Page:Handbook of Meteorology.djvu/43

Rh The same results are seen in the temperature range in Europe, from Athens to Petrograd, or in South America, from Guayaquil to Punta Arenas. Within the tropics and also in polar regions, temperature changes due to latitude are not regular, nor are they great. In the main they are due to causes and conditions more or less local in character.

Mean Temperatures.—The daily, the monthly and the yearly means are required in weather service. The most accurate daily means would require the average of the hourly observations for the day; but the results would not be commensurate with the labor involved. The investigations of General A. W. Greely, while at the head of the U. S. Weather Bureau, showed that the average deduced from readings made at, , and , taking the last named twice and dividing by 4 gave a result very closely approaching the average of hourly means. This method has much to recommend it.

In the various Weather Bureau stations, where temperatures are recorded by regular observers, and in the various military field stations, the daily mean is found by taking half the sum of the daily maximum and the daily minimum. This mean is slightly in excess of the mean deduced by the preceding methods, but the error is so small that it may be disregarded.

The cooperative observer’s day begins and ends with the time that the maximum thermometer is set—usually from sunset to the following sunset. The daily mean thus established, however, does not differ materially from the true mean. The monthly and the yearly means are sufficiently accurate for practical purposes.

The yearly mean is deduced by dividing the sum of the monthly averages by 12. A closer average may be found by adding the monthly sums and dividing by the number of days in the year on which observations are made.

The mean annual temperature of a region is not a key to its temperature conditions or to its habitability. Thus, New York City and San Francisco, both seaports, situated not far apart in latitude, have about the same mean yearly temperature. But while the difference between the winter and the summer means in San Francisco is not more than 8 degrees, in New York it is about 32 degrees, and while the difference