You will have heard a million times forecasters talking of high or low pressure systems, but what exactly are they, and how do they affect our weather? A huge amount of air is pushing down on us at all times, the gases in the air are made of tiny molecules that although we can’t see them, are taking up space and have weight. The reason we can’t feel this is because our bodies also have air, which pushes outwards and this creates a balance between the two. Warm air molecules move quickly and push each other apart, meaning the air gets lighter and rises. Cooler air molecules move slowly, and they take up less space than the warm air molecules, and as the air becomes denser and heavier it sinks. Air pressure is the weight of the air pushing down on the earths surface, when air warms up it rises, when air cools and sinks there is high pressure. Air pressure varies all over the planet as different areas receive different amounts of the suns warmth. Pressure is measured with a barometer, and is recorded as millibars (mb). There is a difference between absolute pressure and sea level pressure. You can find your sea level pressure with a quick google search by typing in your location. My weather station records the actual pressure where I am, and will be different to those nearer the sea. Where there are hurricanes/typhoons the pressure at sea will drop very low, as these storms from from low weather systems.
As stated, when the air cools it sinks, and this means there is more air pushing down on the earths surface, thus the air pressure rises. As the air sink it becomes warmer and drier and the clouds disappear, so areas of high pressure usually bring warm dry weather with clear skies. In weather maps, high pressure will be depicted as red.
When the air warms it rises, leaving less air pushing down on the earths surface, and this leads to the pressure going down. Areas of low pressure, which are usually described as a depression, bring rain, wind and storms, and changeable temperatures. As high pressure is shown as red on weather maps, low pressure is shown as blue.
So generally, high pressure = good, low pressure = bad. Here in Britain, when we have typical weather for the seasons, we will have high pressure in spring/summer, and low pressure in autumn/winter. The reason we haven’t had the typical weather patterns lately is because of the jet stream, because it has been sat to the south of the UK for much of the past 18 months it has allowed low pressure to sweep right across the UK, usually in summer it sits above us, therefore drawing away low pressure above us and allowing high pressure to build. This is the kind of weather we have been seeing lately, the jet stream is in its more typical place for this type of year so the low pressure is being dragged north of us. Let’s all hope it will stay there throughout July and August!