Science 10 Weather Dynamics Fronts, Highs, and Lows Science 10 Weather Dynamics

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.

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)

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.

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)

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.