Imagens das páginas
PDF
ePub

thermometers placed on a wooden stand (fig. 246). One of the bulbs is covered with muslin, and is kept continually moist by being connected with a reservoir of water by means of a string. Unless the air is saturated with moisture, the wet bulb thermometer always indicates a lower temperature than the other, and the dif ference between the indications of the two thermometers is greater in proportion as the air can take up more moisture.

According to Glaisher, the temperature of the dew point may be obtained by multiplying the difference between the temperatures of the wet and dry bulb by a factor which depends on the temperature of the air at the time of observation, and subtracting the product thus obtained from this last-named temperature. The following are the numbers :—

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][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][merged small][merged small][merged small]

These are often known as Glaisher's factors. The temperatures are expressed on the Fahrenheit scale. As an example: if the temperature of the wet bulb is 49° and that of the dry bulb 54°, then the dew point is 44°-that is, at this temperature the moisture present in the atmosphere is just sufficient to saturate it.

-284]

Causes which modify Temperature.

297

CHAPTER XII.

METEOROLOGICAL PHENOMENA WHICH DEPEND UPON HEAT.

282. Meteorology.-Meteorology is that branch of physics which is concerned with the phenomena which occur in the atmosphere; such, for instance, as variations in the temperature of the air, wind, rains, storms, electrical phenomena, etc. Though of recent origin, this science is an important application of the physical sciences, and furnishes useful information to navigation, to agriculture, and to hygiene.

283. Mean temperature.—The mean daily temperature, or simply temperature, is that obtained by adding together 24 hourly observations, and dividing by 24. A very close approximation to the mean temperature is obtained by taking the mean of the highest and lowest temperatures of the day and of the night, which are determined by means of the maximum and minimum thermometers (209). These ought to be protected from the sun's rays, raised above the ground, and be far from all objects which might influence them by their radiation. The lowest daily temperature is at 4 A.M., and the highest at 2 P.M.

The temperature of a month is the mean of those of 30 days, and the temperature of the year is the mean of those of 12 months. The highest mean monthly temperature is in July, and the lowest in January. The temperature of a place is the mean of its annual temperature for a great series of years. The mean temperature of London is 10°35 C., or 50°·63 F. The temperatures in all cases are those of the air and not those of the ground.

284. Causes which modify the temperature of the air.— The principal causes which modify the temperature of the air are the latitude of a place, its height—that is, its distance above the sea-the direction of the winds, and the proximity of seas.

Influence of the latitude. The temperature of the air and of the ground diminishes from the equator towards the poles. This is due to the fact that the sun's rays, which are perpendicular at the equator, are more and more inclined as we come nearer the poles.

Now, the more acute is the angle under which the rays of heat fall upon a body, the less is the body heated; hence the heat absorbed decreases from the equator to the poles, for the rays are then more and more oblique. Yet, as in summer the days are longer as we get nearer the north, the loss due to the increasing obliquity of the sun is partially compensated by the sun remaining longer above the horizon. Under the equator, where the length of the days is constant, the temperature is almost invariable; in the latitude of London, and the more northerly countries, where the days are very unequal, the temperature varies greatly; but in summer it sometimes rises almost as high as under the equator. The lowering of the temperature produced by the change in latitude alone is small; thus in a latitude of 115 miles north of ours, the temperature is only 1° C. lower.

Influence of altitude. The height of a place has a much more considerable influence on the temperature than its latitude. In the temperate zone an ascent of 540 feet corresponds in the mean to a diminution of 1° C.

The cooling as we ascend in the atmosphere has been observed in balloon ascents, and a proof of it is seen in the perpetual snows which cover the highest mountains, even under the torrid zones. The height at which snow remains unmelted through the year, or the line of perpetual snow met with, differs in different places. On the Andes it commences at a height of 14,760 feet, and on the Alps at 8,880 feet.

Direction of winds. As winds share the temperature of the countries which they have traversed, their direction exercises great influence on the air in any place. In our climate the hottest winds are the south, then come the south-east, the south-west, the west, the east, the north-west, north, and, lastly, the north-east, which is the coldest. The character of the wind changes with the seasons; the east wind, which is cold in winter, is hot in summer.

Proximity of the seas. The neighbourhood of the sea tends to render the temperature of the air uniform, by heating it in winter, and cooling it in summer. The average temperature of the sea in equatorial and polar countries is always different from that of the atmosphere. With reference to the uniformity of the temperature, it has been found that in temperate regions, that is, from 25 to 50 degrees of latitude, the difference between the maximum and minimum temperature of a day does not exceed, on the sea, 2° to 3° C. ; while upon land it amounts to 12° to 15°. In islands the uniformity of

-287]

Climate.

299

temperature is very perceptible, even during the greatest heats. In continents, on the contrary, the winters for the same latitudes become colder, and the difference between the temperature of summer and winter becomes greater.

285. Gulf Stream.-A similar influence to that of the winds is exerted by currents of warm water. The mildness of the climate in the north-west of Europe is usually assigned to one of these, the Gulf Stream. This great body of water, taking its origin in equatorial regions, flows through the Gulf of Mexico, whence it derives its name; passing by the southern shores of North America, it makes its way in a north-westerly direction across the Atlantic, and finally washes the coast of Ireland and the north-west of Europe generally. Its temperature in the Gulf is about 28° C., and is generally a little more than 5° C. higher than the rest of the ocean, on which it floats owing to its lower specific gravity. To its influence is due the milder climate of western Europe as compared with that of the opposite coast of America; thus the river Hudson, which is in the same latitude as Rome, is frozen over three months in the year. It also causes the polar regions to be separated from the coasts of Europe by a girdle of open sea; and hence the harbour of Hammerfest is open the year round. Besides its influence in thus moderating climate, the Gulf Stream is an important help to navigators.

286. Isothermal lines. When all the points on a map whose temperature is known to be the same are joined, curves are obtained, which Humboldt first noticed, and which he called isothermal lines. If the temperature of a place only varied with the obliquity of the sun's rays, that is, with the latitude, isothermal lines would all be parallel to the equator; but as the temperature is influenced by many local causes, especially by the height above the sea level, the isothermal lines are always more or less curved. On the sea, however, they are almost parallel. A distinction is made between isothermal lines, isotheral lines, and isochimenal lines, where the mean general, the mean summer, and the mean winter temperatures are respectively constant. An isothermal zone is the space comprised between two isothermal lines. Kupfer also distinguishes isogeothermal lines, where the mean temperature of the soil is

constant.

287. Climate.-By the climate of a place is understood the whole of the meteorological conditions to which a place is subjected, its mean annual temperature, summer and winter temperatures, and the extremes within which these are comprised. Some

writers distinguish seven classes, of climates according to their mean annual temperature—a hot climate from 30° to 25° C. ; a warm climate, from 25° to 20° C.; a mild climate, from 20° to 15° C.; a temperate climate, from 15° to 10° C.; a cold climate, from 10° to 5° C. ; a very cold climate, from 5° to zero; and an arctic climate, where the temperature is below zero.

Those climates, again, are classed as constant climates where the difference between the mean and summer and winter temperature does not exceed 6° to 8°; variable climates, where the difference amounts to from 16° to 20°; and extreme climates, where the difference is greater than 30°. The climates of Paris and London are variable; those of Pekin and New York are extreme. Island climates are generally but little variable, as the temperature of the sea is constant; and hence the distinction between land and sea climates.

There is a great difference between a land and a sea climate (284). The former is characterised by a greater range of temperature than the latter. Thus in the north-east of Ireland ice scarcely forms in winter, and the myrtle flourishes as in Portugal; yet this is in the same latitude as Königsberg in Prussia, where the mean annual temperature is 5° C., the range being from -3°5, the mean monthly temperature in January, to 13° 6, that of July. Winter in Plymouth is not colder than in Florence, yet grapes do not flourish in the open air, for while they can stand a somewhat severe cold in winter, they require a hot summer to ripen them.

The reason of this is that the land absorbs and radiates heat easily; it thus becomes more easily heated and more rapidly cooled than the sea, which mainly from its great specific heat is not so rapidly heated, but on the other hand does not so soon again part with the heat it has acquired.

But the temperature is by no means the only characteristic which influences climates; there are, in addition, the moisture of the air, the quantity and frequency of the rains, the number of storms, the direction and intensity of the winds, and the nature of the soil.

FOG. RAIN. DEW.

288. Fogs and mists.-When aqueous vapour, rising from a vessel of boiling water, diffuses in the colder air, it is condensed; a sort of cloud is formed which consists of a number of small hollow vesicles of water, which remain suspended in the air. These are usually spoken of as vapour, yet they are not so, at any rate

« AnteriorContinuar »