How to interpret a weather radar

When making decisions on the weather, one of the key tools available to meteorologists, aviators, and the aviation industry in general is the weather radar.  There is a good network of radars that cover the UK and Ireland.  Lets take a look at how to interpret this radar imagery.

Showers

This radar image shows a day with heavy showers and thunderstorms.  Showers and thunderstorm form small “blobs” on the radar image with the higher intensity precipitation showing in the centre of the blob.  Clusters or lines of heavy showers/thunderstorms can also develop in certain weather situations.

radar-showers

If you look at a time series of radar images then it is possible to watch the showers / thunderstorms develop (or dissipate), and identify which direction they are moving in.

radar

On this animation it is possible to see showers / thunderstorms starting to develop close to the area marked ‘Exeter Airport’ that are growing in size and intensity with time, and moving in a south-easterly direction.

It is not always possible to easily differentiate between rain and snow but our meteorologists know the subtle signals to keep an eye out for!

Drizzle

Drizzle will usually appear on the radar image as patchy / speckled pixels in at the lightest intensity.

radar-drizzl

Weather Fronts

A weather front will appear as a larger, more continuous area of rain, often forming a linear band that is typically thousands of miles long.

radar2

On this animation it is possible to see a weather front moving in from the west.  The rain is not of a uniform intensity, and is lightest on the leading edge and heaviest to the west.  The rainfall intensity can also be enhanced as the front crosses larger hills and mountains.

Limitations of radar

A weather radar works by scanning around the horizon using a beam that is angled a few degrees above the horizon.  By measuring how long it takes to get a return signal, and by looking at the strength of this return signal it is possible to turn this into a radar image.  It is not always the absolute truth however due to the way it works. A couple of examples are shown in the diagrams below:

limitations-of-radar

In order to minimise problems like this, the radar images you see on the Met Office website are composite images that use data from our whole network.  This means that the data coming from each radar overlaps with its neighbours and we can do some clever processing to produce the most realistic final map.

The latest radar imagery can be found on the Met Office website and on our range of aviation specific products.

Karen Shorey- Aviation Product Manager

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