CIRCULATION AND CONVECTION IN GASES. 57 



Water- Vapour aids Convection-Currents. Convection arising 

 from heating of the lower layers of the atmosphere is greatly aided by 

 the increased amount of water-vapour which the air takes up, the vapour 

 being much lighter than the air which it displaces. As an example of 

 the joint-effect of water-vapour and expansion through heat to produce 

 convection, we may probably instance the formation of a thundercloud. 

 When a storm is first gathering, an observer a short distance away may 

 see enormous piled-up masses of cloud rising far into the upper regions. 

 These show that a great volume of light, damp air has risen by 

 convection, expanding in the ascent, and therefore cooling through the 

 work done in expansion until the temperature of cloud-deposition is 

 reached. At the same time, it frequently happens that there is a 

 surface indraught towards the storm area from the surrounding region, 

 the upper strata with the clouds, moving in the direction opposite to 

 that of the strata immediately below them. 



These examples will prepare us for the general statement that 

 winds are convection-currents in the atmosphere due to local diminu- 

 tions of air density, either through heat, or increased evaporation, or 

 both. Since the weather depends so largely on the direction of the 

 wind, it is, of course, of the utmost practical importance that the nature 

 and origin of all atmospheric movements should be investigated and 

 explained as thoroughly as possible. But though the above general 

 statements may be made with confidence, meteorologists have not yet 

 succeeded in discovering, except in a few cases, how particular winds 

 arise, i.e. what share in their origin is to be assigned to heat, and what 

 share to evaporation. They are still further from foretelling what winds 

 will be formed from a given distribution of the atmosphere with known 

 temperature and amount of water-vapour. The problem is one of 

 enormous difficulty, which will probably only be fully solved in the 

 distant future. 



Weather Forecasting in the Case of Cyclones. At present, the 

 art of weather-forecasting depends largely on the fact that a given distri- 

 bution of weather travels onwards irf a definite course, and that certain 

 definite types of weather have been recognised and their movements 

 studied. As an example, we may give a short account of the simplest 

 type, that of the Cyclone, and the mode in which English weather is 

 forecast when a cyclone is approaching our shores. 



There are about fifty meteorological stations scattered over the north- 

 western part of Europe, at which meteorological observations are taken 

 at stated times every day. These include the readings of the barometer, 

 the direction and force of the wind, the kind of weather, and the 

 temperature. The results are at once telegraphed to London. The 

 barometer readings are marked on a map containing all the stations, 

 each reading at its own station. Curves are then drawn on the map, 

 joining all points where the pressure is the same, one curve for every 

 fifth of an inch. There will thus be a curve for 29 inches, another for 

 29'2, another for 29*4, and so on. It generally happens that these 

 curves do not pass exactly through the stations, but their position may 

 be ascertained from the known readings. If, for example, the reading 

 at London is 29 '25, and at Dover 29'15, it is assumed that the 29'2 line 

 passes about halfway between these places. The map with these curves, 



