Paul Kahler, age 14, of Whitesboro, New York, for his question:

Why is it cooler in the mountains?

This is a popular summertime question, when sizzling city folk turn their minds to cooler locations. In the California valleys, people may look longingly at patches of winter snow still clinging to the distant mountains. This seems odd because those lofty peaks are closer to the sun and one would expect them to get more of its heat. However, the behavior of the sun's radiation is very complex. Strange to say, it is directly related to those glorious pictures of the earth from outer space.

Seen from a spacecraft, our home in the heavens is a hazy ball, glowing with moving patches of blues and greens and trailing clouds of wispy white. At least, this is what the sunlit side of the globe looks like. And all this bright beauty is actually reflected radiation that comes from the sun. The solar radiation reaching our upper atmosphere is estimated to equal about 23 trillion horsepower. But almost half of this radiant energy is reflected back into space. It does not reach down to warm the earth  ¬but it does add that hazy glow of gorgeous colors seen from outer space.

Solar radiation is electromagnetic energy, traveling in a blend of assorted wavelengths. The portion that pierces the upper atmosphere runs into various obstacles as it zooms down to the surface of the planet. Different objects absorb and reflect its assorted wavelengths in different amounts. Clouds of water droplets in the atmosphere may reflect back as much as 75 per cent of the light waves. About 15 per cent of the solar radiation is absorbed by fragments of atmospheric dust and water vapor, ozone and carbon dioxide. Hardly any at all is absorbed by the bulk of airy gases. Hence, most of it reaches the surface, where certain wavelengths are converted to heat.

These wavelengths are absorbed by hot pavements and sandy deserts, by buildings and other surfaces basking in the sunshine. The molecules of these substances use this solar energy to gain speed    and heat, of course, is generated by molecular motion. And, heat spreads by radiation. Warm objects on the surface of the earth spread their warmth to the air around them and the lower level of the atmosphere gets hot. But only a portion of this heat is carried aloft.

This simplified explanation of how the earth copes with solar radiation indicates why mountain tops stay cooler than the plains and valleys, even though they are closer to the sun.. The atmosphere lets through the light waves, but it converts almost no wavelengths into heat. The air gets almost all its warmth from heat radiated by the land and sea and not much of this heat spreads high enough to warm the tops of the mountains.

Some surface heat is carried aloft by winds and convection currents. But this light, warm air is expanding    and the expanding process causes it to lose its heat and become cooler. Even a hot high wind has little warmth to share with a lofty mountain peak. This is why many high mountains in the tropics never get a chance to melt their snowy crowns. In our more temperate zones, the mountains stay cooler when the summer sun scorches the lower plains and valleys.