How to read synoptic weather charts

Ever wondered what all the lines, letters, numbers and shapes mean on a synoptic weather chart?

The word “synoptic” means ‘forming a general summary’ and in the case of a synoptic weather chart we are looking at a summary of the weather including, pressure, fronts, wind speed and direction.  Synoptic weather charts are produced to cover large areas giving an overall view of the weather and how the weather will change or evolve over the coming days.  Synoptic weather charts are produced every 12 hour – 24 hours and can show the forecast for up to 120 hours.

Pressure pattern
The circular lines on the chart are isobars – isobars join areas of an equal barometric pressure. Isobars are important because they can be used to work out the wind speed and direction, and if a weather system is a high or low pressure system.

Warm air rising produces an area of low pressure while cooler air falling produces and area of high pressure.  As pressure is always trying to become equal, air is always moving from an area of high pressure to an area of low pressure.  The difference between a high and low pressure provides us with a pressure gradient, the greater the gradient the stronger the wind.  Isobars show the barometric pressure so the closer the isobars are together the greater the pressure gradient therefore the stronger the wind.

In the Northern hemisphere wind flows around a high-pressure system in a clockwise direction, and anti clockwise around a low-pressure system allowing us to work out the wind direction.

Warm and cold fronts
As low and high pressures systems interact the different temperature air masses are forced to mix.  The boundary between the different temperatures creates fronts of warm and cold air, and these are known simply as ‘warm’ and ‘cold’ fronts.  Warm fronts are represented by semicircles and is the leading edge of an area of warm air while cold fronts are represented by triangles and is the leading edge of an area of cold air.  It is usual to see an increase in the amount of cloud and rainfall along the front itself.  The area between a warm and cold front is known as the ‘warm sector’ and typically low cloud with patchy light rain or drizzle are seen.

Occluded fronts
In general cold fronts move faster than warm fronts and so they catch up with each over time.  When the fronts merge it creates an ‘occluded’ front.  It is represented by semi circles and triangles along the front.  A weakening front is represented by crosses on the front line.

Black lines with no semi-circles or triangles are known as troughs and mark areas where the air is particularly unstable. This means that the air is quite turbulent or moving around a lot, for example, warm air beneath cold air that wants to rise. Due to the turbulent nature of the air, there is no clearly defined boundary in the same way a front has a boundary between warm and cool air. Showers are often the result of this type of air.

Synoptic Weather Chart

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Weather forecasting and how to read synoptic charts is covered in our RYA Day Skipper Theory course.
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