Mean annual temperature, °F Figure 2. Temperature preferences of the world's population.

not only the human desire for comfort that dictates such a narrow temperature range, but also that these mean temperatures are best tolerated by the agricultural crops and domesticated animals on which man depends for food.

Temperature is obviously not the only parameter that governs where people want to live or do live. Such considerations as water supply, humidity, precipitation patterns, soil conditions, the availability of energy supplies, harbors, altitude, topographical roughness, political pressures, and proximity to other people are also highly important. But temperature is the overriding factor.

There are many instances of living organisms that are tolerant to high temperatures and of other organisms that are tolerant to low temperatures. Some examples follow (Spector, 1956): Certain cyanophyta (blue-green algae), notably Oscillatoria filiformis, have been observed living in water at a temperature of 185°F. Another cyanophyte, Mastigolocladus laminosus, encounters a maximum of 149°F in its natural environment and is said to tolerate a minimum of —2°F. Ducks have survived 16 days at — 40°F air temperature. Cattle have survived 2 weeks at 9°F and, in other experiments, 24 hours at 106°F. Nadrix sipedon, a water snake, is reported as having a temperature tolerance range from 32°F to 110°F. The American cockroach (Periplaneta Americana) has survived for 24 hours at 120°F. The arctic fir (Abies excelsa) is said to carry on photosynthesis from —40°F to +86°F, while the reindeer lichen (Cladonia rangiferina) is reported as having an active photosynthesis range from —4°Fto + 100°F. The morning-glory has been observed in active respiration from 50°F to 140°F.

Other examples of extreme temperature tolerance are as follows (Troitskaia, 1952): The majority of plants cease their functions at temperatures below 32° F but are capable of surviving through temperatures as low as — 60°F or — 80°F. The seeds of some plants can withstand temperatures even down to — 310°F.

Many more examples could be cited to show the remarkable adaptability of some forms of plant and animal life. Nevertheless, the vast majority of important food crops require temperatures within the range of 50°F to 86°F during their growing seasons. The seeds of most herbaceous and woody plants require a temperature within the range of 50° F to 86°F for seed germination. Most fish cannot survive water temperatures much below 32°F or above 86°F. In summary, although a few hardy species of plants and animals can withstand more or less prolonged exposures to very high or very low temperatures and a few species have become adapted to hot or cold environments, most of the plants and animals important to man as sources of food and as suppliers of oxygen through photosynthesis require temperatures above the freezing point of water and below 86°F for survival and active growth.

In addition to the mean-annual-temperature limitation, there is also a limitation because of the daily temperature extremes experienced at the warmest and coldest seasons of the year. It is more difficult to set sharp limits on daily temperature extremes. However, on the basis of human tolerances (see Figure 3) and assuming that human beings would not want to remain indoors constantly for long periods of time, it can be seen that mean daily temperatures of 104°F (40°C) and 14°F (-10°C) at the hottest and coldest seasons might represent reasonable limitations. It should be remembered that daily temperature extremes could typically go far above and below these mean daily values. Thus, these limits represent more rigorous conditions than might seem initially to be the case.

Furthermore, a reasonable fraction of a planet's surface area (say, at least 10 per cent) should possess the required conditions of temperature in order for it to be called habitable.

Regarding the temperature requirements, therefore, we specify that a region is habitable only if the mean annual temperature lies between 32°F (0°C) and 86°F (30°C), if the highest mean daily temperature during

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