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3/25/14 Supercooling of Water Precipitation Ice generally melts at 0oC but doesn’t necessarily freeze. Tiny droplets of water can remain liquid at temperatures well below freezing. This is called “supercooling.” Droplets can remain in liquid form until -40oC Precipitation Coalescence Cloud droplets collide Form larger droplets Become too heavy Fall Precipitation Types Rain Collection of cloud droplets Snow Forms as ice crystals Deposition (gas to solid) Sleet Rain freezes on the way down Freezing rain Rain freezes on contact with the surface Hail Convection currents carry rain up and down through freezing and nonfreezing air Processes That Lift Air Orographic lifting Air is forced to rise over a mt. Frontal wedging Warm, less dense air forced over cool dense air Convergence Processes That Lift Air Orographic lifting Air is forced to rise over a mt. Cools condenses Clouds form Then air descends Cool air warms Creates Rain shadow desert Converging winds force air upward Localized convective lifting Warm air rising 1 3/25/14 Processes That Lift Air Frontal wedging Warm, less dense air forced over cool dense air Front- 2 different air masses collide Processes That Lift Air Convergence Converging winds force air upward Warm Front Faster moving warm air meets cold air Warm air gets forced upward Processes That Lift Air Localized convective lifting Stability Warm air rising Ability of an air mass Hot sunny afternoon Warm air pocket over thunderstorms to resist rising cool surface Heat transfers from the warm air to the cool air and surface Warm air cools Resists rising Stability Unstable air Cool air and warm surface Warm air rises Heats the air pocket Air pocket rises Cools, condenses, and forms clouds Atmospheric Stability Stability Stable Air Parcel of air Cooler than surroundings Resists vertical movement Doesn’t rise Unstable Air Parcel of air Warmer than surroundings Continues to rise 2 3/25/14 Atmospheric Stability Different Environmental Lapse Rate Decrease in temp with altitude Troposphere 5oC per 1000m Stability and Daily Weather Stable Air Widespread clouds Horizontal clouds Little to no precip. Adiabatic Rate When air expands, it cools When air is compressed, it warms Dry adiabatic rate 10oC per 1000 m Wet adiabatic rate Varies 5OC per 1000 m What are the conditions necessary for a T-storm to form? Unstable Air Towering clouds Heavy precip. What are the 3 stages of T-storms? Moisture Water vapor condenses and forms clouds Latent heat Cumulonimbus clouds Lifting Mechanism Convective lifting, orographic lifting, etc. Unstable Air Rising air Cools, condenses, and forms clouds The Cumulus Stage Cumulus Stage Air rising – Updrafts Latent heat Condensation The Mature Stage Precipitation Cools air – sinks Downdrafts Equal Updrafts Downdrafts 3 3/25/14 The Dissipation Stage Downdrafts Dominate Rain tapers Updrafts Cutoff 4