Step 1. Gather basic symbol definitions 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 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
Step 3. Download current maps and high seas text reports 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 |