ch 16 weather

WeatherWeather

Weather is the state of the Weather is the state of the atmosphere at a specific time atmosphere at a specific time

and place.and place.

Includes such conditions as Includes such conditions as air pressure, wind, air pressure, wind,

temperature and moisture in temperature and moisture in the air.the air.

The Sun’s heat evaporates The Sun’s heat evaporates water into the atmosphere water into the atmosphere

forming clouds; water forming clouds; water returns to Earth as rain or returns to Earth as rain or

snow; the Sun also heats air.snow; the Sun also heats air.

Temperature is a measure Temperature is a measure of air molecule movement.of air molecule movement.

The Sun’s energy causes air The Sun’s energy causes air molecules to move rapidly; molecules to move rapidly;

temperatures are high and it temperatures are high and it feels warmfeels warm

When less of the Sun’s When less of the Sun’s energy reaches air energy reaches air

molecules, they move less molecules, they move less rapidly and it feels cold.rapidly and it feels cold.

Wind –air moving in a Wind –air moving in a specific direction.specific direction.

As the Sun heats air, it As the Sun heats air, it expands, becomes less expands, becomes less

dense, rises, and has low dense, rises, and has low atmospheric pressure.atmospheric pressure.

Cooler air is Cooler air is denser and denser and sinks, sinks, causing high causing high atmospheric atmospheric pressure.pressure.

Air moves from high Air moves from high pressure areas to low pressure areas to low

pressure areas, causing pressure areas, causing wind.wind.

Humidity –the amount of Humidity –the amount of water vapor in the air.water vapor in the air.

Warmer air can hold more Warmer air can hold more water vapor, tending to make water vapor, tending to make

it more humid.it more humid.

Relative humidity –the Relative humidity –the amount of water vapor in amount of water vapor in

the air compared to what it the air compared to what it can hold at a specific can hold at a specific

temperature.temperature.

When air cools, it can’t hold When air cools, it can’t hold as much water vapor, so the as much water vapor, so the water vapor condenses to a water vapor condenses to a liquid or forms ice crystals.liquid or forms ice crystals.

Dew point- the temperature at Dew point- the temperature at which air is saturated and which air is saturated and

condensation forms.condensation forms.

Clouds form as warm air is Clouds form as warm air is forced upward and cools. forced upward and cools.

then the water vapor then the water vapor condenses in tiny droplets condenses in tiny droplets

that remain suspended in the that remain suspended in the air.air.

The shape and height of The shape and height of clouds vary with clouds vary with

temperature, pressure and temperature, pressure and the water vapor in the the water vapor in the

atmosphere.atmosphere.

Cloud ShapeCloud Shape Stratus: smooth, even sheets or Stratus: smooth, even sheets or

layers at low altitudes.layers at low altitudes.

Cloud ShapeCloud Shape Cumulus: puffy, white clouds, often Cumulus: puffy, white clouds, often

with flat bases.with flat bases.

Cloud ShapeCloud Shape Cirrus: high, Cirrus: high,

thin, white thin, white feathery clouds feathery clouds made of ice made of ice crystals.crystals.

Cloud HeightCloud Height Cirro: high cloudsCirro: high clouds Alto: middle-elevation cloudsAlto: middle-elevation clouds Strato: low cloudsStrato: low clouds

Nimbus clouds are dark and Nimbus clouds are dark and so full of water that so full of water that

sunlight can’t penetrate sunlight can’t penetrate them.them.

Precipitation: water falling Precipitation: water falling from clouds.from clouds.

When water droplets in When water droplets in clouds combine and grow clouds combine and grow

large enough precipitation large enough precipitation falls to Earth.falls to Earth.

Air temperature determines Air temperature determines whether droplets form rain, whether droplets form rain,

snow, sleet, or hail.snow, sleet, or hail.

Section 2: Weather Section 2: Weather PatternsPatterns

Because air and moisture Because air and moisture move in the atmosphere, move in the atmosphere,

weather constantly weather constantly changes.changes.

Air mass –large body of air Air mass –large body of air with properties like the with properties like the

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

Highs and LowsHighs and Lows Stormy weather is associated with Stormy weather is associated with

low pressure areas.low pressure areas.

Fair weather is associated with high Fair weather is associated with high pressure areas.pressure areas.

Air pressure is measured by a Air pressure is measured by a barometer.barometer.

Front –a boundary between Front –a boundary between two different air masses.two different air masses.

Clouds, precipitation, and Clouds, precipitation, and storms occur at frontal storms occur at frontal

boundaries.boundaries.

Cold front- where colder air Cold front- where colder air advances under warm air.advances under warm air.

Warm front –where warmer Warm front –where warmer air advances over colder airair advances over colder air

Occluded front –involves Occluded front –involves three air masses of three air masses of

different temperatures.different temperatures.

Stationary front –air mass Stationary front –air mass and their boundaries stop and their boundaries stop

advancing.advancing. Warm, moist air is forced rapidly Warm, moist air is forced rapidly upward, where it cools and upward, where it cools and condenses.condenses.

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

Thunderstorms occur inside Thunderstorms occur inside warm, moist air masses and warm, moist air masses and

fronts.fronts.

LightningLightning Movement of air inside a storm Movement of air inside a storm

cloud causes parts of the cloud to cloud causes parts of the cloud to become oppositely charged.become oppositely charged.

Current flows between the regions Current flows between the regions of opposite electrical charge, of opposite electrical charge, forming a lighting bolt.forming a lighting bolt.

Thunder –lighting Thunder –lighting superheats the air, causing superheats the air, causing

it to expand rapidly and it to expand rapidly and then contract, forming then contract, forming

sound waves.sound waves.

Tornado –a violent, whirling Tornado –a violent, whirling wind that moves in a narrow wind that moves in a narrow

path over land.path over land.

Hurricane –a large, Hurricane –a large, swirling, low-pressure swirling, low-pressure system that forms over system that forms over

tropical oceanstropical oceans..

Blizzard – a winter snow storm Blizzard – a winter snow storm with strong winds, cold with strong winds, cold temperatures, and low temperatures, and low

visibility, that lasts more than visibility, that lasts more than three hours.three hours.

Severe Weather SafetySevere Weather Safety

A National Weather Service watch A National Weather Service watch means conditions are favorable for means conditions are favorable for severe weather to develop.severe weather to develop.

A warning means that severe A warning means that severe weather conditions already exist.weather conditions already exist.

Section 3: Weather Section 3: Weather ForecastsForecasts

Meteorologists study and Meteorologists study and predict the weather.predict the weather.

The National Weather The National Weather Service makes weather Service makes weather

maps.maps.

Station models show Station models show weather conditions at a weather conditions at a

specific location.specific location.

Temperature and Temperature and PressurePressure Isotherms are lines Isotherms are lines

on a weather map on a weather map connecting points connecting points of equal of equal temperature.temperature.

Isobars are lines Isobars are lines on a weather map on a weather map that connect points that connect points of equal of equal atmospheric atmospheric pressure.pressure.

Weather fronts move from Weather fronts move from west to east.west to east.

THIS IS DUE TO THE SPIN OF THE THIS IS DUE TO THE SPIN OF THE EARTH AND THE GLOBAL EARTH AND THE GLOBAL “PREVAILING WESTERLY WINDS”“PREVAILING WESTERLY WINDS”