[Illustration: Fig. 10.--Diagram to illustrate a cyclone travelling towards the east. The two concentric circles represent isobars, that is, they are lines drawn through points where the barometer registers the same (low) pressure. Into the area of low pressure so formed the winds blow strongly in the direction known as counterclockwise, and are represented by the arrows whose double barbs signify their strength. It will be noted that in the rear of the cyclone the winds are northerly. They thus chill the air here, and by chilling it _raise_ the pressure. The winds to the front of the cyclone are warm because southerly, they therefore tend to _lower_ the pressure here by warming the air, and the result is that the isobars tend to be displaced towards the east, and at the same time become deformed. In other words, the cyclone moves to the east.]
We may take British weather, which has become a proverb on account of its variableness, as a text for a brief discussion of the subject.
The daily variations in our weather, as all who have read weather reports know, are chiefly determined by the movements of areas of low pressure or cyclones, which mostly come to us from the Atlantic, and continue eastwards past us, often towards the Baltic. We have already noted the occurrence of what we have called a permanent area of low pressure in the North Atlantic, but this "permanent area" in point of fact is due chiefly to the constant passage here of cyclones, or moving areas of low pressure.
The causes of the eastward displacement of these depressions are interesting. One cause is the general eastward movement of the atmosphere in this region, produced in the fashion already described. This movement necessarily raises the pressure to the west of the depression, owing to the influx of fresh air, while the onward movement of the air in front of the depression lowers the pressure there, and so produces displacement. Again, the air is sucked into a depression in the direction opposite to the hands of a clock, and a moment's reflection will show that this means that the winds to the east of the depression are southerly and those to the west of it northerly. The warm southerly winds in front tend to lower the pressure by warming the air, while the cold northerly winds behind it raise the pressure by cooling the air. This again produces a displacement of the depression towards the east (see fig. 10).