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Pressure cells
Weather patterns are driven by the sun’s radiation. Latitude, season and cloud cover contribute to the amount of heat reaching the earth’s surface. Varying amounts of heat are reflected back into the atmosphere. This depends both on cloud cover and the type of surface such as ocean, grassland, forest and bare earth or sand.
The layer of air that surrounds the earth is held close to the surface by gravity. As the earth’s surface heats and cools unevenly so does the air above it. Large masses of hot air rise, spread, cool and fall back to earth creating convection currents. A low pressure cell is formed where air is rising and therefore becoming less dense. A high pressure cell occurs where air is sinking and becoming denser.
Air pressure is caused by the motion of billions of gas molecules in the atmosphere. These molecules move randomly in all directions bouncing against anything they meet. The more gas molecules there are in a certain space, the more collisions there will be and therefore the higher the air pressure.
It is very difficult to measure these collisions in an open space. Therefore air pressure, also called atmospheric or barometric pressure, is actually measured as the weight of the column of air above a point on the earth’s surface. The more molecules of air there are and the greater the weight of the column of air, the higher the pressure is. Air pressure is measured in hectopascals (hPa).
Air moves out of high pressure cells and into low pressure cells in a spiral fashion as shown below in Figure 1. The high pressure cell is marked “H” and the low pressure “L”. In the southern hemisphere air circulates clockwise around a low pressure cell and anticlockwise around a high pressure cell. The opposite occurs in the northern hemisphere.
Figure 1: Circulation of air around and between pressure cells
The concentric lines around the high and low pressure cells are called isobars and indicate places of equal air pressure.
Winds are caused by the flow of circulating air out of high pressure and into low pressure systems. Wind speed is indicated by the distance between the isobars. The closer the isobars are together, the stronger the winds will be. Where isobars are drawn relatively far apart there are only light winds or the weather is calm.
Wind direction is also indicated by the isobars. The arrows drawn on the isobars in Figure 1 show the change in wind direction as the air circulates around the pressure cells. The wind is named according to the direction from which it comes, using points of the compass as a reference.
For example, Point A to the north of the low pressure cell in Figure 1 is experiencing a westerly wind. To the south of the low at point B the wind is an easterly. The compass points are also subdivided to provide wind directions such as south-easterlies at Point C or north-westerlies at point D.
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