On a weather chart, lines joining places with equal sea-level pressures are called isobars. Charts showing isobars are useful because they identify features such as anticyclones (areas of high pressure), depressions (areas of low pressure), troughs and ridges which are associated with particular kinds of weather.

	High pressure or anticyclone Weather is usually settled and cloudless, but in winter clear skies and light winds can mean frost or fog. Winds blow clockwise around an anticyclone in the northern hemisphere. If high pressure persists over northern Europe in winter, then this can mean a spell of very cold east winds for Britain. In summer, however, high pressure over the British Isles or the continent usually brings warm, fine weather.
	Low pressure or depression Usually associated with disturbed weather — strong winds and rain belts, with perhaps snow in winter. Winds blow anticlockwise around a depression in the northern hemisphere.
	Pressure lines or isobars Winds blow along these lines: strong when the lines are close together, lighter when they are apart. The figures show the pressure (in millibars) at all points along each line. The term hectopascal (hPa) is often used instead of millibar; 1 millibar equals 1 hectopascal.

The side of the front which has the 'humps' or 'triangles' indicates the direction in which the front is moving.

Warm front

Ahead of the warm front is a belt of thickening cloud, gradually developing into moderate rain and cloud. The belt of rain extends 100-200 miles ahead of the front. Behind the front the rain usually becomes lighter, or ceases, but the weather remains cloudy. The warm front does not necessarily bring higher temperatures. This is particularly so in summer, when the cloudy weather behind the front cuts off the heat from the sun. In winter, however, the south-westerly winds that usually blow after a warm front has moved through bring milder conditions. The advance of a warm front is usually the way in which a cold winter spell is broken down. Ahead of a warm front pressure falls steadily, and sometimes very rapidly. After the passage of the front the barometer usually steadies, or falls less rapidly.

Cold front

This usually brings a narrower belt of cloud and rain. It is called a cold front because the moist south-westerly winds ahead of it are replaced by cooler, drier north-westerly winds. When a cold front moves through an area it usually brings brighter, clearer weather behind it, but this brighter weather is sometimes mixed with showers. Pressure falls ahead of the cold front, but usually rises after its passage and the veer of the wind.

Occluded front or occlusion

The characteristics of an occlusion are similar to those of a cold front in that the rain belt is narrow, and the winds generally veer to the north-west behind it. There is usually a clearance to the west after the front has moved through.

Sometimes fronts do not bring much in the way of rain or cloud, and are then known as 'weak'. Thunderstorms can occur with all three types of front, but are most likely to accompany a cold front. However, many thunderstorms are not linked with frontal activity.

Station circle

This shows the state of sky or amount of cloud.

Pressure

Isobars are drawn at intervals of four millibars, unless there is a particularly deep depression where too many isobars would be difficult to read. In this case they are drawn at eight-millibar intervals.

Wind

The arrow indicates the direction from which the wind is blowing. A system of 'feathers' and 'pennants' is used to indicate wind speeds; full feathers indicate 10 knots, half feathers 5 knots and solid pennants 50 knots.

Temperature

This is given in degrees Celsius.

Visibility

This is indicated by two figures plotted to the left of the station circle, and of any weather symbols. In the range 01 to 50 the figures give the visibility in tenths of kilometres, e.g. 20 = 2,000 m = 2 km. Visibilities of greater than 5 km are recorded in whole kilometres plus 50, e.g. 56 = 6 km, 60 = 10 km, 70 = 20 km.

Weather symbols

Example station entry

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