High and low pressure systems cause day-to-day changes in our weather. In this article, we look at how they are defined and how they form.
The Earth's atmosphere exerts 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.