Page images
PDF
EPUB

during the years 1853–1862 shows a mean humidity in January of 90, while the liighest monthly mean for the four years at Batavia was 88; and while the lowest of the monthly means at Clifton was 79:1, the lowest at Batavia was 78:9. These figures however represent an immense difference in the quantity of vapour in every cubic foot of air. In January at Clifton, with a temperature of 35° to 40° Fahr., there would be only about 4 to 1. grains of vapour per cubic foot of air, while at Batavia, with a temperature from soo to 90° Fahr., there would be about 20 grains in the same quantity of air. The most important fact however is, that the capacity of air for holding vapour in suspension increases more rapidly than temperature increases, so that a fall of ten degrees at 50° Fabr. will lead to the condensation of about 1) grains of vapour, while a similar fall at 90° Fahr. will set free 0} grains. We can thus understand how it is that the very moderate fall of the thermometer during a tropical night causes beavier dews and a greater amount of sensible moisture than are ever experienced during much greater variations of temperature in the temperate zone.

It is this large quantity of vapour in the equatorial atmosphere that keeps up a genial warmth throughout the night by preventing the radiation into

space of the heat absorbed by the surface soil during the day. That this is really the case is strikingly proved by what occurs in the plains of Northern India, where the daily maximum of heat is far beyond anything experienced near the equator, yet, owing to the extreme dryness of the atmosphere, the clear nights are very cold, radiation being sometimes so rapid that water placed in shallow pans becomes frozen over.

As the heated carth, and everything upon its surface, does not cool so fast when surrounded by moist as by dry air, it follows, that even if the quantity and intensity of the solar rays falling upon two given portions of the earth's surface are exactly equal, yet the sensible and effective heat produced in the two localities may

be

very different according as the atmosphere contains much or little vapour. In the one case the heat is absorbed more rapidly than it can escape by radiation; in the other case it radiates away into space, and is lost, more rapidly than it is being absorbed. In both cases au equilibrium will be arrived at, but in the one case the resulting mean temperature will be much higher than in the other.

Influence of Ilinds on the Temperature of the Equator. --The distance from the northern to the southern tropics being considerably more than three thousand miles, and the area of the intertropical zone more than one-third the whole area of the globe, it becomes hardly possible for any currents of air to reach the equatorial belt without being previously warmed by contact with the earth or ocean, or by mixture with the heated surfacc-air which is found in all intertropical and sub-tropical lands. This warming of the air is renilered more certain and more effective by the circumstance, that all currents of air coming from the north or south have their direction changed owing to the increasing rapidity of the carth's rotational velocity, so that they reach the equator as casterly winds, and thus pass obliquely over a great extent of the heated surface of the globe. The causes that produce the westerly monsoons act in a similar manner, so that on the equator direct north or

suuth winds, except as local land and sea breezes, are almost unknown. The Batavia observations show, that for ten months in the year the average direction of the winil varies only between 5° and 30° from due cast or West, and these are also the strongest winds. In the two months--arch and October--when the winds are northerly, they are very light, and are probably in great part local sea-breezes, which, from the position of Batavia, must come from the north. As a rule, therefore, every current of air at or near the equator has passed obliquely over in immense extent of tropical surface and is thus necessarily a warm wind.

In the north temperate zone, on the other hand, the winds are always cool, anil often of

very low temperature even in the height of summer, due probably to their coming from colder northern regions as easterly winds, or from the upper parts of the atmosphere as westerly winds; and this constant supply of cool air, combined with quick radiation through a dryer atmosphere, carries off the solar heat so rapidly that an equilibrium is only reached at a comparatively low temperature. In the cquatorial zone, on the contrary, the heat accumulates, on account of the absence of any medium of sufficiently low temperature to carry it off rapidly, and it thus soon reaches a point high enough to produce those scorching effects which are so puzzling when the altitude of the sun or the indications of the thermometer are alone considered. Whenever, as is sometimes the case, exceptional cold occurs near the cquator, it can almost always be traced to the influence of currents of air of unusually low temperature. Thus in July near the Aru islands, the writer experienced a strong south-cast wind which

almost neutralisail the usual effects of tropical heat although the weather was bright and sunny. But the wind, coming direct from the southern ocean during its winter without acquiring heat by passing over land, was of an unusually low temperature. Again, Jr. Bates informs us that in the Upper Amazon in the month of May there is a regularly recurring south wind which produces a remarkable lowering of the usual equatorial temperature. But owing to the increased velocity of the carth's surface at the equator a south wind there must have been a south-west wind at its origin, and this would bring it directly from the high chain of the Peruvian Aules during the winter of the southern hemisphere. It is therefore probably a cold mountain wind, and blowing as it does over al continuous forest it has been unable to acquire the usual tropical warmth.

The cause of the striking contrast between the climates of equatorial and temperate lands at times when both are receiving an approximately cqual amount of solar heat may perhaps be made clearer by an illustration. Let us suppose there to be two reservoirs of water, each supplieil by a pipe which pours into it a thousand gallons a day, but which runs only during the daytime, being cut off at night. The reservoirs are both leaky, but while the one loses at the rate of nine hundred gallons in the twenty-four hours the other loses at the rate of eleven hundred gallons in the same time, supposing that both are kept exactly half full and thus suljected to the same uniform water-pressure. If now both are left to be supplied by the above-mentioned pipes the result will be, that in the one which loses by leakage less than it receives the water will rise day by

:

day, till the increased pressure caused the leakage to increase so as exactly to balance the supply ; while in the other the water will sink till the decreasing pressure causes the leakage to decrease so as to balance the supply, when both will remain stationary, the one at a high the other at a low average level, cach rising during the day and sinking again at night. Just the same thing occurs with that great heat-reservoir the earth, whose actual temperature at any spot will depend, not alone upon the quantity of heat it receives, but on the balance between its constantly varying waste and supply. We can thus understand how it is that, although in the months of June and July Scotland in latitude 57° north receives as much sun-heat as Angola or Timor in latitude 10° south, and for a much greater number of hours daily, yet in the latter the mean temperature will be about 80° Falır., with a daily maximum of 90° to 95°, while in the former the mean will be about 60° Fahr. with a daily maximum of 70° or 75°; and, while in Scotland exposure to the full nonday sun produces no unpleasant heat-sensations, a similar exposure in Timor at any time between 9 A.M. and 3 P.M. would blister the skin in a few minutes almost as cffectually as the application of scaldiny water.

Ileat Due to the Condensation of Atmospheric Vapour. - Another cause which tends to keep up a uniform high temperature in the equatorial, as compared with the variable temperatures of the extra-tropical zones, is the large amount of lcat liberated during the condensation of the aqueous vapour of the atmosphere in ihe form of rain and dew. Owing to the frequent ncar approach of the equatorial atmosphere to the

« EelmineJätka »