Humidity

“ When the air is kept at 65 to 70 per cent. humidity in winter one will feel comfortable in a much lower temperature—about 68°—than when the air is extremely dry, as it usually is in the average living apartment. With a relative humidity of 30 to 40 per cent. which one now often finds in warm houses in winter, the temperature may be forced up to 75° or over and still one may feel cold, because of the rapid evaporation from the pores of the skin, and the cold created inside the clothing by the heat lost in the process of evaporation. ”

“ 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. ”

“ 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 humidity of the atmosphere, although invisible, has a special and strong influence in absorbing radiant energy, whether from the sun or from the earth. It therefore converts the dry air into a powerful obstructor to the passage of radiant heat, and by this means the humid atmosphere is made to act as a blanket, keeping the earth and the lower air much warmer than it would otherwise be. ”

“ It rains day and night for weeks at a time, till everything is penetrated with dampness; and when the hot sun comes out upon the soaked and steaming earth, it is far more injurious than in the dry season. One cannot wonder at the prevailing sickness, for the humidity seems to permeate everything with subtle power. ”

“ I answer, even supposing that humidity, by means of its natural quality, does not obstruct, nobody knows in what proportion, or to what degree, the brain, for the best exercising its functions should be moist; and therefore it is vague to say, there is a greater proportion of it in women than in men, or in men than in women.
XCIV. There may be opposed however, to this doctrine of humidity, the opinion of many, who [Pg 258] affirm, that the humid and cloudy countries produce heavy dull spirits; and, on the contrary, that in the bright and clear countries, are born ingenious and sprightly ones. ”

“ The increase of humidity—favorable to precipitation—is noted by the gathering of moisture on cold objects, the collection of perspiration on our own skins owing to diminished evaporation, and the dampness of many hygroscopic substances. The last effect is illustrated by the packing of salt, the tightening of cordage and of strings of musical instruments, the dull or damp appearance of stone walls and columns, the settling of smoke, and by a number of other similar phenomena, all of which have been appealed to, with more or less justification, as evidence of a gathering storm. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ Forty-five to sixty per cent humidity is desirable in a house. This means forty-five to sixty per cent as much water as the air is capable of taking up at room temperature. Cold air is usually dryer than warmer air because cold air cannot take up as much humidity as warm air. ”

“ In fact, the geologists have found that the different epochs in earth’s evolution are best characterized by their humidity or dryness. To arrive at a clear conception in these matters we shall briefly survey our present knowledge of the importance for the evolution of life on earth that we should attach to the humid and to the dry periods or localities.
We are all familiar with the heavy, moisture-laden warmth which meets us when we enter a hothouse. It is particularly favourable to the growth of plants and to the prosperity of the lower animals. ”

“ 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. ”

“ The qualities hitherto sought in a hygrometer, or instrument to discover the degrees of moisture and dryness [136] in the air, seem to have been, an aptitude to receive humidity readily from a moist air, and to part with it is as readily to a dry air. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ This indeed appears so from the color of the "heavens." And experience teaches that the air is humid by nature. Philosophers also say that if there were no sun the air would be a perpetual humidity. But they assert that the air is both humid and warm, but that it is humid from its own nature, because the heaven was made out of waters, and that therefore it is, that it rains and contains a fructifying moisture; but because the light and heat of the sun are added to it, the humid nature of the air is so tempered that it is also warm. ”

“ 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 ”

“ 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. ”

“ The daily range of temperature is also influenced by humidity. Because of the effect of water vapor in the atmosphere on insolation47 and radiation, a rock surface becomes hotter in the day and cooler at night beneath a dry atmosphere than beneath a moist one. Aridity therefore favors the disruption of rock by changing temperatures. ”

“ 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 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. ”

“ 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. ”

“ The moisture sensation of the air is its humidity. Before saturation is reached, the moisture is in the form of vapor; at the point of saturation it may appear in the air as fog, or cloud; when the ground temperature reaches the point of saturation condensation takes place in the form of dew ”

“ ABSOLUTE HUMIDITY The amount of vapor actually present in the atmosphere is termed the absolute humidity, expressed usually either in the expansive force that the vapor exerts or in its weight in grains per cubic foot of air. ”

“ In warm moist air strength is lost and there is a distinct tendency to intestinal troubles. In moist cold air perspiration is checked, and rheumatic and joint affections are very common. The main differences between mountain air and that of the plains depend on the former being more rarefied, colder, of a lower absolute humidity, and offering less resistance to the sun’s rays. ”

“ The modern science of ventilation concerns itself with these other factors; chief among them is the factor of moisture. As we shall see in the next chapter our bodies are constantly giving off from the lungs and by evaporation from the sweat glands water vapor into the air. This causes the humidity to go up rapidly in rooms where people are congregated, and particularly so where there are many people present. Also everyone gives off a great deal of heat. ”

“ 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. ”

“ If we add moisture to a sunless spot, we have the ideal environment for germs to breed and flourish in. There is always moisture or humidity in the air if the altitude is low, and if it is near the ocean, or any large body of water, the moisture is relatively greater. For this reason we send patients with pulmonary disease to the mountains, where the altitude is much higher, where there is no moisture, and consequently where there are practically no germs. ”

“ 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. ”

“ Cold humidity is the normal trait of the winter climate; catarrh is almost permanently epidemic; many of the inhabitants can echo the declaration of one their frank fellow-citizens, who says: 'Depuis que je me connais, je suis enrhumée. ”

“ 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. ”