gas to liquid to solid: releases energy, warming the surrounding air.
• Energy gained or released is called latent heat.
• Water can also change directly from gas to solid (deposition in the UK) or solid to gas (sublimation).

Latent Heat Example

• Spilling volatile liquid like AVGAS fuel on skin: the liquid evaporates, absorbing heat from the skin, causing a cooling sensation.

Temperature Lapse Rates

• Temperature lapse rate: Rate at which temperature changes with height.
• Average lapse rate: 2°C per 1000 ft.
• The lapse rate is variable and changes with atmospheric conditions.

Environmental Lapse Rate (ELR)

• Weather forecasters use radiosonde balloons to collect temperature data throughout the atmosphere.
• The actual rate of temperature change with height is the Environmental Lapse Rate (ELR).

Dry Adiabatic Lapse Rate (DALR)

• When dry air rises, its lapse rate is a constant 3°C per 1000 ft. This is the Dry Adiabatic Lapse Rate (DALR).

Saturated Adiabatic Lapse Rate (SALR)

• When saturated air rises, its lapse rate is less than the DALR because condensation releases heat.
• The Saturated Adiabatic Lapse Rate (SALR) is approximately 1.8°C per 1000 ft in a temperate climate.
• The SALR varies depending on the amount of latent heat released.

Adiabatic Process

• 'Adiabatic' means no energy is gained or lost by the air parcel itself.
• Condensation in rising saturated air releases energy, resulting in a lower SALR compared to the DALR.

Atmospheric Stability Based on Lapse Rates

• ELR < SALR (<1.8°C): Absolutely stable atmosphere.
• ELR > DALR (>3.0°C): Absolutely unstable atmosphere.
• SALR < ELR < DALR (>1.8°C, <3.0°C): Conditionally unstable atmosphere.

Atmospheric Stability

• Atmospheric stability is a major factor in dictating the weather.
• Stability describes the air's resistance to vertical motion.

Stable Atmosphere

• In a stable atmosphere, a displaced parcel of air returns to its original level.

Unstable Atmosphere

• In an unstable atmosphere, a displaced parcel of air continues to move vertically, forming air currents.

Analogy

• Stable air: Calm ocean.
• Unstable air: Rough ocean.

Stability and Lapse Rates

• Unstable Atmosphere: ELR is greater than both DALR and SALR.

  • Rising air is always warmer than the environment, so it continues to rise.

• Stable Atmosphere: ELR is lower than the SALR.

  • Rising air is colder than the environment, so it sinks back down.

• Conditionally Unstable Atmosphere: ELR is between the DALR and SALR (1.8°C and 3.0°C per 1000 ft).

  • Stability depends on whether the air cools at the DALR or SALR.
    • If unsaturated (cooling at DALR): stable.
    • If saturated (cooling at SALR): unstable.

• Neutrally Stable: ELR is same as SALR (saturated) or DALR (unsaturated).

  • Air neither rises nor sinks; very unlikely in real-world conditions.

Atmospheric Stability and Cloud Formations

• Stability is affected by surface heating or cooling.
  • Heating from below: Unstable atmosphere, choppy flying conditions, convective clouds, heavy showers, good visibility outside showers.
  • Cooling from below: Stable atmosphere, smooth flying conditions, flat layer-type clouds, long-lasting precipitation, poor visibility.

Temperature Inversion

• Temperature inversion: Temperature increases with increasing height.

Formation of Temperature Inversions

• Surface cooling during cold nights in winter.
• At weather fronts when warm air settles over cold air.
• Above 'temperature inversion turbulence', due to mixing.
• From subsidence in stable air masses.