1. Sep 2012 Lesson 4.5 Meteorology Humidity, Temperature & Stability

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

3. Introduction Humidity and temperature, and their distribution throughout different altitudes influence air stability.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.Whether air is stable or unstable means different weather characteristics for aircraft, which pilots must be aware of.

4. Outline HumidityHumidity TemperatureTemperature Lapse RatesLapse Rates Air StabilityAir Stability

5. Humidity HumidityHumidity –Amount of water vapour in air Phase ChangesPhase 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 NucleiCondensation Nuclei –Microscopic particles that water droplets and ice crystals need to form around

6. Relative Humidity SaturationSaturation –When mass of air holds maximum amount of water vapour DewpointDewpoint –Temperature air must be cooled (at constant pressure) to become saturated Relative HumidityRelative 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 decreasesIf given air mass is heated, then relative humidity decreases If given air mass is cooled, then relative humidity increasesIf given air mass is cooled, then relative humidity increases If water vapour is added, then relative humidity increasesIf water vapour is added, then relative humidity increases If water vapour is lost, then relative humidity decreasesIf water vapour is lost, then relative humidity decreases

7. Temperature Freezing point for water = 0°CFreezing point for water = 0°C Boiling point for water = 100°CBoiling point for water = 100°C IsothermsIsotherms –Lines on weather map joining places of equal temperature Temperature effect on air densityTemperature effect on air density –Cold air = More dense (heavier) –Warm air = Less dense (lighter)

8. Atmospheric Heating Atmosphere is heated from below (ground)Atmosphere is heated from below (ground) Radiation HeatingRadiation Heating –Ground heats up from sun during day, releases heat into air above ConductionConduction –Heat gradually transfers from one air mass to another, from hot to cold –Very minor role in weather, air not a good conductor

9. Atmospheric Heating ConvectionConvection –Warm air rising, causes cold air to replace it. –Vertical circulation distributes heat AdvectionAdvection –Cold air moving over warm surface gets heated TurbulenceTurbulence –Mechanical turbulence distributes heat through vertical air movement Compression (AKA Subsidence, AKA Adiabatic Heating)Compression (AKA Subsidence, AKA Adiabatic Heating) –When air sinks, it compresses and heats

10. Atmospheric Cooling Radiation CoolingRadiation Cooling –Ground cools during night, cools air above Advection CoolingAdvection Cooling –Warm air moving over cold surface gets cooled Expansion (AKA Adiabatic Cooling)Expansion (AKA Adiabatic Cooling) –When air rises, it expands and cools

11. Adiabatic Lapse Rates Lapse RateLapse Rate –Decrease of temperature with altitude Lapse RatesLapse Rates –ICAO Standard = 1.98°C / 1000 ft –Dry Adiabatic = 3°C / 1000 ft –Saturated (Wet) Adiabatic = 1.5 °C / 1000 ft

12. Inversions Increases of temperature with altitudeIncreases of temperature with altitude Isothermal LayerIsothermal 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 aboveInversions and Isothermal Layers can occur on clear, still (calm) night when ground cools air above

13. Air Stability Stable AirStable Air –Tendency of air to stay at its horizontal level when moving –Air resists upward or downward movement Unstable AirUnstable Air –Tendency of air to continue to move away from original horizontal level when disturbed StableUnstable Cause Cooling from below Heating from below Lapse Rate Shallow or inversion Steep Cloud Stratus Type Cumulus Type PrecipitationSteadyShowers VisibilityPoorGood

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