272 PHYSICAL LIMITATIONS HAVE BEEN OVERCOME often lose their homogeneity and contriists at the surface to the extent that it is difficult to identify them, but it has been found that the upper hiyers of these moving air masses change less than the lower layers and that certain properties of these upper layers may be used to identify these air masses. This is one of the chief purposes in using the radiosonde. Weather experts expect forecasting to be greatly improved by this study of the movements of air masses on a three-dimensional scale. The chief sources of these large air masses are the polar and tropical regions. When air masses from these source regions meet, the warm, moist masses are forced above the cold, dry masses and thus form clouds and precipitation as they are cooled by expansion. It is these great vertical motions which are responsible for practically all precipitation. Slight precipitation may sometimes take place when air currents of different temperatures are mixed, but this happens on a very small scale compared with the amount of precipitation brought about by the expansional cooling of rising air. A Knowledge of Meteorology Is of Major Importance to Aviators. Airplane pilots are advised to avoid flying through thunderstorms. Lightning seldom does any more harm than temporarily blinding the pilot. It is the severe turbulence in thunderstorms and the possibility of hail and icing conditions that may damage the airplane or throw it out of control that cause pilots to fly around thunderstorms if possible. It is difficult to fly over a thunderstorm because the upward convection currents ascend to such high altitudes. The formation of ice on aircraft is caused by flying through rain or cloud particles which are below a temperature of 32° F. Supercooled water droplets exist in the majority of clouds when the temperature is below 32° F. This supercooled water is in a very unstable state and freezes quickly when disturbed. It is possible to form ice at temperatures between 32° and 35° F. because of the expansional cooling of air over the airfoil (wind surfaces). Formation of ice on propellers and wings decreases the "lift" of the airplane and should be avoided. Some airplanes are equipped with de-icing devices; in some cases small portions of the wing surfaces are heated electrically or by engine exhaust gases. Pneumatic (rubber) covering the leading edges of the wings which can be alternately inflated and deflated causes the ice to break off. Propeller blades may be protected against ice by allowing de-icing fluids to which the ice will not adhere to stream over the blades. In general, however, the pilot avoids, if possible, layers of air in
which ice will form. WEATHER-FORECASTING 273 Weather predictions and observations made while in flight enable the pilot to know when to expect ice formation. Weather-forecasting Benefits Man in Many Ways. Warnings of storms are displayed at more than four hundred points along the coasts and the Great Lakes. A white flag means fair weather, a square dark-blue flag means rain or snow, while a red flag with a black center is the signal to prepare for a violent storm. Two such flags give warning of the dreaded hurricanes. Such warnings enable vessels at sea to prepare for the hurricane as far as possible or make for the nearest port. the danger is over. Millions of dollars are saved by detaining ships until Frost warnings are very important to market gardeners, fruit-, tobacco-, and cranberry-growers, for they are thus enabled to prepare to save their crops by the use of smudge pots and other means. Transportation companies are enabled to protect perishable shipments en route or to refuse to accept such products for shipment until the danger is past. Warnings of snowstorms enable highway departments and railroads to be ready with snowplows and decrease or halt traffic if necessary. Sheep and cattle ranches must prepare to protect their livestock against blizzards and heavy snows. Week-end winter-sport carnivals are planned and called off on the basis of weather forecasts. One of the most important services of the Weather Bureau is that rendered to aerial transportation. The United States Weather Bureau maintains about fifty airport weather stations, while a great many more airports maintain their own weather service. STUDY QUESTIONS 1 What data are required for successful weather prediction? 2. How far ahead may the weather be fairly reliably predicted today? 3. What is meant by air-mass analysis? Of what value is it? 4. Give a few examples to show the value of weather prediction. 5. What are cyclones and anticyclones? 6. What does a pronounced barometric change indicate? Give a reason for your answer. 7. Discuss the value of the observation of clouds in weather prediction. 8. What causes the ring around the moon? What is its significance? 9. Why is it difficult to forecast weather? 10. 11. How are data concerning air masses at high altitudes collected? Why should airplane pilots avoid thunderstorms? 12. What causes ice to form on airplane wings? 13. What is the ceiling, and how is it measured? 14. What is the radiosonde?