Basic Geography/Climate/Weather Fronts

Weather Fronts

There are three types of weather front:

• Warm Front

A warm front is warm air moving into an area of cold air this forces the warm air to rise over the cold air. As the warm air lifts into an area of low pressure clouds form as flat sheets

• Cold Front

A Cold front is cold air moving into an area of warm air. The cold air pushes under the warm air forcing the warm air to rise. The warm air then condenses producing rain.

• Occluded Front

An occluded front is where two fronts, a warm and a cold meet. If the cold front cuts under the warm front it is known as a cold occlusion. If the cold front rises over the warm one it is known as a warm occlusion. Both types of occluded fronts can bring rain.

Global Air masses

Globally there are very large areas of high and low pressures. They form bands around the Earth that are parallel with the line of latitude. At the equator (0°) there is a low pressure band with two more, one near the Arctic (60° North) and one near the Antarctic (60° South). Between the low pressure lie high pressure belts one at 30° North and one at 30° South, just north and south of the tropical latitudes.

As air moves from high to low pressures it produces the global prevailing winds, known as trade winds. The trade winds are strongest between the high and low pressures with wind speeds relatively low at the pressure belts.

The trade winds do not blow in a straight line towards the poles as they are blown of course by the spinning of the Earth. In the northern hemisphere the winds are blown to the right and to the left in the southern hemisphere. Therefore making prevailing winds mainly south westerly in the North.

Atmospheric circulation

The low pressure area at the equator is the driving force for the whole current system of the Earth. As warm air rises at this low pressure area it cools and condenses forming clouds and precipitation. The low pressure then takes cold air from the north and south to replace the rising air. This cold air is then itself heated pushing the now dry warm air further north and south. This dry air starts to sink back to Earth at the tropics creating the High Pressure areas. This falling dry air makes the areas have a much drier climate forming deserts and savannas.

This dry air is then sucked back towards the equator where the process is repeated. The areas where this cycles of winds happen is known as the Hadley Cells.