Sep 2012Lesson 4.5

Meteorology

Humidity, Temperature & Stability

Reference

From the Ground UpChapter 6.5:Humidity, Temperature and StabilityPages 136 - 140

Introduction• Humidity and temperature, and their

distribution throughout different altitudes influence air stability.

• Whether air is stable or unstable means different weather characteristics for aircraft, which pilots must be aware of.

Outline• Humidity• Temperature• Lapse Rates• Air Stability

Humidity• Humidity

– Amount of water vapour in air

• Phase Changes– Condensation = Water vapour to water droplets– Evapouration = Water droplets to water vapour– Sublimation = Ice crystals to water vapour– Deposition = Water vapour to ice crystals

• Condensation Nuclei– Microscopic particles that water droplets and ice crystals

need to form around

Relative Humidity• Saturation

– When mass of air holds maximum amount of water vapour

• Dewpoint– Temperature air must be cooled (at constant pressure) to

become saturated

• Relative Humidity– Ratio of water vapour in given air mass compared to amount

it could hold if saturated• If given air mass is heated, then relative humidity decreases• If given air mass is cooled, then relative humidity increases• If water vapour is added, then relative humidity increases• If water vapour is lost, then relative humidity decreases

Temperature• Freezing point for water = 0°C• Boiling point for water = 100°C

• Isotherms– Lines on weather map joining places of equal

temperature

• Temperature effect on air density– Cold air = More dense (heavier)– Warm air = Less dense (lighter)

Atmospheric Heating• Atmosphere is heated from below (ground)

• Radiation Heating– Ground heats up from sun during day, releases

heat into air above

• Conduction– Heat gradually transfers from one air mass to

another, from hot to cold– Very minor role in weather, air not a good

conductor

Atmospheric Heating• Convection

– Warm air rising, causes cold air to replace it.– Vertical circulation distributes heat

• Advection– Cold air moving over warm surface gets heated

• Turbulence– Mechanical turbulence distributes heat through vertical air

movement

• Compression (AKA Subsidence, AKA Adiabatic Heating)– When air sinks, it compresses and heats

Atmospheric Cooling• Radiation Cooling

– Ground cools during night, cools air above

• Advection Cooling– Warm air moving over cold surface gets cooled

• Expansion (AKA Adiabatic Cooling)– When air rises, it expands and cools

Adiabatic Lapse Rates• Lapse Rate

– Decrease of temperature with altitude

• Lapse Rates– ICAO Standard = 1.98°C / 1000 ft– Dry Adiabatic = 3°C / 1000 ft– Saturated (Wet) Adiabatic = 1.5 °C / 1000 ft

Inversions• Increases of temperature with altitude

• Isothermal Layer– Temperature remains constant throughout a

layer of some depth

• Inversions and Isothermal Layers can occur on clear, still (calm) night when ground cools air above

Air Stability• Stable Air

– Tendency of air to stay at its horizontal level when moving– Air resists upward or downward movement

• Unstable Air– Tendency of air to continue to move away from original horizontal

level when disturbed

Stable UnstableCause Cooling from below Heating from belowLapse Rate Shallow or inversion SteepCloud Stratus Type Cumulus TypePrecipitation Steady ShowersVisibility Poor Good

Next Lesson

4.6 – MeteorologyAir Masses & Fronts

From the Ground UpChapter 6.6 & 6.7:Air Masses & FrontsPages 140 - 147