How to read synoptic weather charts

Key elements of synoptic charts

1. Isobars and pressure patterns

• Isobars are circular black lines connecting areas of equal barometric pressure.
• The spacing of isobars indicates wind strength: closely spaced isobars mean strong winds, while widely spaced ones suggest lighter winds.
• Air moves from high to low pressure, and the direction of movement is influenced by the pressure gradient.
• Around high-pressure areas, air circulates clockwise; around low-pressure areas, it moves anticlockwise.

2. Fronts

• Fronts mark boundaries between different air masses (warm, cold, dry, moist).
• Warm fronts are shown as red lines with semi-circles pointing in the direction of movement; cold fronts use blue lines with triangles.
• The direction of the semi-circles or triangles shows where the front is heading.
• Warm air follows a warm front, cold air follows a cold front, and both are typically associated with increased cloud and rainfall.
• If a front’s line is broken by crosses, it means the front is weakening and the difference between air masses is less pronounced.
• When a cold front overtakes a warm front, an occluded front forms, shown as a purple line with alternating semi-circles and triangles.
• The area between these fronts, called the ‘warm sector,’ often brings low cloud and patchy light rain.

3. Troughs

• Troughs are black lines without semi-circles or triangles, marking regions of unstable, turbulent air.
• These areas are often associated with showers but do not represent boundaries between air masses like fronts do.

Synoptic weather charts use symbols and lines to summarise atmospheric conditions. By learning to read isobars, fronts, occluded fronts, and troughs, anyone can better understand and anticipate weather changes.