A Brief History of Weather Map Analysis

Surface weather maps have been an invaluable part of conveying weather information for some time. In the United States, the U.S. Army Signal Service, a predecessor to today’s National Weather Service, began publishing surface map analyses in 1871. These maps contained information on wind, weather, sky cover, temperature, and air pressure. Analysis of the maps for pressure revealed the Highs and Lows that we still use today. A lot of information for individual weather stations is contained in what is called a station plot. Maps may contain plots for hundreds of stations. Generally, a station plot consists minimally of: A circle showing the station location. Cloud cover which is denoted by how the circle is filled. In this case, the sky is one-quarter covered by clouds. A line emanating from the circle, which indicates which direction the wind is blowing from. In this case, the wind is from the southeast. Barbs indicating the wind speed. A short barb equals five knots, a long barb ten. In this example, the wind is 25 knots. To the upper left of the circle is the temperature. The lower left is the dew point, which is a measure of moisture in the air. To the upper right is coded air pressure. In our example, the nine-nine-five means the air pressure is 999.5 millibars. Directly to the left, is a symbol for weather. There are many symbols that may be used here. In our case, the three stars indicate moderate snow. When these were hand plotted, they had to fit all this information into an area the size of dime. A staple of today’s weather maps are fronts. This was not always the case. Cold fronts are the blue line with triangles on it, and warm fronts are the red lines with half circles on it. Fronts mark boundaries between air masses with differing characteristics, often between cold air and warm. The first preconception of fronts was brought forth by a fellow named Elias Loomis in 1841, but the theory of fronts didn’t really gain traction until the early 1900s when a group of Norwegians put forth a model of cyclone structure called the Norwegian model. Weather maps in the United States didn’t begin to take on their modern form until August 1st 1941, when the Weather Bureau, that’s the direct predecessor to today’s National Weather Service, began to include fronts in their daily weather maps. Following features like fronts and highs and lows through time allows you to see how weather systems move and evolve. Map analyses have traditionally been done by hand. An analyst will draw in lines of constant pressure, kind of a game of connect–the-dots. The result is something similar to a topographic map showing elevation, except that it is high and low pressure that is revealed. Using the pressure pattern for guidance, but also considering things like wind direction and speed, temperature, dew point, and pressure changes with time, fronts are then drawn in. Other data such as satellite imagery and radar data may aid in finding the exact position of fronts. Most maps you see today are analyzed by computers. But there is still a place for hand analyzed maps with forecasters, as it gives them a feel for the weather that may be missing from just a quick look at the computer generated maps.