chapd1.2

Air and Weather

Air and the Sun

• weather begins w/ the sun– provides energy for making weather– amount of energy reaching Earth different in different places

• more at equator less at poles

– uneven heating causes air to move - makes weather

• most of the sun’s energy never reaches Earth– lost in space

• the energy that does reach Earth – 3/10 reflected back into space– 3/10 warms the air– 4/10 warms land and ocean

• atmosphere traps the heat– greenhouse effect

• w/o greenhouse effect most of the sun’s energy would be reflected back into space and the surface would be too cold to support life

Air Masses• air masses: huge bodies of air, covering

thousands of km– has same general properties as the land or water

over which it forms• moisture content and temperature

• moist air masses form over water

• dry air masses form over land

• air masses that form over the poles are cold

• air masses that form over the equator/tropics are warm

Air Masses Meet• when 2 air masses meet they usually don’t mix

– form a border called a front– most weather happens along fronts

• cold front forms when cold air mass catches up to a warm air mass– colder air mass forces the warmer air up into the

atmosphere– as warmer air is pushed upwards it cools and

makes clouds– rain develops - thunderstorms occur along cold

fronts

• warm front forms when warm air mass catches up to cold air mass– warm air slides over the colder, thicker air– clouds form– steady snow or rain may fall as the front

approaches and passes– then the sky becomes clear and temps rise

• if a front stops moving it is a stationary front– can stay in one spot for several days– constant fall of rain or snow

COLD FRONT

WARM FRONT

Air Masses Move

• wind speed often increases as a front approaches – direction also changes

• air pressure changes as air masses move over an area– front moves closer - air pressure drops– Front moves over the area - air pressure

rises

• temp changes as a front moves over an area– warmer air comes from a warm front– cooler air comes from a cold front

Summary

• sun provides energy to make weather

• atmosphere traps heat near Earth’s surface - greenhouse

• air masses form over continents and oceans– when 2 air masses meet they form a front– fronts: areas where most weather happens