Isobar weather maps explained

Isobars are lines linking points on the earth’s surface (or at fixed heights above the earth’s surface) where the air pressure remains constant. Isobars are like contour lines on an ordnance survey map that mark points on the earth’s surface that are at the same height above a datum point (usually sea level).

The wind is caused by air under higher pressure moving towards the area of the earth where the air is under less pressure. Areas of warm air are low-pressure areas and areas of high pressure contain cooler air. 

Air would normally move at right angles to the pressure line (the isobar) from higher pressure to lower pressure but due to the Coriolis effect, in the Northern Hemisphere air is rotated clockwise (to the right) and in the Southern, counterclockwise (to the left) with reference to the movement of the wind direction. The friction between air and sea is lower than it is when the air is passing over land, so you will usually find that wind is deflected more by the Coriolis effect over the sea than overland. The angle is usually 40-50° above land and can reach 70-80° above sea - then the wind blows practically along the isobar.

The closer together the isobars are, the steeper the change in air pressure (in the same way that an ordnance survey map with contours close together would illustrate a steep hillside).

Warmer air is generally less stable and occupies an area of low pressure known as a cyclone or depression. This will usually bring cloudy and unsettled weather. Cooler air will create an area of relatively high pressure, bringing more stable conditions, less cloud and clear skies.

In the northern hemisphere, warmer air moves anti-clockwise around a low-pressure system (or cyclone) along the isobars, with the wind blowing about 10 - 20 degrees in towards the centre of the depression. 

In the northern hemisphere, cooler air moves clockwise around a low-pressure system (or anticyclone) along the isobars, with the wind blowing about 10 - 20 degrees outwards from the centre of the depression. 

Where warm and cold air masses meet we have fronts marked by red half-moons or blue triangles, with the half-moons representing the warm front and the blue triangles representing the cold front. A front with half-moons and triangles alternating across its line on the same side represents an occluded front. When the triangles and half-moons are on opposite sides of a front line, this represents a stationary front. 

By using the scale on a weather chart you can measure the distance between isobars and therefore estimate the forecast wind speed.

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