Page images
[merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small]

Causes of the Uniform Iligh Temperature near the Equator.--- It is popularly supposed that the uniform high temperature of the tropics is sufficiently explained by the greater altitude, and therefore greater heatingpower, of the midday sun ; but a little consideration will show that tiis alone by no means accounts for the phenomenon. The island of Java is situated in from six and a half to cight and a half degrees of south latitude, and in the month of June the sun's altitude at noon will not be more than from 58° to 60'. In the same month at London, which is fifty-two and a half degrees of north latitude, the sun's noonday altitude is 62°. But besides this difference of altitude in favour of London there is a still more important difference; for in Java the day is only about eleven and a half hours long in the month of June, while at London it is sixteen hours long, so that the total amount of sun-beat received by the earth must be then very much greater at London thau at Batavia. Yet at the former place the mean temperature of the day and night is under 60° Falır., while in the latter place it is soo Falır., the daily maximum being on the average in the one case about 68o and in the other about S9o.

Neither does the temperature at the same place depend upon the height of the sun at noon ; for at Batavia it is nearly vertical during October and February, but these are far from being the hottest months, which are May, June, and September ; while December, January, and February are the coldest months, although then the sun attains nearly its greatest altitude. It is evident, therefore, that a difference of 30° in thic altitude of the sun at noon has no apparent influence in raising the temperature of a place near the cquator, and we must

therefore conclude that other agencies are at work which often completely neutralise the effect which increased altitude must undoubtedly exert.

There is another important difference between the temperate and tropical zones, in the direct heating effect of the sun's rays independently of altitude. In England the noonday sun in the month of June rarely inconveniences us or produces any burning of the skin ; while in the tropics, at almost any hour of the day, and when the sun has an clevation of only 40° or 50°, exposure to to it for a few minutes will scorch a European so that the skin turns red, becomes painful, and often blisters or peels off. Almost every visitor to the tropics suffers from incautious exposure of the neck, the leg, or some other part of the body to the sun's rays, which there possess a power as new, as it is at first sight inexplicable, for it is not accompanied by any extraordinary increase in the temperature of the air.

These very different effects, produced by the same amount of sun-hcat poured upon the carth in different latitudes is due to a combination of causes. The most important of these are, probably,--the constant high temperature of the soil and of the surface-waters of the ocean,—the great amount of aqueous vapour in the atmosphere,-the great extent of the intertropical regions which cause the winds that reach the cquatorial zone to be always warm,—and the latent heat given out during the formation of rain and dew. We will briefly consider the manner in which each of these causes contributes to the degree and the uniformity of the equatorial temperature,

Influence of the Ileat of the Soil.---It is well known that at a very moderate depth the soil maintains a uniform temperature during the twenty-four hours ; while at a greater depth even the annual inequalities disappear, and a uniform temperature, which is almost exactly the mean temperature of the locality, is constantly maintained throughout the year. The deptlı at which this uniform temperature is reached is greater as the annual range of temperature is greater, so that it is least near the equator, and greatest in localities near the arctic circle where the greatest difference between summer and winter temperature prevails. In the vicinity of the equatur, where the annual range of the thermometer is so small as we have seen that it is at Batavia, the mean temperature of about soo Fahr. is reached at a depth of four or five feet. The surplus heat received during the day is therefore conducted downwards very slowly, the surface soil becomes greatly superheated, and a large portion of this heat is given out at night and thus keeps up the high temperature of the air when the sun las ceased to warm the carth. In the temperate zones, on the other hand, the stratum of uniform carth-temperature lies very cleep. At Geneva it is not less than from thirty to forty feet, and with us it is probably fifty or sixty fect, and the temperature found there is nearly forty degrees lower than at the equator. This great body of cool carth absorbs a large portion of the surface heat during the summer, and conducts it downwards with comparative rapidity, and it is only late in the year (in July and August) when the upper layers of the soil have accumulated a surplus store of solar heat that a sufficient quantity is radiated at

[ocr errors]

night to keep up a high temperature in the absence of the

At the equator, on the other hand, tliis radiation is always going on, and earth-licat is one of the most important of the agencies which tend to equalise the cquatorial climate.

Influence of the aqueous Vapour of the atmosphere. -The aqueous vapour which is always present in considerable quantities in the atmosphere, exhibits a singular and very important relation to solar and terrestrial heat. The rays of the sun pass through it unobstructed to the earth; but the warmth given off by the heated carth is very largely absorbed by it, thus raising the temperature of the air ; iind as it is the lower strata of air which contain most vapour these act as a blanket to the carth, preventing it from losing heat at niylit by radiation into space. During a large part of the year the air in the equatorial zone is nearly saturated withi vapour, so that, notwithstanding the heat, salt and sugar become liquid, and all articles of iron get thickly coated with rust. Complete saturation being represented by 100, the daily average of greatest humidity at Batavia reaches 96 in January and 92 in December. In January, which is the dampest month, the

range of humidity is small (77 to 96), and at this time the range of temperature is also least ; while in September, with a greater daily range of humidity (62 to 92) the range of temperature is the greatest, and the lowest temperatures are recorded in this and the preceding month. It is a curious fact, that in many parts of England the degree of humidity as measured by the comparative saturation of the air, is as great as that of Batavia or even greater. A register kept at Clifton

« EelmineJätka »