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Why does it rain when pressure is low? Why does high pressure lead to fine weather? Watch the Met Office explain how pressure affects the weather.

Atmospheric pressure is the weight of air in a column above your head, we don’t think of air as having weight because we don’t feel it, but actually there is the weight of a small car bearing down on us all of the time. Standard sea level pressure is 1013 hPa (this stands for hecto Pascals, but used to be called millibars).

Diagram showing a man with a column of air above his head weighing down on him indicated by a large blue arrow in the downward direction Weight of air above your head. Pressure decreases with height

This is a synoptic chart. I’m sure you are used to seeing these on TV or online. This particular chart is an analysis chart, and has been drawn by one of the Met Office’s Chief Meteorologists using a combination of data from observations including satellite and radar data, and the computer model.

A Met Office Analysis Chart valid 0000 UTC Mon 10 April 2017. It shows a map centred on the UK, with isobars and fronts marked on. A large area of high pressure sits in the Atlantic marked with an x at the centre labelled 1035 hPa. Low pressure centred over Scandinavia results in weather fronts across the UK, with a trailing cold front across central areas, and 2 occlusions across Scotland. The isobars across the UK, indicate a northwesterly wind bringing a polar maritime air mass. Met Office Analysis Chart

We’ll learn more about all the features you see on these weather charts later on, but for the moment, we want to focus on pressure. The thin black lines that are labeled are isobars. An isobar is a line that connects all areas of equal pressure. H is the centre of an area of high pressure, and L is the centre of a low. There is no particular number that constitutes an area of high or low pressure, they are relative to each other, they generally tell us whether the air is rising or sinking. Let’s have a look at this in more detail.

High pressure

Photograph of the Sun shining in a blue sky

Highs, also called anticyclones, tend to be very large, often a few thousand kilometres across. In a high, the air is generally sinking, this leads to mostly dry and settled weather, usually with light winds, however cloud amounts can be very variable and quite tricky to forecast. The descending air can also trap pollutants and moisture and so can lead to rather hazy conditions. In the winter, highs can produce widespread fog, which can be slow to clear due to the long nights, and sometimes sticks around for days on end. But equally, high pressure can lead to glorious blue-sky summer days.

Weather graphics showing a map of the North Atlantic and Northwest Europe. A large area of high pressure covers much of the UK, with a low centred over Iceland and cloud and rain to the west of the UK

Low pressure

Low pressure systems, also called cyclones, tend to be smaller and more compact. As the air rises in a low, it will cool, allowing water vapour to condense. This forms clouds, which can lead to precipitation – rain, snow or hail.

Pressure and temperature

Highs and lows can, and often do, develop as a result of temperature. Where heating occurs, air will rise, and this results in low pressure. Equally, if air stagnates over a cold area for a long time, this results in cooling and sinking, and therefore high pressure. You can start to see that weather is a complex business, because temperature is leading to pressure changes, and those pressure changes result in weather and therefore can determine the temperature.

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