Fig. 24.—Temperature. Sixth Day.
Study the maps individually at first. Describe the temperature distribution on each map. Ask yourself the following
questions in each case: Where is it coldest? Where warmest? What is the lowest temperature on the map? What is the highest? At what stations were these readings made?
Then compare the successive maps and answer these questions: What changes have taken place in the intervening 24 hours? In what districts has the temperature risen? What is the greatest rise that has occurred? Where? In what districts has the temperature fallen? What was the greatest fall in temperature and where did it occur? Has the temperature remained nearly stationary in any districts? In which? You will find it a help in answering such questions to make out a table of all the stations, and to indicate in columns, after the names of the stations, the number of degrees of rise or fall in temperature at each place during the 24-hour interval between the successive maps. When the temperature is higher at any station than it was on the preceding day, note this by writing a plus sign (+) before the number of degrees of rise in temperature. When the temperature has fallen, put a minus sign (-) before the number of degrees of fall. Thus, New Orleans, La., had a temperature of 48° on the first day. On the second it had 33°. Therefore the change at New Orleans was -15° in the 24 hours. At Key West, Fla., the change was +11° in the same time.
Write a brief account of the temperature distribution on each day of the series, and of the changes which took place between that day and the one preceding, naming the districts and States over which the most marked falls and rises in temperature occurred, with some indication of the amount of these changes. Note especially the changes in position, and the extent, of the districts with temperatures below -20°; between 0° and -20°, and between 30° and 0°. Write out a clear, concise statement of the temperature distribution and changes shown on the whole set of six maps.
Cold Waves.—The series of charts for these six days furnishes an excellent illustration of a severe cold wave.
A cold wave, as the term is now used by the Weather Bureau, means, during December, January, and February, a fall in temperature of from 20° to 16° in 24 hours, with a resulting reduction of temperature to between 0° and 32°, and, during the months from March to November inclusive, a fall of from 20° to 16° in 24 hours, with a reduction of temperature from 16° to 36°. During December, January, and February a cold wave means the following falls and reductions of temperature. Over the Northwestern States, from western Wisconsin to Montana, including Wyoming, Nebraska, and western Iowa, and over northeastern New York and northern New Hampshire, northern Vermont and northern Maine, a fall of 20° or more to zero or below; over southern New England and adjoining districts, the Lake region, the central valleys and west to Colorado, including northern New Mexico and northwestern Texas, a fall of 20° or more to 10° or below; over southern New Jersey, Delaware, eastern Maryland, Virginia, western North Carolina, northwestern South Carolina, northern Georgia, northern Alabama, northern Mississippi, Tennessee, southern Kentucky, Arkansas, Oklahoma, and southern New Mexico, a fall of 20° or more to 20° or below; over eastern North Carolina, central South Carolina, central Georgia, central Alabama, central Mississippi, central and northern Louisiana and central and interior Texas, a fall of 18° or more to 25° or below; along the Gulf coasts of Texas, Louisiana, Mississippi, and Alabama, over all of Florida, and over the coasts of Georgia and South Carolina, a fall of 16° or more to 32° or below. From March to November inclusive a cold wave means falls of temperature of the same amounts over the same districts, with resulting temperatures of 16°, 24°, 28°, 32°, and 36° respectively.
Notice that the region from which the greatest cold came in this cold wave is Canada. In that northern country, with its short days and little sunshine, and its long, cold nights, everything is favorable to the production of very low temperatures.
Cold waves occur only in winter. In the summer cool spells, with similar characteristics, may be called cool waves.