Pressure, Fronts, air masses

Air Pressure

The weight of air pressing down on a given area of Earth’s surface

Measured with barometer Changes in air pressure

Falling: stormy weather is coming Rising: fair weather is coming Steady: existing weather will continue

WINDWARD SIDE• Warm air cools as it goes over the mountain.• Moisture is released - precipitation

LEEWARD SIDE(other side of the mountain)Cool dry air slides downthe other side of the mountain creating a warm, dry climate.

WIND

LOW ATMOSPHERIC PRESSURE (less)

HIGH ATMOSPHERIC PRESSURE (More)

Warm Moist Air

Warm Air Rises

Warm Air Contracts CoolsMoisture condenses

Moisture is pulled down by gravity

Air rising has released the moisture,And the air becomes warm - less dense. The other side of the mountain will have a great afternoon.

PRESSURE SYSTEMS

HIGH - Clear/fair weather - densely packed molecules warming the air. Warming air decreases relative humidity and water vapor evaporates, before clouds form. Winds turn clockwise and away from center of system

LOW - As warm, less dense air rises, it creates a low pressure system.It forms along fronts where warm air meets cold air, causing a weather change. Stormy weather is associated with low pressure areas.

• Different air masses don’t mix together, when they meet and form a front. • Due to the Coriolis effect, winds begin to swirl counterclockwise in the Northern hemisphere. • Rising air forms a low pressure system, cools, reaches Dew point creating clouds, storms and precipitation.

WARM

COLD

Low pressure

Wind Currents Sun drives convection currents in air Wind is moving air, caused by differences in

air pressure Wind moves from high to low pressure The greater the pressure difference the

greater the wind Local winds=local weather (sea breezes and

land breezes) Global winds= circle Earth in wide belts (jet

stream) Wind vane: determines wind direction; points

to the direction from which the wind is blowing

Anemometer: measures wind speed; Beaufort scale (0-12) relates common observations to wind speed

Local Winds

Sea breeze- convection current blows wind from the cooler sea toward the warmer land during the day

Land Breeze- at night, air moves toward the water as the land cools more rapidly than the water

Ocean Currents

Wind blowing over the ocean’s surface pushing the water, producing a current of seawater

2 types (due to differences in density of seawater) Cold: move from polar regions to

equator Warm: move from tropical equatorial

latitudes to polar regions

Humidity and Dew point Humidity

is the moisture in the air Relative humidity is a comparison of the

amount of water vapor in the air to the greatest amount of water vapor that the air could hold at a given temperature

Psychrometer measure relative humidity Dew point

Temperature when more water vapor is condensing than evaporating-when dew forms

Air Masses

Large body of air that has the same properties as the surface over which it develops.

Maritime Tropical-warm, moist air-forms over ocean

Maritime Polar-cool, moist air-forms over cool ocean waters (north of U.S.)

Continental Tropical- hot, dry air-forms over Mexico, moves north in summer

Continental Polar- cold, dry air-moves into U.S. from Canada in winter

FRONTS - a boundary between 2 different air masses. Clouds, precipitation, and storms occur at frontal boundaries.

Rain occurs at all fronts because warm air is cooled, air becomes saturated and water is precipitated.

COLD COLD FRONTFRONT A cold air mass invades a warm air mass. Cold air

forces the warm air rapidly aloft along the steep front.

Brings violent storms followed by fair cooler weather

COLD AIR

WARM AIRCumulus

Cumulonimbus

The front continues moving in the direction of the cold front.

Cumulus

A warm front develops when a warm air mass meets a cold mass.

Warm air (less dense) advances over the colder air. Brings rain and showers followed by warmer, more

humid weather

WARM WARM FRONTFRONT

COLD AIR

WARM AIR

HOT OVER COLD

NIMBOSTRATUS

ALTOSTRATUS

CIRRUS

The front continues to move along in the same direction.

A STATIONARY FRONT develops when warm or cold front quits moving forward. Often brings many days of almost continuous precipitation.The front does not move - it stays stationary.

COLD orWARM AIR

Weather stays the same.

An OCClUDED FRONT develops when 2 cold air masses merge, forcing the warmer air between to rise. Usually produces heavy rain or other precipitation.

COLD AIR WARM AIR COLD AIR

CUMULONIMBUSSTRATOCUMULUS

Climate vs. Weather

Climate Weather

General weather conditions in broad area over long periods of time, includes seasonal weather changes

Factors in climate Latitude Elevation Topography Distance from water Global winds Ocean currents

The atmospheric conditions in a specific area (within 2 weeks)

Meteorology-study of weather; Meteorologist-studies weather (forecaster)

Factors in weather Convection-uneven

heating and cooling of the Earth’s surface

Air masses-certain temp and amount of moisture

WEATHER FORECASTING

Meteorologists - study and predict the weather.

Station models - show weather conditions at a specific location.

Isotherms - are lines on a weather map connecting points of equaltemperature.

Isobars - are lines on a weather map that connect points of equalatmospheric pressure. Indicates locations of high H and L low pressure systems. Weather fronts move from west to east because of the prevailing westerlies.

Weather Map symbols

STATION MODEL

WIND DIRECTION - TEMPERATURE

CLOUD COVER - BAROMETRIC PRESSURE

SEVERE WEATHER SAFETY

WATCH - means conditions are favorable for severe weather to develop.

WARNING - means that severe weather conditions already exist.

•huge, slowly spinning tropical storm that forms over water•Winds of at least 74 MPH or more (large scale cyclone that causes great destruction to coastal areas)•Winds circulate counter-clockwise•Need warm water temp and moist conditions•Builds energy as it moves across the ocean, sucking up warm, moist tropical air from the surface and dispensing cooler air

Hurricane