The motion of air in the upper altitudes may lead to an accumulation in some
regions and a loss of molecules from other regions. The former result in the formation
of regions of high pressure (‘highs’ or anticyclones) and the latter result in regions of
low pressure (‘lows’, depressions, or cyclones). On a weather map, such as that shown
in Fig. 1.11, the lines of constant pressure marked on it are called isobars. Elongated
regions of high and low pressure are known, respectively, as ridges and troughs.