Lesson 4.5
Meteorology
Sep 2012

From the Ground Up
Chapter 6.5:
Humidity, Temperature and Stability
Pages 136 - 140

• 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.

• Humidity
• Temperature
• Lapse Rates
• Air Stability

• 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

• 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

• 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)

• 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

• 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

• 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

• 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

• 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

• 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
Cause
Stable
Cooling from below
Lapse Rate Shallow or inversion
Cloud Stratus Type
Precipitation Steady
Visibility Poor
Unstable
Heating from below
Steep
Cumulus Type
Showers
Good

4.6 – Meteorology
Air Masses & Fronts
From the Ground Up
Chapter 6.6 & 6.7:
Air Masses & Fronts
Pages 140 - 147