Weather

Section 1

  Weather - is the state of the atmosphere at a specific time & place –  Includes such conditions as air pressure, wind,

temperature, and moisture in the air – The Sun’s heat evaporates water into the

atmosphere forming clouds; water returns to the Earth as rain or snow.

  Temperature is a measure of (air molecule) movement. – The Sun’s energy causes air molecules to move

rapidly; temperatures are high and it feels warm – When less of the sun’s energy reaches air

molecules, they move less rapidly and it feels cold.

  Wind - air moving in a specific direction – As the Sun heats air, it expands, becomes less

dense, rises, and has low atmospheric pressure – Cooler air is denser and sinks, causing high

atmospheric pressure – Air moves from high pressure areas to low

pressure areas, causing wind.

  Humidity - the amount of water vapor in the air. –  Warmer air can hold more water vapor, tending to

make it more humid   Relative Humidity - the amount of water vapor in

the air compared to what it can hold at a specific temperature –  When air cools, it can hold as much water vapor, so the

water vapor condenses to a liquid or forms of ice crystals.

  Dew point - the temperature at which air is saturated and condensation forms.

  Clouds form as warm air is forced upward and cools.

  Water vapor condenses in tiny droplets that remain suspended in the air.

  The shape & height of clouds vary with temperature, pressure and the water vapor in the atmosphere

Cloud types

  Shape: – Stratus - smooth, even sheets or layers at low

altitudes – Cumulus - puffy, white clouds, often with flat

bases – Cirrus - high, thin, white, feathery clouds made

of ice crystals

  Height: – Cirro - high clouds – Alto - middle-elevation clouds – Strato - low clouds

  Nimbus - clouds are dark and so full of water that sunlight can’t penetrate them.

  Precipitation - water falling from clouds – When water droplets in clouds combine and

grow large enough precipitation falls to Earth – Air temperature determines whether the

droplets form rain, snow, sleet, or hail.

Weather Patterns

Section 2

  Because air and moisture move in the atmosphere, weather is constantly changing – Air mass - a large body of air with properties like

the part of Earth’s surface over which it formed.

  Highs and lows – Stormy weather is associated with low pressure

areas – Fair weather is associated with high pressure

areas – Air pressure is measured by a barometer

  Front - a boundary between two different air masses – Clouds, precipitation, and storms occur at

frontal boundaries – All fronts are attached to an area of LOW

PRESSURE

Cold Front

  Cold air replaces warm air   Fastest moving front   Cumulonimbus clouds produce intense, but

brief precipitation.   Associated with thunderstorms   Clearing occurs soon afterwards

Warm Front

  Warmer air replaces cold air   Slow moving   Lasts 12 - 24 hours   Light to moderate precipitation   Precipitation begins long before warmer

temperatures arrive

Occluded Front

  Involves 3 different air masses with different temperatures

  Precipitation associated with warm front –  (light to moderate)

  Fast moving cold front catches up with slow moving warm front.

Stationary Front

  air masses move parallel to each other – precipitation similar to a warm front

  (light to moderate)

  May last from 1 - 4 days in a particular area

Draw and label these fronts

Cold Front

Warm Front

Occluded Front

Stationary Front

Severe Weather

  Thunderstorms occur inside warm, moist air masses and at fronts – Warm, moist air is forced rapidly upward, where

it cools and condenses – Strong updrafts of warm air and sinking, rain-

cooled air cause strong winds.   Is the parent storm for all severe storms:

(hurricanes, tornadoes)

  Lightning – Movement of air inside a storm cloud causes

parts of the cloud to become oppositely charged – Current flows between the region of opposite

electrical charge, forming a lightning bolt   Thunder - lightning superheats the air,

causing it to expand rapidly and then contract, forming sound waves.

  Tornado - a violent, whirling wind that moves in a narrow path over land

  Hurricane - a large, swirling, low-pressure system that forms over tropical oceans (winds must maintain 74 mph)

  Blizzard - winter storm with 35 mph winds, and visibility reduced to 1/4 mile for a period of 3 hours.

  Severe weather safety – A National Weather Service watch means

conditions are favorable for severe weather to develop.

– A warning means that severe weather conditions already exist.

Weather Forecasts

Section 3

  Meteorologist - study and predicts the weather.

  The National Weather Service makes weather maps. – Station models - show weather conditions at a

specific location

  Temperatures and pressures –  Isotherms - are lines on a weather map

connecting points of equal temperatures –  Isobars - lines on a weather map that connect

points of equal atmospheric pressure   Weather Fronts move from west to east