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Rh late frosts prevail, fruit growers have learned to take advantage of this fact. The difference between the temperature of a low spot and higher ground a few rods away is at times the difference between freezing and non-freezing temperature.

In tropical regions where mountains lie against the coast the difference between sea level temperature and that of the foot-hills a thousand feet higher is very marked. Thus, the temperature of the business district of Victoria, Hongkong, is almost intolerable to Europeans; on the Peak, a few hundred feet higher, the climate is pleasant. The same difference is noticeable between Rio Janeiro and its suburb, the Corcovado; it is even more noticeable in comparing the climate of Vera Cruz with that of Puebla or Orizaba.

On the other hand, extremely hot days in the foot-hills of the Sierra Nevada Mountains are apt to be cool days along the coast. The explanation is not hard to find: the ascending hot air of the foot-hills is replaced by cold air blowing in from the ocean.

In many instances the difference between low valley and hill stations is quite as much hygienic as climatic. It is the difference between moist, dusty and miasmatic air on the one hand; and clear, dry air on the other.

Temperature and Latitude.—In general, the mean temperature of the air decreases as latitude increases. In the southern hemisphere, which has chiefly an ocean surface, the decrease is quite regular and the direction of the isotherms does not vary much from that of the parallels. In the northern hemisphere the decrease is by no means regular, and the isotherms wander greatly from the parallels.

The following illustrates the decrease in the United States, as affected by latitude. In column I, the stations from south to north are approximately along the ninety-sixth meridian; in column II they are situated along the Atlantic Coast.