How to Understand a Pressure Chart

BBC Weather

Met Office

The light coloured pressure lines on the map are called Isobars. Points along each line have the same pressure. Wind runs anti-clockwise around a Low Pressure System (Indicated on the weather map by the letter "L" followed by the pressure in millbars). Winds move clock-wise around a High Pressure System (Indicated on the weather map by the letter "H" followed by the pressure in millbars)

Low Pressure Systems. (Cyclones)

As the pressure system moves across the country the wind direction experienced on the ground changes. Where the isobars are closest together the wind speed is strongest.

This symbol represents a cold front.

This symbol represents a warm front.

 

A Low Pressure System

 

Fronts rotate around a Low pressure system in an anti-clockwise direction. Both are associated with a change in temperature,cloud, a lot of rain and a shift in wind direction. The diagrams below show the formation of an occluded front.

 

 

When a depression or a low pressure system forms, it usually consists of a warm front and a faster moving cold front. This can be seen in diagram 1. To the north of the warm front is the cool air that was in the area before the depression developed. As the depression intensifies, the cold front catches the warm front. This can be seen in diagram 2. The line where the two fronts meet is called an occluded front. When an Occluded Front passed overhead, you would feel changes in temperature and wind speed.

High Pressure Systems (Anticyclones)

Areas of sinking air which result in high pressure are called anticyclones. The changes in pressure are much smaller than those associated with low pressure. High pressure systems cover huge areas, the rate at which changes occur is slower and the weather patterns usually more stable. The weather conditions depend on the time of year

In Summer

• Hot days with few or no clouds but there can be mist in the mornings.
• Light winds.
• Warm moist air rising from the ground may result in thunderstorms.
• Some cloud covercan form over Eastern England. This is caused by light winds blowing over the cooler North Sea.

In Winter

• Cloudless skies but the low angle of the sun produces less heat
• days arecold and the nights even colder due to lack of cloud cover.
• Fog and frost often form at night.
• Cold air from Russia can bring snow to the east of the country.

 

Summary text file with weather data from Trewern for the past 24 hours
Trewern weather Data Archive
Full Trewern weather display

To access these pages you will need a User Name and Password email

graham@trewern.org.uk