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Meteorology Humidity, Temperature & Stability. Reference. From the Ground Up Chapter 6.5: Humidity, Temperature and Stability Pages 136 - 140. Introduction. Humidity and temperature, and their distribution throughout different altitudes influence air stability.
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Reference From the Ground Up Chapter 6.5: Humidity, Temperature and Stability Pages 136 - 140
Introduction • Humidityand 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
Next Lesson 4.6 – Meteorology Air Masses & Fronts From the Ground Up Chapter 6.6 & 6.7: Air Masses & Fronts Pages 140 - 147
