From summer sunshine to autumn storms, many of the changes we experience in the UK’s weather are connected to atmospheric pressure.
Areas of high and low pressure influence whether our weather is settled or unsettled, while their position and movement can affect everything from temperature and cloud cover to rainfall and wind speed. But what exactly is atmospheric pressure, and why does it matter?
What is atmospheric pressure?
Atmospheric pressure is the force exerted by the Earth’s atmosphere on its surface. It is usually measured in hectopascals, written as hPa, although the term millibars is also sometimes used.
Pressure varies from place to place and over time. Meteorologists are particularly interested in areas where pressure is higher or lower than the surrounding atmosphere, rather than simply looking at one pressure reading in isolation.
These patterns are displayed on surface pressure charts using isobars. These are lines joining places with the same sea-level pressure. High-pressure centres are normally marked with an “H”, while low-pressure centres are shown with an “L”. Weather fronts are also displayed, helping forecasters follow areas of cloud and rain.
What causes high and low pressure?
Pressure systems are closely linked to the vertical movement of air.
In an area of low pressure, air rises through the atmosphere. As it rises it cools, allowing water vapour to condense into cloud and, in many cases, rain. Low-pressure systems, also known as depressions, are therefore commonly associated with unsettled weather and weather fronts. Air circulates anticlockwise around a depression in the northern hemisphere.
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High pressure, also known as an anticyclone, is associated with descending air. This sinking motion tends to limit cloud development, often bringing drier and more settled conditions. Winds circulate clockwise around an anticyclone in the northern hemisphere and are often lighter near its centre.
However, high pressure does not always guarantee blue skies. During autumn and winter, moisture can become trapped near the surface beneath the sinking air. With lighter winds and less daytime heating, low cloud, mist or fog may persist, producing what is sometimes described as anticyclonic gloom.
Why does pressure affect the wind?
Air is naturally driven from areas of higher pressure towards areas of lower pressure. The difference in pressure over a particular distance is known as the pressure gradient.
Where isobars are widely spaced, the pressure gradient is relatively weak and winds are generally lighter. Tightly packed isobars indicate a steeper pressure gradient and usually stronger winds. The Earth’s rotation also deflects moving air, helping to create the familiar spiral patterns around high- and low-pressure systems.
This is why a deep depression with tightly packed isobars can bring strong winds, while a broad area of high pressure may produce several days of light winds.
When pressure patterns become blocked
Weather systems usually travel broadly from west to east across the North Atlantic. Sometimes, however, an area of high pressure becomes slow-moving and disrupts this progression. This is known as atmospheric blocking.
A block can keep Atlantic fronts away from the UK, allowing dry and settled weather to continue for an extended period. Elsewhere, fronts may become slow-moving, resulting in repeated rain or prolonged windy conditions. The position and strength of the jet stream play important roles in determining whether these blocked patterns develop and how long they last.
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How pressure changes from summer to autumn
During summer, high pressure can bring long, sunny days, dry spells and very warm or hot conditions. If it persists, it can also contribute to heatwaves, drought, poor air quality and high pollen levels. Summer low pressure, meanwhile, can produce showers, thunderstorms and prolonged rainfall.
The move from summer into autumn does not cause pressure to change on a particular date. Instead, it marks a gradual shift in the wider atmospheric pattern. Meteorological autumn begins on 1 September, while astronomical autumn normally begins around 22 or 23 September. Autumn is often associated with falling temperatures, shorter days and an increasing chance of unsettled or stormy conditions as the season progresses.
As Atlantic depressions become more influential, areas of low pressure and their associated fronts can bring more frequent spells of rain and stronger winds to the UK. The jet stream helps steer these systems, so changes in its position can determine which parts of the country experience the wettest and windiest weather. High pressure can still dominate during autumn, bringing dry and calm spells, although these may increasingly feature fog, low cloud and chilly nights rather than the widespread sunshine and heat associated with summer.
Pressure is therefore central to understanding the UK’s changeable weather. By following the highs, lows, fronts and isobars on a weather chart, we can begin to see not only what conditions are approaching, but why the forecast is changing.
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