Meteorology: The Science of the Atmosphere

by Charles Fitzhugh Talman

Among the meteorological elements that unmistakably pertain to weather the most important is the temperature of the air. The thermometer, with which temperature is measured, is, in its common form and in its essential features, too familiar to require description here; but we may remark that, as in the case of the barometer, several methods of graduating this instrument have been used. Besides numerous obsolete systems, there are three different thermometric scales—the Fahrenheit, the Centigrade, and the Absolute. The first is still the prevailing one in English-speaking countries, and the second prevails in all other countries. The Absolute scale, long familiar to physicists, has recently come into somewhat limited use in meteorology. It starts at the “absolute zero”—the temperature of a body totally devoid of heat. This temperature has been nearly attained in laboratory experiments with liquid helium. One advantage that the Absolute scale possesses over the others is that it has no below-zero readings. Such readings are a source of occasional errors when temperature is recorded on the Fahrenheit or the Centigrade scale.

The freezing point of water is 32° Fahrenheit = 0° Centigrade = 273° Absolute. The boiling point of water, at sea level, is 212° Fahrenheit = 100° Centigrade = 373° Absolute.

While the layman is well acquainted with the thermometer, he sometimes fails to understand certain differences between the scientific and unscientific methods of using this instrument for weather-measuring purposes. On a hot summer day he is, perhaps, inclined to feel aggrieved because the official record of temperature does not adequately express the state of his feelings, to say nothing of being at odds with the impressive instrument displayed at the corner drug store. Hence the following explanation is in order:

It is the function of the official thermometer to indicate the true temperature of the air. A thermometer exposed to direct sunshine records its own temperature—i. e., the temperature of the glass and mercury—and nothing else. A thermometer “in the shade”—under a tree, for example—comes nearer to showing the true air temperature; but it is exposed to radiation from surrounding objects and its readings will vary with the nature and location of these objects. The meteorological thermometer is nearly always installed in a kind of latticed screen, or shelter. It is thus largely protected from radiation, while the air circulates freely around it. Only when thermometers are exposed under such standard conditions is it possible to obtain comparable readings of the temperature at different places, so that, for instance, maps may be drawn showing the distribution of this element over a country. The best location for the thermometer screen is a few feet above sod. Many thermometers of the United States Weather Bureau are installed on the roofs of tall buildings; not because this is an ideal location, but because no better is available in the heart of a large city, where, for practical reasons, the office has to be placed. In many small towns the site of the station is such that the thermometer screen (or “instrument shelter,” as it is called in the Weather Bureau) can be placed close to the ground, and at the same time get ample ventilation and be free from the radiation of buildings. In certain large cities the Bureau maintains a branch station in a park or in the suburbs, where a satisfactory exposure for all instruments can be secured.

The artificial temperature of a city street is too local and indefinite a thing to be inscribed on weather maps, utilized by the forecaster, or embodied in climatic statistics. As a concession, however, to the demand of the “man in the street” for a record of conditions prevailing in his own sphere, the Weather Bureau has installed in several cities little pavilions in which working meteorological instruments are displayed for the benefit of the public. The thermometers in these so-called “kiosks”—which are modeled, with improvements, after the weather pavilions found at European health resorts—always read several degrees higher in hot weather than the thermometer at the regular Weather Bureau station in the same vicinity. Such records are erratic, at best, and present indications are that the kiosks will eventually be abolished.

MAXIMUM AND MINIMUM THERMOMETERS

Besides the ordinary thermometer, there are instruments that answer the questions “How hot was it to-day?” and “How cold was it last night?” These are known, respectively, as the maximum and the minimum thermometer. They hang almost horizontally in the screen. The former has a constriction just above the bulb, which prevents the mercury from retreating after it has reached the highest reading for the day. It can be reset by whirling it on a pivot. The minimum thermometer is filled with spirit instead of mercury. A little index inside the column is carried toward the bulb by the surface of the alcohol as the temperature falls. When the temperature rises the index remains behind, marking the lowest point reached. The highest and lowest temperature of the day, as well as the temperature at any moment of the day, can be read from the thermograph, or self-registering thermometer. In the commonest type of thermograph changes of temperature alter the curvature of a flexible metal tube filled with spirit, and the movements of the free end of the tube are communicated by levers to a recording pen.

On an average day, in our climates, the air is coldest about sunrise. The appearance of the sun checks the atmospheric cooling due to the loss of heat from the earth that has been going on through the night, and the air begins to warm up. As long as the amount of incoming heat from the sun is greater than the amount of outgoing heat from the earth, the temperature will continue to rise. After noon, when the sun is highest, the supply of solar heat diminishes, but it is still greater, for a time, than the heat loss from the earth, and for this reason the temperature, as a rule, keeps on rising until some time toward the middle of the afternoon, when the maximum temperature of the day occurs.

Humidity is an element of weather that is more often talked about than understood. Atmospheric humidity is the state of the atmosphere with respect to the amount of moisture it contains in a gaseous form, not in the form of a liquid. This gaseous moisture is called water vapor, and it is not directly perceptible to the senses, as liquid water is. As we have explained elsewhere, the capacity of the air for water vapor increases with the temperature. The actual amount present at any time, per unit volume, is called the absolute humidity, and the ratio of this amount to the maximum amount the air can hold at the same temperature is called the relative humidity. The latter is generally expressed in percentage. When the air is charged to its full capacity with aqueous vapor its relative humidity is 100 per cent.