270 PHYSICAL LIMITATIONS HAVE BEEN OVERCOME It Is Important to Observe the Clouds in Weather-forecasting. The best known clouds are the fair-weather cumulus clouds. These snow-white billowy piles which project upward into the sky have dark, flat bases. In "muggy" weather these clouds become thunderheads. Stratus clouds are low, flat, foglike clouds of wide extent. Another type is the nimbus, from which precipitation takes place. are very high, wispy, rapidly moving clouds of fine ice crystals. Cirrus clouds They often appear as the advance guard of a cyclone, having outstripped the Fig. 89. Weather map retouched to show lines of equal pressure and equal temperature. Compare this map with the one shown in Fig. 88. storm that is following. As the storm gets nearer, these clouds form slightly lower, extensive, white veil-like layers called cirro-stratus clouds. The velo cloud of southern California is the very thick high fog that drifts in from the sea. As it reaches the land, the lower portions are evaporated by the heat from the land, leaving the higher portion to be dissipated later by the heat of the sun. The Zuiii Indians had an adage: "When the sun is in his house it will rain soon." Large, nearly colorless rings around the sun or moon, known as halos, are produced by the refraction of light by the ice crystals of very tenuous cirrus clouds. The larger the rings appear to be, the closer is the approach of the storm, because the ring, which in reality is always of the same diameter, gives the appearance of an increase in diameter as the clouds come closer to the earth when the storm is approaching.
WEATHER-FORECASTING 271 Air-mass Analysis Is Expected to Improve Weather-forecasting. Airplanes were formerly used to obtain data concerning the upper atmosphere. The airplanes carried an aero-meteorograph, which recorded the pressure, humidity, and temperature encountered at different altitudes. Adverse flying weather and the time required to ascend to the 16,000 to 20,000 feet at which observations were made limited the use of airplanes. During the past few years the radiosonde or radio-meteorograph has replaced the use of the airplane. The radiosonde makes the same observations that the aero-meteorograph did, but it contains a radio transmitter that sends radio signals to a ground receiving station. The radiosonde weighs only two pounds, and one instrument costs less than it costs to make a single high-altitude flight with an airplane. The instrument is carried aloft by a balloon which bursts at very high altitudes; the instrument is carried to the ground by a parachute. In addition to the use of the radiosonde the direction and force of the wind are noted at many stations by observations of pilot balloons with special instruments called theodolites. The azimuth and ^ elevation angles are read each minute by the observer and communicated by phone to the computer at the plotting-board in the office. From these angles the wind velocity and direction are computed for various altitudes. The nephoscope, which consists of a black mirror in a circular frame graduated in degrees and a movable sighting eyepiece stand, is used in the determination of the direction and motion of clouds. The ceiling is the distance from the ground to the base of the clouds. At night the ceiling is determined by throwing vertically upward a beam of light from a point five hundred to a thousand feet or more from the point of observation. Knowing this fixed horizontal distance, one can calculate the ceiling from the angular elevation of the spot of light on the cloud from the observing-point. In the daytime balloons are used for ceiling observations. The time that it takes for the balloon to disappear into a cloud and the known rate of ascent of the balloon furnish the data required to calculate the ceiling. It has been known for many years that extensive masses of air, fairly homogeneous as to temperatures and humidity, often move from certain source regions out over the face of the earth. The weather at any particular place depends largely upon the type of air mass present, the modifications it has undergone in its history, and the way it is interacting with adjacent masses. As these air masses move, they ' The azimuth is measured in degrees east or west along the horizon from the south point or the north point.