Did you ever seek relief from the heat of a midsummer sun by going to the beach or shore of some large body of water? If you have, you no doubt enjoyed the cool breezes that swept in from the water during the middle of the day when such cool winds are most welcome. You weren't much interested in the reason for these cool breezes as long as they continued to bring relief from the heat, but the same sun whose warmth the wind tempered was itself responsible for the breezes that worked to your advantage. Here is the "why" of the natural mechanics of this feature. Air is heated by radiation from land or reflection from water, and moves from place to place in accord with its attempt to adjust the various differences in temperature brought about by unequal heating -- and so we have wind. Water absorbs heat slowly and is equally reluctant to release it. Land heats quickly and as rapidly cools when the source of heat is removed. For that reason, the land bordering large bodies of water is warmed quickly by the sun while the lake or sea increases in temperature relatively slowly. Consequently the air above the land becomes uncomfortably warm in contrast to the cooler air above the water. The warm air over the land rises and the cool air over the water sweeps in to replace it. The result is a breeze blowing landward as long as the air above the earth is warmed and rising. At night the opposite may be true. The land cools more rapidly than the water. The air above the land becomes cooler than that over the sea and moves out over the water producing a breeze taking the opposite direction from that blowing during the daytime.
Thus the different effect of the sun's energy upon land, and water, and air accounts for many things that are well known to us. Areas far removed from large bodies of water and their modifying effect upon temperature have greater climatic extremes. The land heats up excessively during the day and cools rapidly at night. The effect of large bodies of water upon climate is profound, for the coldest regions of the world are not along the coast near either of the poles but inland where the moderating influence of the sea cannot be felt.
But our weather is dependent upon still another factor - the ability of the air to absorb, hold, and transport moisture. If the atmosphere were not so constituted, much of the land-mass of our earth would be destitute of life, for without water plants and animals cannot live. The great bodies of water would lie dormant, their moisture unavailable because of lack of means of transporting it to dry localities. A trip through an arid region where irrigation has partially overcome the natural shortage of water will effectively point out the dependence of living things upon moisture. In the case of our weather, we have the oceans and similar bodies of water as "reservoirs"; the prevailing winds are the medium by which this moisture is transported; and the "valves" or "floodgates" by which the flow of moisture is regulated are the temperature differences of the atmosphere as it ascends, descends, and moves about from place to place.
The ability of the atmosphere to do these things accounts for rain, fog, snow, frost, dew, clouds, and many other phenomena. Many times, no doubt, you have made the characteristic remark, "It's not the heat - it's the humidity". What you really mean is that the air about you is carrying a heavy charge of water vapor, and can absorb so little that the natural process of body perspiration - which is Nature's cooling system for the human body - is hindered. The air feels thick, sticky, or "muggy", and you are uncomfortable. In addition to the absorption of moisture from bodies of water, the air picks it up from land, and plants, and animals as well. Air that contains very little water as it sweeps over the earth seeks moisture from the area over which it passes, drying the soil and the living things thereon. Desert plants are constructed to resist dehydration, and thus are able to survive under such conditions. It is because of these facts that we have our deserts and our "great plains", almost entirely devoid of trees. The prevailing winds that sweep eastward we find dry regions for the winds, sweeping inland, are "bled" of their moisture in passing over the mountains. Thus, east of the Cascades and the Sierra Nevadas arid or semi-arid conditions prevail.
So the weather, although it is often exasperating, is responsible for many of our blessings. More about this in the next issue of NATURE NOTES. (C.F.B.)
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