Fronts, Highs, and Lows

Science 10

Fronts Air masses eventually move around

due to worldwide wind currents, and overtake one another.

The zone between the air masses is called a front.

Meteorologists define four general types of fronts: cold, warm, stationary, and occluded.

Fronts video

Cold Fronts A moving cold air mass overtakes a

warmer air mass. Because the cold air is more dense than

the warm air, it moves in below the warm air and pushes the warm air upwards.

This causes a cumulus or cumulonimbus cloud to form.

Rain, thunderstorms, and tornadoes result from cold fronts.

Cold Fronts (cont’d)

Warm Fronts A warm air mass overtakes a cooler

one. The warm air gently rides up over the

trailing end of the cold air mass. Stratus and nimbostratus clouds form

and can bring light rain or snow lasting longer than it does with a cold front.

After the front passes, it leaves warmer temperatures than before it arrived.

Warm Fronts (cont’d)

Cold Front vs. Warm Front

Stationary Fronts Cold and warm air masses are in

contact, but neither is moving. Warm air drifts up and condenses to

form stratus clouds and often rain. The front may remain stationary for

several days. The result is a dreary, gloomy sky.

Stationary Front Video

Occluded Fronts A cold air mass catches up with a

slower-moving warm front. Many types of clouds are present,

one after the other. Occluded fronts often cause a

period of steady precipitation.

Occluded Fronts (cont’d)

Occluded Front Video

High Pressure Systems Air descends and exerts more pressure

on the region below. When an air mass forms over cold

ground, the air cools, becomes more dense, and settles lower to the ground. This creates more pressure, so the system draws more air in from above. Air at the bottom is forced out and to the right, due to the Coriolis effect.

High Pressure Systems (cont’d)

High Pressure Systems (cont’d) The overall result is a clockwise

rotation of air called an anticyclone. Because the cool descending air has

little moisture, high pressure systems create a clear sky.

They may be hundreds of km across. They cause weather conditions to

stay the same for several days.

Low Pressure Systems Very intense heating of the ground can

create a low pressure system. The heated air rises leaving less dense

air below. As the air rises, it pulls more air

beneath it. The Coriolis effect causes the air to

curve to the right. This results in a counterclockwise

rotation of air known as a cyclone.

Low Pressure Systems (cont’d)

Low Pressure Systems (cont’d) Lows are generally smaller in size

than highs. They bring unstable conditions,

which cause changing weather, precipitation, and often storms.