Atmospheric Sounding

the determination of the vertical or horizontal distribution of temperature, humidity, pressure, wind, and other physical parameters of the atmosphere. Vertical atmospheric sounding has the greatest significance. Many methods of vertical atmospheric sounding exist; among them are the radiosonde method, the optical method (with a laser beam), the acoustic method (by means of sound), the radar method, and the rocket method. In acoustic sounding the temperature and wind distribution are determined from measurements of the time and direction of arrival of sound waves from the explosions of small grenades ejected by rockets.

The most common method of vertical sounding is done with radiosondes, which are miniature weather stations that are lifted to heights of 30–40 km by rubber or polyethylene balloons filled with hydrogen or helium. The temperature is measured with thermistors (less frequently, with bimetallic deformation thermometers), the pressure is measured with membrane manometers, and the humidity is measured with film or electrochemical hygrometers. A radiosonde transmits the results of the measurements continuously by radio; the results are recorded at the release point. The velocity and direction of the wind in the layer to which the radiosonde ascends are determined by means of radar sets that continously determine the device’s spatial coordinates. Radio-sondes are released several times a day at specific hours. The atmospheric sounding results produced at more than 800 points by radiosondes in various geographcial regions are the main initial materials for the compilation of weather forecasts. For scientific research purposes, in addition to the large-scale use of radiosondes, special radiosondes are launched periodically to measure the composition of the atmosphere, radiation fluxes, and other quantities.

At great altitudes (up to 100 km or more) atmospheric sounding is conducted by means of meteorological rockets that carry in their nose cones instruments that are released on a parachute after the maximum altitude is reached. Measurements are made of the density, temperature, wind, and on scientific research launchings, the composition of the air and the intensity and spectrum of the solar radiation. Some of the measurements are made during the rocket’s ascent; others, while the instruments are descending by parachute. The results are transmitted by radio and are processed on electronic computers. The temperature is determined by electrical thermometers or from the air density data; at altitudes above 80–90 km it can be calculated from the diffusion velocity of artificial clouds emitted by the rocket. Wind measurements are made by radar tracking of the drift of the rocket’s nose cone (during its descent by parachute) or of clouds of artificial reflectors.

Since the stations for radiosonde and rocket atmospheric sounding provide only 20 percent of the meteorological information required for weather forecasting, with the vast ocean, polar, and mountain regions remaining almost uncovered, a very important role is played by weather satellites, which make possible the collection of meteorological data over all areas of the globe. The wind in the free atmosphere is determined by analyzing data on the shapes of clouds and their drift that is obtained from photographs taken by satellites by daylight or infrared light. The vertical temperature profile can be calculated from measurements of the spectral distribution of the outgoing thermal radiation from the earth-atmosphere system, since the radiation intensity is a definite function of the temperature. The measurements are made in narrow portions of the spectrum that correspond to the absorption bands of the gases with a stable and thoroughly studied vertical distribution in the atmosphere. The absorption bands of CO2 (4.3 and 15 microns) and O2 (5 mm) are used for this purpose. The vertical profiles of water vapor, ozone, and other variable parts of the atmosphere’s gaseous composition may be calculated for a known temperature distribution from the measurement data of the outgoing radiation in the absorption bands of these gases.

Methods of atmospheric sounding have been developed using lasers and also radio waves of various wavelengths. Horizontal sounding is done occasionally for scientific research purposes or for weather reconnaissance. Instruments are lifted on automatic free balloons that drift for a long time at specific heights and automatically transmit the results of their measurements by radio. Horizontal atmospheric sounding is also conducted by means of airplanes equipped with recording apparatus; aerial photographs of the clouds are sometimes also made in flight.