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30 November 2023
What is Air Pressure?
What is Air Pressure?
Air pressure is a force that is exerted on the Earth’s surface by the atmosphere
How do we get High and Low Pressure?
Temperature in the atmosphere affects what the air pressure is. As air cools, it becomes more dense and descends. This leads to high pressure at the surface because there are more air molecules hitting the Earth’s surface. Air that is warming up, becomes less dense and ascends which causes low pressure on the Earth’s surface.
Surface Pressure Charts
Surface pressure charts indicate solid lines of equal pressure which are known as isobars. These are measured in millibars. The standard air pressure at sea level is 1013.2mb, anything above is high pressure and anything below is low pressure. These maps also indicate areas of high and low pressure and give meteorologists an indication of what weather conditions we are going to get. For example, the closer the isobars are together, the stronger the winds are going to be.
You may also notice on these charts that there are dashed lines as well. This coloured map shows them as red dashed lines and they indicate thickness lines – the vertical distance between the 1000mb and 500mb pressure levels. Warm air will be ‘thicker’ and cold air will be ‘thinner’. On the map they are shown as decametres so 5280 metres is shown as 528. We refer to this as the Snow Line. Any areas that are North of the 528 line, the precipitation is likely to fall as snow whereas for areas situated South of the 528 line, it is more likely to fall as rain.
Isobars
Now there may be some people in the world who believe the Earth is flat, but there is no doubt that the Earth’s surface is not! The air pressure decreases as altitude increases. So how is it that we can create isobars, lines of equal pressure when the Earth’s surface has all different elevations? What we do is take measurements of air pressure and convert it to a value that would be observed if that instrument was located at sea level. We use sea level as the common denominator which allows us to draw equal lines of pressure and create isobars.
What Weather Conditions can we expect in High and Low Pressure?
Typically, weather conditions in high pressure are more settled, sunnier, drier with less wind. This is because air is sinking. In low pressure, warm air is rising which then condenses to form clouds and precipitation. Therefore in general, you will get cloudier, wetter and windier conditions in low pressure.
What does this mean in Winter?
High pressure in winter will lead to a greater chance for clear skies overnight and frost developing. Lighter winds can create fog and in colder temperatures, freezing fog can occur. Low pressure in winter normally signifies milder spells of wind, cloud and rain. However, if we have low pressure in a colder air mass, then this will bring a significant risk of snow, freezing rain and icy conditions as any precipitation that falls out of the sky is likely to freeze on the ground overnight. Don’t worry though, Ice Watch will have you covered in any wintry weather conditions!
