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• Air pressure is sometimes also called atmospheric pressure. This is the total weight of the air in the atmosphere pushing down on the surface of the earth.

• Air can be heavier than you think – warm air is usually a bit lighter than cold air. Normal air pressure is 1000 millibars (mb).

• Low pressure happens when the air pressure goes below this, and high pressure is when the air pressure is higher than 1000mb.

• The instrument used to measure air pressure is called a barometer. Usually this will have a small vacuum box that will be squeezed when there is a low pressure system and will expand when there is a high pressure system.

• This will then point to a place on a dial that will indicate whether good weather (high pressure) or bad weather (low pressure) is expected.

• The weather maps that we see on the news are sometimes called synoptic charts or surface pressure charts.

• They often contain a series of lines called isobars. Isobars are lines that are drawn to join places that have the same atmospheric pressure. These are usually drawn for every 4 millibars (eg 996 or 1004).

• Usually, this chart will also indicate the places where the centre of the low and high pressure can be found. The closer the isobars are together, the windier it will be.

• Out in the Atlantic Ocean, warm air (from the south) and cold air (from the north) meet but do not easily mix. The lighter, warm air will start to rise over the denser colder air.

• When air rises in the atmosphere it will start to cool and its moisture condenses to form clouds. Once these clouds become thick enough, they will produce precipitation such as rain, hail or snow.

• Depressions often move across Ireland and the UK from the south-west to the north-east.

• Depressions can last between 3 and 5 days as they pass over the UK. As the depression passes – people will experience changes to the weather on the ground.

NOTE: The sequence describes the change in weather if you were standing above where it says east on the diagram (above).

The weather changes as a depression passes overhead in the following ways:

1. At first the clouds are high fair-weather, cirrus clouds.
2. As the warm front (marked by red semi circles) passes it will push air up over the cold air and this will bring some rain which will eventually become drizzle and then clearer skies.
3. The front edge of the cold air is called the cold front (marked by blue triangles), and this will cause the air to rise quickly and will bring heavy rain from cumulus and cumulonimbus clouds.
4. As the depression moves away to the east, the sky will clear of clouds, rain will turn to showers and the winds will be lighter.

• Anticyclones are caused by massive high pressure systems that usually are pushed up past Spain and France to sit above the British Isles for a period of time.

• Sometimes these weather systems can bring dry, settled weather for up to three weeks.

• Anticyclones can bring different types of weather in the winter compared with the summer.

• They are caused by high pressure and often there is very little cloud cover, light winds (and calm conditions) and usually have very little precipitation.