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J. Perelman
"Entertaining physics". Book 1.
Chapter 6. Thermal phenomena


Have you ever wonder how ice formed icicles, which we often see hanging from the roof?

In what weather formed icicles: thaw or frost? Beli in the thaw, how could freeze water at a temperature above zero? If cold, it could take from the water on the roof?

You can see that the task is not as simple as it seems at first. That could be formed ice icicles, should at the same time to have two temperatures: for melting is above zero and freezing below zero.

Actually it is: the snow on the slope of the roof melts, because the sunlight heats it to a temperature above zero, and dripping water from the roof edge freeze, because here the temperature is below zero. (Of course, we are not talking about the case of the formation of icicles, which is caused by heat heated under the roof of the room.)

Imagine the following scene. Clear day; frost just 1 to 2 degrees. The sun pours all his rays; however, these oblique rays do not heat the earth so that the snow could melt. But on the slope of the roof, facing the Sun's rays are not hollow, as on the ground, and the steeper angle, closer to direct. It is known that the light and heat rays is greater, the greater the angle be-rays with the plane on which they fall. (The action of rays is proportional to the sine of this angle ; for the case shown in Fig. 87, the snow on the roof receives heat 2.5 times more than an equal area of snow on a horizontal surface, because the sine of 60° more sinus 20° 2.5 times.) That's why the roof is heated stronger and the snow can melt. The thawed water flows and drops hanging from the roof edge. But under the roof temperature is below zero, and drop, cooled to the same evaporation, freeze. On the frozen drop flows the following, also freezing, then the third drop, and so on; gradually formed a small ice hummock. Another time when the same weather these ice flows more elongated, and the result is the formation of icicles growing like limestone stalactites in caves. So there are icicles on the roofs of sheds and generally unheated spaces.

The Sun's rays warm the sloping roof is stronger than the horizontal surface (the numbers indicate the magnitude of the angles).

The same reason causes our eyes and more grandiose effects: because of the difference in climatic zones and seasons is due largely But not entirely: the other important reason is the different duration of the day, i.e. the period of time during which the Sun warms the Earth. Both reasons, however, due to one astronomical fact: the tilt of earth's axis to the plane of rotation of the Earth around the Sun] by changing the angle of incidence of sun rays. The sun from us winter is almost the same distance as in summer; it is equally removed from the poles and the equator (the difference in distance is so insignificant that it does not matter). But the tilt of the sun's rays to the Earth's surface near the equator is greater than at the poles; in the summer this angle is higher than in winter. This causes noticeable differences in the temperature of the day and, therefore, the life of all nature.

Entertaining physics J. Perelman


System Orphus


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