Relative humidity

“ Types of Change of Relative Humidity with Altitude.—As in the temperature types, the continuous lines represent the records of the ascent, and the broken lines the records of the descent, generally under changing conditions. Lines inclining upward to the left show a decreasing humidity, and to the right an increasing humidity.
Type 1 may be called a normal type of curve when there are clouds. ”

“ Ordinarily, the air which may be at the dewpoint at daylight becomes relatively dry at midday because its higher temperature gives it what is popularly termed “a greater capacity for moisture.” But if the relative humidity remains high in the middle of the day it is evident that the absolute humidity has increased, and unsettled weather may be expected. ”

“ He then shows how the relative humidity is greatest just before sunrise of each day, and the vapor tension or absolute humidity is least; that as the day advances the relative humidity diminishes and the absolute humidity increases, till the maximum temperature of the day is reached, when absolute humidity is greatest and relative humidity is least. ”

“ The absolute humidity as a whole decreases as the temperature falls. The relative humidity averages 90%, more or less, over the oceans, and is high under the clouds and rain of cyclonic storms, but depends, on land, upon the wind direction, winds from an ocean or from a lower latitude being damper, and those from a continent or from a colder latitude being drier. ”

“ Relative humidity is simply the ratio of the actual vapor pressure present in a given space to the vapor pressure when the space is saturated with vapor at the given temperature. It matters not whether air be present or not. One hundred per cent humidity means that the space contains all the vapor which it can hold at the given temperature—it is saturated. Thus at 100 per cent humidity and 212 degrees Fahrenheit the space is saturated, and since the pressure of saturated vapor at this temperature is one atmosphere, no air can be present under these conditions. ”

“ Yet even in the arid continental interiors in summer the absolute vapour content is surprisingly large, and in the hottest months the percentages of relative humidity may reach 20% or 30%. At the low temperatures which prevail in the winter of the higher latitudes the absolute humidity is very low, but, owing to the cold, the air is often damp. ”

“ Towards the margins of the zone, with increasing annual ranges of temperature, seasons in the extra-tropical sense gradually appear.
Physiological Effects of Heat and Humidity.—Tropical heat is associated with high relative humidity except over deserts and in dry seasons. The air is therefore muggy and oppressive. The high temperatures are disagreeable and hard to bear. The “hot-house air” has an enervating effect. Energetic physical and mental action are often difficult or even impossible. The tonic effect of a cold winter is lacking. ”

“ Check instruments. Humidity is so readily affected by temperature, wind, and pressure, that a knowledge of these factors is essential to an understanding of its fluctuations. Pressure, disregarding daily variation, is taken account of in the tables for ascertaining relative humidity, and is determined once for all when the altitude of a station has been carefully established. The temperature is obtained directly from the dry bulb reading. ”

“ It is an element of much practical interest, because it is one of the main factors in determining the drying power of the air, the other important one being wind. The air feels dry when evaporation proceeds rapidly from our skin, either on account of low relative humidity, brisk air movement, or both. People are hardly conscious of high relative humidity except when, in hot weather, it retards the evaporation of perspiration, and the latter collects in liquid form on the skin. ”

“ This is often a more crucial factor than temperature, and one that is more troublesome to change. Relative humidity is a measure of the amount of moisture in the air as compared with the maximum amount of moisture the air can hold at a given temperature. ”

“ There is less water vapor contained in the air at high altitudes, the air being cooler. A large amount of rainfall does not necessarily produce an increase in the relative humidity. That is to say, a country with a very high average of rainfall is not necessarily a damp country so far as the atmosphere is concerned. ”

“ A. Diurnal Variation of Relative Humidity under Different Conditions.—Readings of the wet and dry-bulb thermometers in the instrument shelter, or of the sling psychrometer, several times during the day, will furnish data for determining the diurnal variation of relative humidity. Classify your observations according to the weather conditions under which they were made, and by months or seasons. Summarize the results of your investigation, paying special attention to the relation between the diurnal variation of relative humidity and the temperature. ”

“ Relative humidity varies greatly, and very low readings have often been recorded. Cloudiness seems to decrease somewhat towards the inner polar areas, after passing the belt of high cloudiness in the higher latitudes of the temperate zones. In the marine climates of high latitudes the summer, which is the calmest season, has the maximum cloudiness; the winter, with more active wind movement, is clearer. ”

“ A depression in the percentage of relative humidity accompanies this surge of warmer air. On some nights winds apparently disturb, or mix, the layers of air at ground level. On such nights the reradiation of energy is not apparent in the traces of the thermographs. ”

“ The air of mountains is (1) cooler than that of lower districts with the exception already named. (2) It is less dense in proportion to the altitude; its pressure at the height of 16,000 feet being only half that at the sea level. (3) Its absolute humidity is decidedly diminished; there is some difference of opinion as to the relative humidity. (4) The air is as a rule purer. It is generally free from dust, and to a large extent aseptic (that is, free from microbes) . (5) The amount of ozone is commonly greater than in lower regions. ”

“ However, if water vapor, or air containing water vapor, having a temperature , and an absolute humidity of , be cooled to , it will become saturated, and if cooled further, moisture will be precipitated. Therefore is termed the dew point of air having a temperature and an absolute humidity, , or a corresponding relative humidity, . ”

“ Relative humidity does not owe its importance in human affairs solely to its physiological effects, for78 it plays a prominent part in numerous industries—textile, metallurgical, chemical, leather, food, and all those employing drying processes. ”

“ When the air is nearly saturated, the temperature of the air rarely goes above 74°; but, if so, the moisture in the air prevents the passing away of insensible perspiration, and the joint action of heat and humidity exhausts the vital powers, causing sun-stroke, as it is called. ”

“ An increase in the humidity of the atmosphere, while the temperature remains constant, will produce the same result, since increased humidity of the atmosphere diminishes evaporation. A decrease of temperature, decreasing the melting, and an increase of humidity, decreasing the evaporation, would cause the ice to advance farther than either change alone, since both changes decrease the wastage. ”

“ These two types can be taken as the normal change of humidity with change of altitude in cloudy or partly cloudy weather. The humidity increases steadily to the base of the cloud, then there is complete saturation in the cloud, and above it is a sudden fall of humidity, on entering the dry air above the cloud, into which the ascending currents from the ground have not penetrated. ”

“ 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 the [Pg 76] heat. 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. ”

“ According to the theory, nothing is required for the production of rain besides the mixture of portions of the atmosphere with humidity, and of mixing the parts that are in different degrees of heat. But we have seen the causes of saturating every portion of the atmosphere with humidity and of mixing the parts which are in different degrees of heat. Consequently, over all the surface of the globe there should happen occasionally rain and evaporation, more or less ”

“ The air is rendered humid by containing INVISIBLE vapor, and it becomes drier by the condensation of such vapor into fog, composed of solid globules or of hollow vesicles of water—for it is a disputed point whether the particles of fog are solid or vesicular. Hence, though the ambient atmosphere may hold in suspension, in the form of fog, water enough to obscure its transparency, and to produce the sensation of moisture on the skin, the air, in which the finely divided water floats, may be charged with even less than an average proportion of humidity. ”

“ The humidity is about 77 per cent. In the rainy season the mean temperature is 85°, and the humidity varies from 86 per cent to complete saturation. The annual rainfall is seldom less than 125 inches. A man six feet in stature standing on the shoulders of another man of equal height, would just about be able to keep his shoulders above water if the two were placed in a reservoir which would catch and hold the entire rainfall of the year. But in spite of heat and humidity and precipitated moisture, existence can be made passably comfortable. ”

“ By moistening the air woodlands also moderate the extremes of heat and cold. It is seen on the sea-shore how beneficently humidity operates in allaying the severity of winter, and in summer the evaporation of dew and rain gives us cool breezes when they are most needed. ”

“ When the air contains as much water vapor as it can hold, it is said to be saturated. Its humidity is then high. When the air is not saturated, evaporation goes on into it from moist surfaces and from plants. ”

“ These observations indicate that moisture is deposited on the dust-particles from air, which is not saturated, and that the condensation takes place while the air is comparatively dry, before the temperature is lowered to the dew-point. There is, then, no definite demarcation between what seems to us clear air and thick haze. The clearest air has some haze, and, as the humidity increases, the thickness of the air increases. ”

“ We now see that, as a living organism, it tends, on the one hand, to diminish the humidity of the air by sometimes absorbing moisture from it, and, on the other, to increase that humidity by pouring out into the atmosphere, in a vaporous form, the water it draws up through its roots. ”

“ And this relation of cause and effect, manifest in individuals, is one which we may suspect holds in races—other things being equal. In temperate regions, differences of constitutional activity due to differences of atmospheric humidity, are less traceable than in torrid regions, the reason being, that the inhabitants are subject to a tolerably rapid escape of water from their surfaces, since the air, though well charged with water, will take up more when its temperature, previously low, is raised by contact with the body. ”

“ In other cases, from injudicioua husbandry, or the diversion or choking up of natural water-courses, it may become more highly charged with humidity. An increase or diminution of the moisture of a soil almost necessarily supposes an elevation or a depression of its winter or its summer heat, and of its extreme if not of its mean annual temperature, though such elevation or depression may be so slight as not sensibly to raise or lower the mercury in a thermometer exposed to the open air. ”

“ Science answers: By their evaporated moisture influencing the atmosphere. This may save us from the blighting influence of frost, by enveloping the frozen vegetation in a wet blanket of fog; enabling it to be thawed in the dark, as it were, by which we avoid the influence of a bright sunshine, that would have destroyed the tissues had they been suddenly exposed to it when frozen. An equally important result is derived from the direct influence of the humidity of the atmosphere, which modifies the temperature remarkably, as in the immediate vicinity of large bodies of water. ”

“ Even when there is no precipitation the moisture in the air influences its activity in certain ways. Just as iron “rusts” more readily in moist air than in dry, so changes in other mineral substances are influenced by atmospheric humidity. Where precipitation takes place the results are more obvious. The passing shower works changes in the surface of the land, striking in proportion to the rate and amount of precipitation. The rains feed the streams, and every stream is modifying its bed, and with increasing rapidity as its current is swollen. ”

“ The ranges of temperature to which the human body may become accustomed are very broad, so that the limitations to the dispersion of the race seem to be set by the food supply rather than the degree of heat or cold. [80] All authorities agree, therefore, that the regions where acclimatization is most difficult are to be found in the areas of excessive humidity, or, roughly, where there is the maximum rainfall. ”