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Rh between the warmest and the coldest month in San Francisco is 10 degrees, in New York it is 44 degrees.

In studying the temperature of a locality, therefore, in addition to the question of mean annual temperature, various other elements must be taken into consideration. In the main, these are the daily range, the range of monthly means, and the seasonal range. These are affected in turn by latitude, by altitude above sea level, by the direction of prevailing winds, and by distance from the sea. In a minor way they are also affected by the moisture and the smoke content of the air.

The mean annual temperature of a place varies but little from year to year. In New York City, the range of yearly means has varied about 6 degrees in ninety-seven years. The average of each ten-year period for that time varies but a trifle from the normal of 52°. The records of Cooperstown, New York, have been kept continuously since 1854. The averages of ten-year periods show neither apparent gain nor loss in temperature.

Temperature Ranges.—The daily, monthly, yearly and extreme ranges all have an important bearing on the climate of a region. The daily range is a part of the records of every Weather Bureau station; and the greatest daily range in each month is an item of separate record.

The various ranges are usually, though not at all stations, least in tropical regions and greatest in inland regions where the humidity is low. In temperate latitudes they are lower on the coasts than in the interior. In the United States the average daily range is somewhat less along the Pacific Coast than along the Atlantic Coast; and the daily ranges of inland stations are greater than those of coast stations. The reason therefor is that the drier air of inland stations permits greater radiation of heat at night and greater absorption during the day. For a similar reason the daily ranges at stations of considerable altitude are apt to be greater than those at or near sea level.