Practical Exercises in Elementary Meteorology

by Robert DeCourcy Ward

Fig. 8.

The air always has more or less moisture in it. Even the hot, dry air of deserts contains some moisture. This moisture is either invisible or visible. When invisible it is known as water vapor, and is a gas. When visible, it appears as clouds and fog,

or in the liquid or solid form of rain, snow, and hail. The amount of moisture in the air, or the humidity of the air, varies according to the temperature and other conditions. 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. Water which changes to vapor is said to evaporate.

This process of evaporation needs energy to carry it on, and this energy often comes from the heat of some neighboring body. When you fan yourself on a very hot day in summer, the evaporation of the moisture on your face takes away some of the heat from the skin, and you feel cooler. The drier the air on a hot day, the greater is the evaporation from all moist bodies, and hence the greater the amount of cooling of the surfaces of those bodies. For this reason a hot day in summer, when the air is comparatively dry, that is, not saturated with moisture, is cooler, other things being equal, than a hot day when the air is very moist. Over deserts the air is often so hot and dry that evaporation from the face and hands is very great, and the skin is burned and blistered. Over the oceans, near the equator, the air is hot and excessively damp, so that there is hardly any cooling of the body by evaporation, and the conditions are very uncomfortable. This region is known as the “Doldrums.”

The temperatures that are felt at the surface of the skin, especially where the skin is exposed, as on the face and hands, have been named sensible temperatures. Our sense of comfort in hot weather depends on the sensible temperatures. These sensible temperatures are not the same as the readings of the ordinary (dry-bulb) thermometer, because our sensation of heat or cold depends very largely on the amount of evaporation from the surface of the body, and the temperature of evaporation is obtained by means of the wet-bulb thermometer. Wet-bulb readings at the various stations of the Weather Bureau are entered on

all our daily weather maps. In summer (July) the sensible (wet-bulb) temperatures are 20° below the ordinary air temperature in the dry southwestern portion of the United States (Nevada, Arizona, Utah). The mean July sensible temperatures there are from 50° to 65°; while on the Atlantic coast, from Boston to South Carolina, they are between 65° and 75°. Hence over the latter district the temperatures actually experienced in July average higher than in the former.

Unless the air is saturated with water vapor, the evaporation from the surface of the wet-bulb thermometer will lower the temperature indicated by that instrument below that shown by the dry-bulb thermometer next to it, from which there is no evaporation. The drier the air, the greater the evaporation, and therefore the greater the difference between the readings of the two thermometers. By means of tables, constructed on the basis of laboratory experiments, we may, knowing the readings of the wet and dry-bulb thermometers, easily determine the dew-point and the relative humidity of the air—important factors in meteorological observations (see Chapter XXVI). In winter, when the temperature is below freezing, the muslin of the wet-bulb thermometer should be moistened with water a little while before a reading is to be made. The amount of water vapor which air can contain depends on the temperature of the air. The higher the temperature, the greater is the capacity of the air for water vapor. Hence it follows that, if the temperature is lowered when air is saturated, the capacity of the air is diminished. This means that the air can no longer contain the same amount of moisture (invisible water vapor) as before. Part of this moisture is therefore changed, condensed, as it is said, from the condition of water vapor into that of cloud, fog, rain, or snow. The temperature at which this change begins is called the dew-point of the air.

The relative humidity of the air is the ratio between the amount of water vapor which the air contains at any particular time and the total amount which it could contain at the temperature

it then has. Relative humidity is expressed in percentages. Thus, air with a relative humidity of 50% has just half as much water vapor in it as it could hold.

It is found that the readings of the wet-bulb thermometer are considerably affected by the amount of air movement past the bulb, and that in a light breeze, or in a calm, the reading does not give accurate results as to the humidity of the general body of air outside the shelter.