In view of man's dependence upon the primary producers, it seems logical that we should select those components of climate that are vital to plant growth. Plants occupy both marine and terrestrial environments. Because plant life of the lands is most directly exposed to the atmosphere for exchanges of energy and matter, we will want to concentrate on climate of the land-atmosphere interface.
Climate classification - Plants require exposure to solar radiation in order to carry on photosynthesis (the process by which carbohydrate molecules are synthesized from CO2 and H2O); plants require sensible heat as measured by air and soil temperatures within specific limits; plants require water in the capillary water form in the root zone of the soil. Although land animals are consumers, they too require fresh water and a tolerable range of surrounding (ambient) temperature.
Our problem in climatology is therefore to select categories of available information that correlate closely with the needs of life on the lands. We can use the same basic information that enters into the global balances of radiation, heat, and water. Two or more categories of information may then be combined and refined into a finished climate classification system.
If we have done our work well, the regional units within the climate system will strongly reflect the control of the atmosphere upon terrestrial life and will give some indication in the opportunities and constraints that the atmospheric environment imposes on man as he seeks to increase his food supplies and water resources at the same time that he extends his areas of urban and industrial land use. The climate classification we seek must have utility in guiding land-use planning and population growth, as well as describing the environment.
Let us now review some of the categories of information that may prove useful in producing meaningful system of climate classification.
Temperature as a basis of climate classification - Temperature of the lower air layer, has long provided the first variable quantity in leading systems of climate classification. Monthly data based on daily readings of the maximum-minimum thermometer have been accumulated for many decades at thousands of observing stations the world over. Thus, availability has been an important factor in favoring the use of air temperature in climate classification.
Using monthly mean air temperature, three major climate groups can be defined:
(1) winterless climates of low latitudes,
(2) climates of middle latitudes with both a summer and a winter season,. and
(3) summerless climates of high latitudes. A winterless climate is commonly defined as one in which no month of the year has a monthly mean temperature lower than 64.4°F (18°C). Note that the isotherm shows a considerable latitude range, bending equatorward over the cool west coast ocean currents and over the land in North Africa and Australia.
A summerless climate is commonly defined as one in which no month has a monthly mean temperature higher than 50°F (10°C). The 50°F (10°C) isotherm of the warmest month closely coincides with the northernmost limit of tree growth, hence it separates the regions of boreal forests from the treeless arctic tundra.
Climate having both a summer and a winter season lie in the regions between the two boundary isotherms described above and constitutes a middle latitude group.
Air temperature is an important environmental factor in plant physiology and reproduction; it enters into many activities of animal life as well (hibernation, migration). For man, air temperature is an important physiological factor and relates directly to the quantity of energy expended in space heating of buildings. Nevertheless, air temperature alone does not define meaningful climate classes, since the ingredient of water availability is missing. The winterless climates, for example, include extremes of dry and moist environments.
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