Weather Intro

Weather Intro - Climate and Weather Basic Elements of...

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Climate and Weather Basic Elements of Climate and Weather Weather : conditions of atmosphere at particular place and time Temperature, air pressure, humidity, precipitation, air motion Varies over space and time Climate : weather averaged over long time Varies over space Significant effects on hazards, involving coastal erosion, hurricanes, thunderstorms, tornadoes, wildfires Basics 1: Hydrologic Cycle Water in oceans covers more than 70% of Earth’s surface Less than 3% of water on Earth is fresh Derived from evaporation of ocean water Some water precipitates over continents as rain or snow Most rain that is not taken up by plants soaks in to become groundwater Some groundwater seeps back to surface into lakes and rivers and ultimately flows to sea Rain or snow falling onto continents can: Fall on vegetation, where some water evaporates back into atmosphere Reach ground Run off into streams or lakes Be taken up by plant roots into leaves, be transpired back into atmosphere Soak into ground to become groundwater Groundwater slowly migrates in direction that water table slopes (downhill) into streams, lakes and wetlands Streams, lakes and wetlands eventually drain back into ocean Amount of water vapor that can be dissolved in air depends on air temperature Cold air can dissolve little water vapor Warm air can dissolve a lot of water vapor When air dissolves maximum water vapor that it can hold: It is saturated Its relative humidity is 100% Temperature at which air’s relative humidity would be 100% is dew point If water cools below dew point, water condenses into droplets If enough droplets form, coalesce into larger droplets and precipitate as rain or snow Adiabatic Cooling and Condensation Adiabatic cooling occurs when rising air expands without change in heat content Rate of cooling as air mass rises is adiabatic lapse rate Expansion of air mass distributes heat over larger volume, so air becomes colder During condensation (water vapor into liquid water), air releases same amount of heat as originally required to convert liquid water into water vapor Heat of condensation reduces adiabatic lapse rate for wet air mass to 5 o C per 1000 m of rise, from 10 o C per 1000 m of rise for dry air mass Very wet air cools at half rate of dry air as it rises
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Air moving over continent may be forced to rise over mountain range, so expands and cools in orographic effect Cooler air can hold less moisture, so precipitation occurs as air mass rises Air moving down other side of mountain range is then warmer and drier Rain shadow effect creates deserts on downwind side of mountain ranges Atmospheric Pressure and Weather Air pressure is related to weight of column of air from ground to top of atmosphere Heating an air mass causes it to expand (causes its density to decrease), so air mass rises and has lower pressure Air near ground surface is pulled in toward low-pressure center to replace rising air Opposite occurs in high-pressure systems Air flows from high-pressure to low-pressure areas, causing winds
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Weather Intro - Climate and Weather Basic Elements of...

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