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How to Read Weather Maps

We must start by saying that the very best way to learn weather maps (to our knowledge) is our own Starpath Weather Trainer software. It covers all aspects of each type of map, both for interpreting the map and applying what you see tactically.

For the here and now, however, the following is a review of how you can learn the symbols and conventions used in preparing surface analysis weather maps, and from these determine what the wind speed and direction should be. For marine weather, it is the wind that matters most. It matters for sailors because wind is their engine, but it matters to all craft since the wind makes the waves and the waves are a potential threat to all vessels underway. Most of this info also applies to forecast maps.

Step 1. Gather basic symbol definitions
A first step would be to take a look at the Ocean Prediction Center's Radiofacsimile Charts User's Guide. (Here is a pdf version of that webpage that may be more convenient—right click and save it is 2 MB.) This document describes most of the products that are broadcast. We will want to go into more detail in some respects, but this is an excellent overview and introduction..

National Weather Service (NWS) maps these days show "station model" reports of actual observations, which include abbreviated weather symbols, both are discussed briefly in the OPC Guide above, but they are covered in great detail in Weather Trainer program, in the Resources Book, item 22 Symbols.

A surface weather map is just a topographic map of the air distribution. The amount of air at a given place is measured by its pressure, usually in millibars, abbreviated mb. Where there is a lot of air, the pressure is high and the region is called a High. It will look like a mountain on a topographic chart. Where there is a paucity of air, there will be a hole or indentation in the distribution corresponding to a region of low pressure called a Low.

The contour lines of equal pressure — corresponding to elevation lines on a topo map — are called isobars. The usual convention is to draw these lines every 4 mb, and to label them with the last two digits of the pressure. A 1024 mb isobar would be labeled 24. A 980 mb isobar would be marked 80. The central pressure of a High or Low is usually underlined.

In the Northern Hemisphere, wind flows clockwise around Highs and slightly out of the High, and counterclockwise around Lows and slightly into them. The closer the isobars the stronger the winds.

In the next section you will see specific examples of these things.

Step 2. Compare surface analysis maps with the corresponding text reports
The best way to learn what a weather map means is to let a professional describe it to you. This is easily done, over and over again, by simply comparing a surface analysis map with the corresponding high seas text report that covers the region at that time. These are produced four times a day, around the world. By studying these, you will pretty soon learn what the maps mean. This is a good exercise for training, and a good exercise underway. You will always learn something about the map from the text report, and something about the text report from the map.

To see how this works, we have built in a sample map and text report that you can practice with. Later in Step 3 we explain how to do this for actual current maps around the world.

Go to
Map Reading Exercise .

Step 3. Download current maps and high seas text reports
The main reference for both the maps and the text reports is the Ocean Prediction Center. Here you can find maps for the Atlantic and Pacific, and through their included links, on around the world.

To carry on with live data, you can select out individual maps and the corresponding high seas reports from the data at the Ocean Prediction Center, or use ftpmail to get them directly (see our Discussion group on marine weather).

If you carry out this comparison as explained in the instructions for several different maps in varying conditions, you will indeed develop a practical working knowledge of weather maps. Naturally, we will be very pleased to hear from you on how this approach worked for you.

— David Burch


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