II. The Atmosphere

5. Low pressure and high pressure

The development process of low pressure

Fig. 5: The development process of low pressure.

We often hear the terms high pressure and low pressure in weather reports. Low pressure means that the atmospheric pressure of a region is lower than the surrounding area; when the situation is reversed, it is called high pressure. Therefore, the designation of high and low pressure is only relative like the designation of peaks and valleys in mountainous areas.

Every day meteorologists collect barometric pressure readings measured simultaneously at weather stations all over the world, and then update them on a map with the location of the weather stations marked on it. They then will connect by pencil the weather stations that have the same pressure readings thereby drawing an isobaric line. The chart formed by these isobaric lines is called the synoptic chart (or a weather map). By studying the distribution of the isobaric lines on the chart we can clearly see the location and distribution of the high and low pressure systems on the surface of the Earth.

Usually, low pressure occurs in mid-latitude temperate zones. It is formed by the movement caused by the colliding surface (i.e. front) of two characteristically different warm and cold air masses when they meet. The development of a low pressure system can be divided into four stages: early, mature, decay, and dissipation (Fig. 5). The average life span of a low pressure system is about seven days.

In the Northern Hemisphere, due to the rotation of the Earth and surface friction, the air currents surrounding a low pressure system will flow in a counterclockwise direction, toward the center of the low pressure (Fig. 6). As a result, air flows will gather from the surrounding area and accumulate at the center of the low pressure system, forcing the air in the center to rise, cooling and condensing the water vapor in the uplifted air, forming clouds and eventually rain. Therefore, regions under a low pressure system usually experience bad weather. Air flows surrounding a high pressure system, on the other hand, flow in clockwise direction; the air flows out of the center (Fig. 6) forcing the air over the center to flow down and outward.

wind in a high pressure systemFig. 6: In the Northern Hemisphere, the wind in a high pressure system flows in clockwise direction out of the center, while in low pressure system wind flows in a counterclockwise direction towards the center. Therefore, with his back to the wind, the person will have high pressure at his right and low pressure at his left.