SNOW AND ICE 43 



becomes more and more ice-like, on the house roofs, where the 

 slope is steep, the powdery snow sweeps down in an avalanche. 



But our snow does not last long. We soon see it disappear, 

 except for isolated patches in sheltered situations. As it melts 

 we note that the most effective melting agent is warm rain. We 

 compare this with our own observations in the case of the rain 

 gauge, where we found that the quickest method of turning snow 

 into water was to add to it a small quantity of warm water. 

 Knowing as we already do that the air on hills is colder than in the 

 valleys, we are not surprised to see the snow linger there when it has 

 disappeared below. If the locality gives a view of distant hills of 

 some size, we shall see them still covered with a spotless mantle 

 while the ground near us has returned to its brown tint. 



As a general rule in Britain, at least near the populous parts, 

 the hills are of soft and rounded contours, and are, therefore, in 

 winter snowstorms more or less continuously shrouded in white. 

 They are never very high, and with some exceptions their covering 

 of snow is of relatively short duration. But in some countries 

 the hills are very much higher and much rockier. Some countries 

 also are much colder than ours. While the country is under the 

 influence of a snowstorm, we should then take the opportunity of 

 speaking of other countries where the snow is permanent, either 

 on the high hills only, or right down to the low ground. What 

 happens to snow when it lies for a long time ? We have already 

 in the playground made observations which help us to answer this 

 question. If it falls on a steep slope, as the layer gets thicker, and 

 as the sun comes out when the snowfall ceases, the whole covering 

 starts to move, and rushes down the slope with increasing force as 

 an avalanche. We have not only watched this happening on roofs, 

 but we have noticed that when greenhouses are placed against a 

 house there is always some arrangement to obviate the risk of such 

 an avalanche crashing into the glass roof. 



But if snow lies long on a surface which is not very steep, what 

 happens ? As it lies it freezes and thaws, it has the air squeezed 

 out of it by the weight of the upper layers, it becomes more and 

 more ice-like, more solid, more firm. Ice is transparent, snow is 

 pure white, but our observations have shown us that as the air is 

 squeezed out of snow it becomes more and more transparent, 



