How the Effect of the Seasons Varies with Latitudes.—If we study the people of different latitudes we find that the periods of greatest and least energy occur at different times. In northern Maine or Minnesota, and still more in Canada, there is only one unfavorable period, the winter. People are at their best from July to September; then their health and efficiency decline steadily as the cold winter comes on, and in January and February reach a lower level than during the same months in New York, Chicago, or St. Louis, for ex ample. Farther south, as we have just seen, there are four periods, two of good health in spring and fall, and two of bad health in summer and winter, but the contrast between the good and bad is not so great as in latitude a little higher or lower. In still lower latitudes there are once more only two periods, but in such warm regions the long sum mer is the unfavorable time, while the short winter is favorable. In central Florida, for example, the long warm summer shows a pronounced decline in health lasting about six months, while the short winter is much the best part of the year. The loss of health and strength due to continued warm weather becomes more pro nounced the farther one goes toward the equator.
In the other continents similar conditions prevail. In Europe, in the latitude of Central France and Southern Germany, the seasonal variations of health and strength are much the same as in Boston, New York, Cleveland, and Detroit. That is, people are most healthy and strong in October and early November and again in May and early June, while they are weakest and most subject to disease in January, February, and early March, and again in July or August. There, as here, great variations occur from year to year, according to whether the weather is unusually hot or cold, rainy or dry, variable or monotonous. Farther north, for example, in Scotland, Scandinavia, and Finland, the summer is the best time of the whole year and the winter the worst. To the south, on the contrary, in Italy, Spain and Greece, the harmful effect of the winter decreases and that of summer increases, until finally on the south side of the Mediterranean the winter is much the best time of the whole year, while the long sum mer greatly diminishes people's efficiency and increases disease and deaths.
How the Periods of Mental and Physical Activity Differ.—In general people's mental activity varies from season to season in the same way as physical activity, but there is an interesting difference in one respect. In the latitude of New York, for example, people do the best brain work about the end of November or beginning of December, that is, a month or six weeks after their physical strength is greatest. In the spring, on the contrary, the best mental work is done in March, a month or two before the physical strength reaches its maximum. This means that people's minds are most stimulated in weather somewhat cooler than that which most stimulates their bodies.
The Optimum Temperature.—Three conditions of climate are of
special importance in their influence on health and energy: (1) temperature; (2) humidity; and (3) variability. For each of these there is a certain most favorable or ideal condition which is called the optimum. Every species of plant and animal has an optimum temperature at which it thrives most vigorously, and man is no ex ception. The optimum may varya little from individual to individual, but not much. It is more likely to vary from one type of activity to another. For physical health, among the white race, the best tem perature is an average of not far from 64° F. for day and night together. In other words, people's health andstrength are greatest when the thermometer drops to about 56° to at night and rises to somewhere between 68 and during the middle of the day. For mental activity the optimum temperature appears to be a good deal lower than for physical, being probably about 40°. As a rule, people's minds are most alert and inventive, people do their best thinking and planning' and have the best judgment when the thermometer falls about to freezing at night and rises to perhaps 45° or 50° by day.
All human progress depends on activity of both mind and body; an active engineer, for example, is needed to plan a system of water works, an active day laborer to dig the ditches. Hence the best climates appear to be those with an average temperature of not far from 40° during the winter and of about 64° during the summer, but other conditions, such as variability, humidity, and rainfall may alter this. Southeastern England comes nearer to this ideal condition than any other part of the world.
The Optimum Humidity.—When the temperature averages about and is therefore close to the optimum for health, the best condition of moisture seems to be a relative humidity of about 80 per cent for day and night together. This means that the air contains about 80 per cent as much moisture as it is capable of holding at that particular temperature. When the air grows cool at night, its capacity for moisture becomes less. Hence the water vapor that it contains increases relatively when compared with what it is capable of con taining, and at length becomes 100 per cent at a temperature of Then dew begins to form, since the air must give up some of the moisture. By day, on the other hand, when the thermometer rises to the air contains only about 65 per cent as much moisture as it might if it were saturated. When the relative humidity at noon, however, falls much below 50 per cent, the delicate mucous membrances of the nose and throat begin to suffer, the skin is likely to be too dry, people tend to become nervous, and the general con ditions of health decline. It must be remembered, however, that the outdoor life which is possible in a dry climate makes up for much of the harm due to dryness. If people were wise they would have as much outdoor life and outdoor air in moist climates as in dry.