High and low pressure systems cause the day-to-day changes in our weather. In this article we look at how they are defined and how they form.
What causes high and low pressure?
The Earth's atmosphere exerts a pressure on the surface. Pressure is measured in hectoPascals (hPa), also called millibars. Standard pressure at sea level is defined as 1013hPa, but we can see large areas of either high or low pressure. These areas are all relative to each other - so what defines a high will change depending on the area around it.
On a weather chart, lines joining places with equal sea-level pressures are called isobars. Charts showing isobars are useful because they identify features such as anticyclones (areas of high pressure) and depressions (areas of low pressure).
Areas of high and low pressure are caused by ascending and descending air. As air warms, it ascends leading to low pressure at the surface. As air cools, it descends leading to high pressure at the surface. This is illustrated in the diagram below.
In general, low pressure leads to unsettled weather conditions and high pressure leads to settled weather conditions.
In an anticyclone (high pressure) the winds tend to be light and blow in a clockwise direction (in the northern hemisphere). Also the air is descending, which reduces the formation of cloud and leads to light winds and settled weather conditions.
In a depression (low pressure), air is rising and blows in an anticlockwise direction around the low (in the northern hemisphere). As it rises and cools, water vapour condenses to form clouds and perhaps precipitation. This is why the weather in a depression is often unsettled - there are usually Weather fronts associated with depressions.