Reading a Weather Map

Every day meteorological measurements (pressure, wind, temperature and humidity among others) are made at hundreds of locations around the world. Each station makes recordings at the same time and, after checks are made for errors, its observations enter a global weather database. These observations are plotted on maps that are interpreted in terms of weather (for example fronts, precipitation and clouds). In addition, they will be used as 'initial conditions' in computer models to predict the weather over the next few days. Here I have selected a series of weather maps to illustrate the how weather information is mapped and interpreted. The case study I have chosen relates to a severe storm that produced flooding over south and east Ireland (and over southern England) on November 6th and 7th, 2000.

Once all weather information has been collected at the same time a 'snap-shot' of the state of the atmosphere can be produced. The map shown shows the atmosphere in the western north Atlantic at 6am (Greenwich time). You will notice that there are no weather stations shown on this map - in fact, this weather map has been interpreted by a computer and meteorologists from the weather station information. In addition to the outlines of the continents and the lines of latitude and longitude, there are two other sets of lines: isobars and fronts. Both sets of lines are critical to understanding the information stored on this map. The isobars identify major circulation systems in the atmosphere and the fronts are lines separating different types of air.