Curiosities of Heat

by Lyman Beecher Tefft

“This subject is so well understood,” said Mr. Wilton, “that I need not spend time in explaining it. Every boy knows the difference between setting his wet slate before the fire to dry so that the heat will fall squarely and perpendicularly upon it and placing it edgewise to the fire. Upon the torrid zone the sun shines perpendicularly, upon the temperate zones obliquely, and upon the frigid zones still more obliquely, and during a part of the year the sun is entirely hidden. In proportion as the rays of heat fall obliquely, any given amount of heat is spread, so to speak, over a larger surface, and the larger the space over which it is spread, the feebler it becomes. What is another cause of inequality of temperature?” No one answered. “Samuel, what is the cause of day and night?”

“The turning of the earth upon its axis.”

“And the rotation of the earth upon its axis,” continued Mr. Wilton, “brings not only an alternation of light and darkness, but also of heat and cold. The heat of the sun is withdrawn along with the light. The heat of the sun is not withdrawn from the earth, but one-half of the earth’s surface is constantly turned away from its influence. This must produce a daily change of temperature. This diurnal fluctuation of temperature may be very small or it may amount to seventy or eighty degrees. Samuel, what is a third cause of unequal temperature?”

“The inclined position of the earth’s axis and the revolution of the earth around the sun cause the change of seasons.”

“If it were not for this, the earth would still have her zones of seasons; a part of the earth would have endless summer, a part endless spring, and the rest unbroken winter, but the alternation of seasons at the same place would be unknown. The axis of the earth is now inclined about twenty-three degrees, twenty-seven minutes, twenty-three seconds to the plane of the earth’s orbit, and as this axis maintains constantly the same position, being parallel in one part of the earth’s orbit to its position in any other part of its orbit, during one part of the year the north pole is turned twenty-three and a half degrees toward the sun, while in the opposite part of the year the south pole is in like manner brought into the light and heat. This causes the sun to appear to move to and fro, north and south, twenty-three degrees, twenty-seven minutes, and twenty-three seconds from the equator in either direction. The tropics, or turning-places, mark the limits of the sun’s northern and southern journey. Everywhere between the tropics the sun, at some period of the year, passes through the zenith, that is, exactly overhead at noon. North and south of the tropics the sun seems to rise higher in summer and to sink lower in winter. In summer the sun at midday is about forty-seven degrees nearer the zenith than in winter. Within the polar circles, which are the same distance from the poles as the tropics from the equator, the heat of the sun is entirely withdrawn during a portion of the year, and during another portion of about equal length the sun does not set. The extremes of temperature, caused by the inclination of the earth’s axis and its revolution around the sun, are very great. In the northern part of Minnesota, the temperature rises in summer to one hundred degrees, and in winter sinks to fifty degrees below zero, giving thus an alternation of one hundred and fifty degrees.

“In this connection you may also remember that the sun is nearer the earth in one part of its orbit than in another part. This difference amounts to about 3,000,000 miles. The sun also remains eight days longer north of the equator than south of it. Our summer, therefore, is eight days longer than the summer of the southern hemispheres, and our winters are correspondingly shorter. These differences tend, however, to balance each other, for while the southern summer is shorter, the sun at that time is nearer, and while our summer is longer, the sun is more distant. Peter, you may explain to us the effect upon temperature caused by the division of the earth’s surface into land and water.”

“I learned while studying physical geography that the temperature is more even upon the sea than upon the land. But why, I do not know.”

“The smooth surface of the sea reflects heat better than the rough land: for this reason, a larger proportion of the heat which falls upon the sea is not absorbed, but reflected and lost, so far as the temperature of this world is concerned. Water is also a very poor conductor of heat, and has withal a very high specific heat. For these reasons the sea receives and parts with heat more slowly than the land, and its absorption or radiation causes a smaller variation of temperature. The result is, therefore, that the sea is cooler in summer and warmer in winter than the land, and the average ocean temperature is lower than the mean continental temperature. The land receives heat more readily and parts with it more rapidly; the fluctuations of temperature must therefore be greater. Hence, the interiors of the continents have much greater extremes of temperature than the sea-board. But of the influence of water in equalizing temperature I shall have occasion to speak again more at length, and will pass it by for the present. What effect, Peter, has the unevenness of the earth’s surface upon temperature?”

“The higher we ascend upon mountains, the colder we find it.”

“That is, Peter, the greater the elevation of any place or country above the sea level, the lower the temperature. Almost the whole surface of the earth is an alternation of mountain and hill, valley and plain. One continent has a very much greater mean elevation than another. One region or tract of country lies sloping toward the sun, another is inclined from it. The effect in the one case is the same as if the sun were brought more nearly overhead; in the other case, the sun is depressed toward the horizon. It is all the same as if the region of country were brought nearer the equator or removed farther from it. The effects of the curvature of the earth are obviated or exaggerated. Do clouds tend to produce inequalities of temperature?”