Page:Handbook of Meteorology.djvu/204

 High-air Thermographs.—High-air temperature observations are usually obtained by thermographs secured to kites or balloons. In manned balloons a very sensitive thermograph is contained in a tube through which a current of air is forced. This instrument, the Assmann aspirator, is far more convenient than an ordinary thermograph. Experience has shown that unless the air is in rapid motion, registration is too slow to be trustworthy. A mechanical fan moves the air through the tube at the rate of about 12 feet per second.

In another form of instrument decreasing pressure moves a plate in one direction while the stylus of a bi-metallic thermometer records with a motion at right angles thereto. A clock is not required in this type of instrument; it is therefore lighter and more convenient.

The Black-bulb Thermometer.—This instrument, now little used, consists of a maximum standard thermometer, the bulb of which is covered with a coat of lampblack and encased in a vacuum tube. Originally it was designed for the measurement of solar radiation. A thermometer of this sort, exposed to direct sunshine, registers a temperature many degrees higher than does an ordinary thermometer, but the degree varies according to the thickness and the quality of the lampblack. It is therefore a very imperfect instrument for the purpose designed.

The black-bulb thermometer roughly measures the temperature which popular tradition terms “sensible” heat; exposed to direct sunshine, the temperature registered is from a few degrees to 60 or more degrees higher than the temperature registered by the ordinary thermometer. With a high humidity, or in smoky, dusty or foggy air, the black-bulb thermometer registers much lower than in clean air.