Fig. 14. Velocity of Summer and Winter Winds in Metres per Second
Fig. 14. Velocity of Summer and Winter Winds in Metres per Second
When the weather has been clear and moderately warm for two or more days, and the winds are light and variable, there may occur on the third day a moderate wind from the east, known as the sea-breeze. This occurs during anticyclonic conditions. Preceding the sea-breeze, the winds are very light, there are no clouds, and the temperature rises rapidly during the forenoon. This heating is due to a slow dynamic compression as the air slowly descends and the surface air does not flow away. There is no cooling because there is no evaporation due to air movement. The absolute humidity is low, often less than ten grams per cubic metre. Cumulus clouds do not form because there is no uplift of the lower air and consequently no chance for condensation of whatever water vapor may be present. No thunder-heads form notwithstanding theheat. The heat, while dry, is nevertheless extremely trying to men and animals. Relief comes in the early hours of the afternoon by the arrival of the sea-breeze.
The usual explanation of the origin of the sea-breeze is that the land being excessively warm, the air over a relatively cool ocean moves in to take the place of the warm and therefore lighter air, which it is assumed has risen. Unfortunately for this explanation, the air over the land hasnotrisen; but on the contrary is falling slowly. Again the sea-breeze does not begin at the place where the temperature contrast is greatest, namely, just inside the shore line; but comes in from the sea. Nor does the flow extend far inland, which would be the case if there were up-rising currents. The sea-breeze is very shallow, generally not extending upward more than 200 metres, and often not above 100 metres. Itdoes not penetrate far inland, as a rule not more than 15 kilometres, 9 miles.
The sea-breeze is probably caused by a slow descent of dry, warm air, on an incline sloping from northeast to southwest. As it reaches the surface it is twisted more to the right; that is, becomes an east wind. It carries inland with it some of the air over the ocean which is much cooler and heavily saturated.
There are certain days, more noticeable in summer than at other times, when the air is heavily laden with water vapor; and there is little or no cooling of the body due to evaporation. We perspire freely but as the sweat does not evaporate, there is a constantly increasing amount of water on the skin.
Fig. 15. Blue Hill Observatory During Ice Storm, November 29-30, 1922
Fig. 15. Blue Hill Observatory During Ice Storm, November 29-30, 1922
It is not altogether a question of temperature, for another day may have as high or even higher temperature. It is essentially a matter of ventilation. On muggy days we are somewhat in the condition of the unfortunate prisoners in the Black Hole at Calcutta. They did not die by poisoning, as has generally been accepted, that is, lack of sufficient oxygen and an excess of carbon dioxide; but because they were unable to keep the skin sufficiently cool. There was no ventilation; no movement of the air and the body became over-heated and exhaustion followed. No matter howmuch water there may be on the skin if the surrounding space is saturated, one feels oppressed. A vigorous fanning of the air helps evaporation and cools us. That is why a brisk northwest wind routs a muggy condition.
John Hay wrote of such days spent in Spain. We who live in a land where the winds are more boisterous, occasionally experience what we call a perfect day. Such days have easterly winds of two metres per second or less than five miles an hour. The temperature is midway between freezing and normal body temperature or about 70° F. The relative humidity is approximately 75% and the absolute humidity 12 grams per cubic metre. The table on page 72 explains the paucity of perfect days. The gusty, boisterous winds, Skiron and Zephyros, blow too frequently.
Perhaps certain of our national characteristics may be traceable to this flow of the air and our climatic environment.