Synoptic Chart Uk — free printable diagram

How to read a synoptic chart

A synoptic chart is a weather map showing conditions across a region at one moment. Two things carry most of the information: the isobars, which are lines joining places of equal air pressure, and the front symbols, which mark the boundaries between air masses. Read the pressure first and the fronts second, and the chart tells you what the weather is doing and which way it is heading.

What it shows

Air pressure, fronts and wind across a region at one moment

Isobars

Lines joining places of equal air pressure

Pressure unit

Millibars (mb), also written hectopascals (hPa)

Low pressure (depression)

Unsettled — cloud, rain, stronger winds

High pressure (anticyclone)

Settled — drier, lighter winds

Closely spaced isobars

Strong wind

Front symbols

Triangles for cold, semicircles for warm, both for occluded

• Isobars do for pressure what contour lines do for height: each line is one value, and how close together they run tells you how steeply the quantity changes.
• Isobar spacing is the wind-speed clue. Tightly packed isobars mean a steep pressure gradient and strong wind; widely spaced isobars mean light wind.
• Wind blows roughly along the isobars rather than straight across them. In the northern hemisphere it circulates anticlockwise around a low and clockwise around a high; in the southern hemisphere both are reversed.
• A depression brings rising air, cloud and rain. An anticyclone brings sinking air, which suppresses cloud — giving clear settled spells, hot and dry in summer and frosty or foggy in winter.
• The symbols point the way a front is moving: triangles on a cold front, semicircles on a warm front, and the two alternating on an occluded front, where a cold front has caught up with the warm front ahead of it.
• A good reading order for students: find the centres of low and high pressure, check the isobar spacing for wind strength, then locate the fronts and work out which places they will cross next.