Weather Notes

Section One- What is Weather? Weather is the state of the atmosphere at a

specific time and place. Includes such conditions as air pressure, wind, temperatures, and

moisture in the air. The Sun’s heat evaporates water into the atmosphere forming

clouds and returning the water to Earth as rain or snow; the Sun also heats air

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

Section One- What is Weather? 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’t hold as much water vapor, so the water vapor

condenses to a liquid or forms ice crystals• Dew point--the temperature at which air is saturated and condensation

forms

Section One- What is Weather? Clouds from as warm air is forced upward and

cools. Then the water vapor condenses in tiny droplets that remain suspended in the air

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

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

Section One- What is Weather? 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

Section Two- Weather Patterns

Because air and moisture move in the atmosphere, weather constantly changes

Air mass--a large body of air with properties like the part of Earth’s surface over which it forms

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 Cold front--where colder air advances under warm air Warm front--where warmer air advances under colder air Occluded front--involves three air masses of different

temperatures Stationary front--air masses and their boundaries stop advancing

Section Two- Weather Patterns

Severe weather Thunderstorms occur along warm, moist air masses and at fronts

• Warm, moist air is forced rapidly upward, where it cools and condensates

• Strong updrafts of warm air and sinking, rain-cooled air cause strong winds

Lightning• Movements of air inside a storm cloud causes parts of the cloud to

become opppositely charged• Current flows between the regions of opposite electrical charge,

forming a lightning bolt

Section Two- Weather Patterns

Thunder--lighting 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

Blizzard--a winter storm with strong winds, cold temperatures, and low visibility, that lasts more than three hours

Severe weather safety The National Weather Service watch means conditions are

favorable for severe weather to develop A warning means that severe weather conditions already exist

Section Three- Weather Forecasts

Metorologists study and predict the weather The National Weather Service makes weather maps Station models show weather conditions at a specific location Isotherms are lines on a weather map connecting points of equal

temperature Isobars are lines on a weather map connecting points of equal

atmospheric pressure Weather fronts move from west to east