RAIN AND ITS ORIGIN IN THUNDERSTORMS. 407 



thus the ascending currents which produce stones as large as oranges and melons 

 must be enormous. 



It would therefore appear that those disturbances in the atmosphere which are 

 accompanied by the greatest amount of electrical discharge are also accompanied by 

 violent ascending currents, much larger in all cases than the 8 metres a second 

 necessary to hold water in suspension, and so it cannot be considered to be an 

 unwarrantable assumption that in all thunderstorms a velocity of 8 metres a second 

 occurs. 



A strong vertical current in the atmosphere must have a form something like that 

 of an hour-glass, having a comparatively large cross-section at the bottom where 

 horizontal currents are feeding into it, and spreading out at the top to allow of the 

 escape of the air after ascension. For simplicity in the following discussion we will 

 imagine an ascending current to consist of three parts : (a) a base in which the cross- 

 section is large and the vertical velocities are small ; (6) a column of ascending air of 

 which the cross-section is comparatively small and the vertical velocities are large 

 and more or less constant throughout ; (<) a cap or crown in which the air rapidly 

 spreads out in all directions so that the vertical velocities are very small a short 

 distance above the head of the column. If the air in the base is saturated, then as it 

 rises through the column it will have its temperature reduced at the rate of 

 approximately 0'5 C. for eacli 100 metres of ascent, and there will be considerable 

 condensation of water, which will form drops and tend to fall. If, however, the 

 vertical velocity within the column is 8 metres a second or over" no water can fall, but 

 it will all be carried upwards until it readies the top of the column, where the 

 vertical velocities diminish. Here the water will accumulate in the form of drops 

 which will continually be going through the cycle of growing to 5'5 nun. in diameter 

 and then being broken up into a number of small drops, each of which will grow 

 again. A rough approximation of the rate at which the water accumulates can be 

 formed by assuming certain simple conditions. Thus let us assume that the height 

 of the column is 2000 metres, so that the air which enters the base will be cooled 

 10 C. during the ascent, and let the initial temperature be 15 C. Then by the time 

 the air reaches the top approximately 6 grammes of water will have been precipitated 

 within each cubic metre of air, and if all this accumulates at the top of the column,* 

 6X8, or 48, grammes of water will have collected over every square metre of the 

 column in one second, that is, in 10 minutes the water accumulated would be 

 equivalent to a layer of water 2'9 cm. deep, or if the water is in the form of drops 

 there would be at the end of 10 minutes 36 drops, each of the maximum size of 

 5'5 cm. diameter, over every square centimetre of the cross-section. Thus if the 



* This of course is only assumed for purposes of a rough approximation ; it is not intended to assert 

 that all the water carried up by the current would accumulate at the top of the column, but as the 

 accumulation which would result from rain falling from above has been neglected, the calculation will give 

 some idea of the magnitudes with which we are concerned. 



