Reading a Weather Map

Isobars are lines of equal pressure and they indicate the weight of the atmosphere above any point along that line. The weight or force exerted by the atmosphere is measured in terms of millibars (mb) and the average air pressure at the earth's surface is 1013mb. You will notice that the isobars isolate areas of differing pressure and that the centres of these regions are marked with an 'x' with the central pressure labelled in bold type. You can treat this map in much the same way as a topographic map that shows changes in elevation using contour lines. Wherever the pressure is relatively high (atmosphere weighs more) there is a H printed next to the central pressure, and conversely there is a L placed next to a low pressure centre.

1. Just as a ball will move downhill from a peak into a depression, air will move from a high to a low-pressure centre. Moreover, the ball will move fastest where the slope change is greatest, the air will move fastest where the pressure change is greatest. On the map the pressure slope is greatest where the isobars are closest together.
2. Unlike the ball moving down the slope, the air moving in on a spinning earth does not take the most direct path. In the Northern Hemisphere moving air is deflected to its right. As a result, air entering a low-pressure system acquires a cyclonic spin (that is, in the same direction as the earth spins). Thus, Lows are often called depressions or cyclones and Highs are referred to as anticyclones.
3. As air exits Highs, there must be sinking air to replace it. Consequently, anticyclones are associated with cloudless skies and little rain. Cyclones exhibit the opposite behaviour, air converging on the centre causes rising air motion, clouds and precipitation.