« ZurückWeiter »
This table is constructed from the indications of Daniell's hygrometer; an instrument consisting of a glass tube about six or eight inches long, bent twice at right angles, and terminated, at each extremity, in a bulb. One of the bulbs, which is usually coloured, contains a very delicate little thermometer and a small quantity of ether; the thermometer dips into the ether, which may be driven, by the heat of the hand, into either of the bulbs. Upon cooling the empty one, which may be done by pouring a few drops of ether upon it, the other immediately becomes cooled also, from the evaporation of the fluid inclosed within it, while the thermometer, dipping into it, shews how much it is cooled. When using the instrument, you narrowly watch the coloured bulb while the cooling process is going on; and at the moment when it becomes wetted with dew, you note the degree at which the thermometer included in it stands, and this is the dew point. Sometimes a few drops of ether are sufficient to produce the effect, and the inclosed thermometer falls perhaps only a degree or two; this shews that the dew point is hardly below the temperature of the air, and the atmosphere is damp. At other times it is required to wet the bulb several times; and dew is not produced upon the coloured ball unti) the inclosed thermometer falls ten, fifteen, or twenty degrees below the temperature of the air ; the atmosphere, under these circumstances, being very dry.
This instrument is usually fitted to a little brass pillar, having affixed to it another small thermometer, which shews the temperature of the air ; so that the comparison between this and the dew point may be made at the same moment.
That the object and use of the hygrometer may be thoroughly understood, it should be remembered that the atmosphere consists of two essentially distinct fluids, one consisting of permanently elastic gases—constituting the air, properly so called the other of aqueous vapour, which, within the range of the temperature of the atmosphere, is capable of assuming the æriform, the fluid, and the solid state. It is upon this that all the most important meteorological phenomena depend, such as dew, fog, cloud, rain, hail, or
It is to discover the existing quantity of this aqueous vapour when in its æriform or invisible shape, relatively to the temperature of the air, that Daniell's hygrometer is made use of.
The dew point frequently is as high as the temperature of the air, during heavy rain, in damp weather, in the evening, and at night. The dew point is very often much below the temperature of the air in clear fine weather, and especially with N. E. winds;
but it can never be higher than the temperature of the air: when the latter falls (supposing them at the same point) the former must fall also—the superfluous moisture (i.e., some portion of the invisible vapour) being condensed either into dew, fog, clouds, or perhaps rain.
In autumn, when the temperature of the air is advancing during the day, it often happens that the dew point advances as much, and in the evening, when the air is again cooling, the dew point must fall; its subsidence being accompanied by a copious deposition of dew, and where the lower strata of the atmosphere are chilled by radiation, by the appearance of mist or fog. The form in which moisture under these circumstances is deposited upon the ground, is much modified by the state of the air: if the wind blows strongly, surfaces become moist and perhaps wet ; it is only when the air is calm, that those minute drops, standing at the very tips of the blades of grass and upon every, the minutest fibre, constituting dew properly so called, can be seen.
Air of the same temperature affects our sensations differently ; the impression is greatly modified by the force of the wind and the state of the dew point. When the atmosphere is calm, the temperature moderate, and the dew point very high, it seems close, warm, and oppressive-sensations much diminished by a light breeze ; on the other hand, when the temperature is moderate, the air calm, and the dew point very low, the feeling is cool, bracing, and pleasant; but if the wind blows fresh, we then feel it cold, harsh, and disagreeable ; this last condition is very apt to induce catarrh, sorethroat, and rheumatism in those disposed to these affections. It is when the dew point is high that ladies' hair falls out of curl, a ringlet is an elegant and delicate hygroscope. Every one must have observed, occasionally, the visible condensation of the breath; this is a tolerable indication of the state of the vapour of the atmosphere, such an appearance shewing that the temperature of the air and dew point are quite, or very nearly, the same: it may sometimes be remarked in the open air, before, almost always during, or just after, continued heavy rain.
During the winter season, (and the remark will apply generally to the autumn and spring), a rise in the thermometer and dew point at the same time, is a sure indication of clouds and rain.
It often happens on a clear sunny day after rain, when evaporation is going on with the utmost rapidity, that the dew point does not rise, or, in other words, that the quantity of vapour in the lower regions of the atmosphere does not increase, nor do clouds form in the higher; the vapour, therefore, must be drawn off to distant regions.
Sometimes when the weather is very foggy in the morning, the hygrometer exposed to it will not be dewed or moistened until the temperature of the dark bulb be reduced two, three, or four degrees. When this occurs, the fog is usually succeeded by a clear and fine day.
The following Table of the wind is arranged in accordance with the remarks made in the second volume of The Analyst, p. 221; and it appears that those to the south of the east and west points of the compass, in comparison with those to the north of these points, were, in the former year, as 15 to 10 at Malvern, and in the latter as 18 to 10; and they bore very nearly the same relation to each other in London—the numbers being, for the first year, 15 and a fraction to 10, in the last 19 to 10.
TABLE IV._Of the Wind at Malvern and London for the Seasons of
1834–5 and 1835-6.
Two or more different currents of wind may be frequently detected by observing the clouds. Sometimes the higher ones may be seen nearly stationary, the lower moving rapidly, either at various angles to the slow-moving higher ones, or now and then in an opposite direction. The primary indication of a change in the direction of the wind may be detected sometimes by noticing the appearance of clouds, even before they are influenced in their movements by the ærial current. They display a great variety of circu. lar segments or curls, in a plane apparently parallel to the horizon -the convex face of the curl when below the zenith being turned downwards towards the horizon, whereas the convex arch of the cumulus is turned upward towards the zenith. The appearance here spoken of may be difficult to describe, but it is very characteristic. The convexity of the curl is generally in the direction of the. approaching current.
When the air is calm, evaporation is proportionate to the interval between the dew point and the temperature of the air ; it is ac
celerated by the slightest current, and enormously increased by a brisk or high wind.
In calm weather, if there is no interval between the dew point and the temperature of the air, there is no evaporation ; if the latter rises evaporation commences, but if it falls precipitation begins. If when the dew point and the temperature of the air are the same the wind blows strong, there is then some degree of evaporation, and the air appears to carry with it particles of nascent vapour, something in the same way that it blows up the dust in our roads, the particles of vapour remaining visible, and rendering the air misty or, as it is termed, thick and hazy.
TABLE V.-The number of days on which Rain or Snow fell during each
Season in 1834, 1835, and 1836.
In taking notice of rain and rainy days with reference to season and climate, the length of time during which it is falling is quite as worthy of regard as the quantity. As much rain may fall during a heavy shower in an hour or two as in a whole day's mizzling rain ; and yet the former may be a fine, sunny, and beautiful day; the latter, a wet and miserable one.
The temperature of rain as it descends is, doubtless, very various; it is generally of the same temperature as the dew point. Whatever the temperature of rain may be when it descends for an hour or two, the air and constituent temperature of the vapour will be equalized.
The following remarks, taken from the meteorological journal to which we are indebted for the preceding tables and observations, may not be unacceptable to our readers.
Jan. 23rd, 1834. Notwithstanding the extreme mildness of this winter, and the great quantities of rain which have fallen during the last thirty days, it has not been by any means sickly--all accounts seem agreed in this particularthough coughs, colds, and rheumatic attacks of a minor character, were observed in the early part of the autumn of 1833. Whether the salubrity of the season is at all attri. butable to the very boisterous winds of November and December, we cannot venture to determine.
March, 1834. A remarkably dry month, with cold N. E. winds. Epidemic catarrh very prevalent.
The spring and early period of the summer of 1834 very dry. Rain fell on the 4th of June very seasonably for all kinds of vegetation : the rest of the month was showery. the early part of July was hot, with genial showers; towards the middle of the month, heavy continued rain and floods. August was generally wet, with a marked and continued decline of temperature towards the end. September was remarkably fine, October seasonable, and November mild and dry.
1835. On the 16th and 17th of April, after mild seasonable weather, the thermometer fell, at night, to 28°, or four degrees below freezing. Snow fell in London, and ice was seen in many places half an inch thick. This thermometric depression seems to have been very general, not only in this country, but in France, and other places. At Bour. deaux, the mercury fell, during the night of the 16th, to
25.5, having been for many days previously at 72" or 75." An account of the sudden increase and subsequent rapid decrease of temperature which occurred in the month of June, 1835, will be found in our 3rd volume, p. 175.
July and August, 1835, were hot and dry. Rain fell in quantity for the first time for some weeks on the 24th of the latter month : this was followed by a considerable decline of temperature. The last few days of August and the first week in September were again hot and fine. On the 8th of the latter month heavy rain fell; the remainder of the month was wet, and the temperature rather low.
October was cold, cloudy, and, for the most part, wet, with some heavy rain towards the latter end of the month. The early part of November was cold, with heavy rain ; the middle mild, with showers, and fresh S. W. breezes. During the last two days of the month the rain was incessant. The first few days in December were mild. Frost set in severely on the night of the 19th, with snow, which continued, with fog and a beautiful hoar-frost, till the 28th.
January and February, 1836, were characterised by great
alternations of temperature, with much snow and rain. For several interesting remarks upon the spring of 1836, we must refer to our 3rd volume, p. 348.