Stability, Adiabatic Processes, Precipitation Stability of a parcel of air is tendency to resist displacement (stay put!) 997 998 999 1000 surface Air moves according to pressure gradient force (from High to Low ) So, why don’t air molecules escape into space? Pressure gradient balanced by gravity. “hydrostatic equilibrium” : Upward pressure gradient is balanced by downward force of gravity Atmosphere (in general) is in hydrostatic equilibrium. • But… • In any given place at any given time, there are large bodies of air (parcels) that are not in hydrostatic equilibrium with their surroundings If surrounding air is more dense / colder than the parcel, parcel will rise. warm Colder / more dense surrounding air UNSTABLE A parcel will rise until it gets to an altitude at which surrounding air is at same density / temperature If surrounding air is less dense / warmer than the parcel, parcel will sink. Warmer / less dense air cold STABLE Therefore, stability is determined by temperature difference between parcel and surrounding air. Temperature of surroundings decreases with altitude at AVERAGE environmental lapse rate: (3.5°F / 1000 ft; 6.4°C / 1000 m) What about parcel? As parcel rises, it expands due to lower atmospheric pressure of surrounding air; as parcel expands, temperature drops. As a parcel of air descends, it contracts due to greater atmospheric pressure of surrounding air; as parcel contracts, temperature rises. . These changes in temperature: – NOT due to heat exchange with surroundings, – due to expanding and contracting. called ADIABATIC – (change in temperature with no gain or loss of heat) Dry Adiabatic Lapse Rate Rate at which temperature drops in rising body of unsaturated air: 5.5°F / 1000 ft. OR 10°C / 1000 m. Wet Adiabatic Lapse Rate Rate at which temperature drops in saturated rising air (temperature at dew point): 3.3°F / 1000 ft. OR 6°C / 1000 m. NOTE: this rate is lower than dry rate WHY? Mechanisms causing air parcels to rise: • Spontaneous: – Convergence – Convection • Forced : – Orographic Uplift – Frontal Lifting GOES Cold front Warm front Lifting condensation level Top & bottom elevations of clouds: Bottom: lifting condensation level: temperature of rising air reaches dew point Top: Cloud rises until cloud temperature reaches temperature of surroundings (becomes stable). Cloud thickness depends on thickness of unstable layer. Back to stability…. To test stability, push parcel up and see if it continues to rise. • Several possible results…. Example #1: ELR = 11 degrees / 1000m Parcel is saturated so use Wet AR: (What is it?) 6 degrees / 1000 m Therefore, surroundings cool faster than parcel. Parcel’s tendency will be to RISE. UNSTABLE (ELR>WAR) Example #2: ELR = 11 degrees / 1000m Parcel is unsaturated so use Dry AR: (What is it?) 10 degrees / 1000 m Therefore, surroundings cool faster than parcel. Parcel’s tendency will be to RISE. UNSTABLE (ELR>DAR) Example #3: ELR = 4 degrees / 1000 m Parcel is unsaturated so use Dry AR: (What is it?) 10 degrees / 1000 m Therefore, parcel cools faster than surroundings. Parcel’s tendency will be to STAY PUT. STABLE (ELR<DAR) Example #4: ELR = 4 degrees / 1000m Parcel is saturated so use Wet AR: (What is it?) 6 degrees / 1000 m Therefore, parcel cool faster than surroundings. Parcel’s tendency will be to STAY PUT. STABLE (ELR<WAR) Example #5: ELR = 8 degrees / 1000 m If air is saturated, use Wet AR. If air is unsaturated, use Dry AR. Stable or unstable? Stable or unstable? unstable stable summary If ELR > WAR , DAR : UNSTABLE If ELR < WAR, DAR : STABLE If WAR < ELR < DAR: CONDITIONALLY STABLE (depends on saturation) (Atmosphere is usually conditionally stable). Sinking air warms at dry adiabatic rate. PRECIPITATION (rain , snow, sleet, freezing rain, hail) • Large parcel of air rises and cools – Adiabatic cooling • Cools to dew point • Cloud droplets grow large enough to fall • Why do cloud droplets need to grow? Processes by which cloud droplets/crystals grow : A. Bergeron Findeisen Process Mid- and High latitudes: “Cold Clouds” (tops colder than 0°C) – Saturation vapor pressure is greater over water than ice • It takes more water vapor to saturate air over water than over ice – ice crystals grow at the expense of water droplets Bergeron Findeisen process B. Collision/Coalescence process Low latitudes: “Warm Clouds” (tops warmer than -15°C) – Collision and coalescence: some big drops form; they fall faster than small drops and collect small drops Precipitation Types rain Precipitation as liquid water Height Melting level 0°C Temperature virga snow Snowflake is several aggregated crystals. Height Temperature 0°C graupel “riming”: shell forms around crystal as it passes through tiny suspended water drops: forms “graupel” (ice pellets) Sleet Frozen raindrops; raindrops that freeze as they descend. Height Melting level Temperature 0°C Freezing rain Supercooled droplets hit a surface which is below freezing Height Melting level Temperature 0°C