Climate

I

Introduction

Climate, the long-term effect of the Sun's radiation on the rotating Earth's varied surface and atmosphere. It can be understood most easily in terms of annual or seasonal averages of temperature and precipitation.

Land and sea areas, being so variable, react in many different ways to the atmosphere, which is constantly circulating in a state of dynamic activity. Day-by-day variations in a given area constitute the weather, whereas climate is the long-term synthesis of such variations (both can be viewed as subdisciplines of meteorology). Weather is measured by thermometers, rain gauges, barometers, and other instruments, but the study of climate relies on statistics. Today, such statistics are handled efficiently by computers. A simple, long-term summary of weather changes, however, is still not a true picture of climate. To obtain this requires the analysis of daily, monthly, and yearly patterns. Investigation of climate changes over geological time is the province of palaeoclimatology, which requires the tools and methods of geological research.

The word climate comes from the Greek klima, referring to the inclination of the Sun. Besides the effects of solar radiation and its variations, climate is also influenced by the complex structure and composition of the atmosphere and by the ways in which it and the ocean transport heat. Thus, for any given area on Earth, not only the latitude (the Sun's inclination) must be considered but also the elevation, terrain, distance from the ocean, relation to mountain systems and lakes, and other such influences. Another consideration is scale: a macroclimate refers to a broad region, a mesoclimate to a small district, and a microclimate to a minute area. A microclimate, for example, can be specified that is good for growing plants in the deep shade beneath trees.

Climate has profound effects on vegetation and animal life, including humans. It plays statistically significant roles in many physiological processes, from conception and growth to health and disease. Humans, in turn, can affect climate by changing their environment, both through the alteration of the Earth's surface and the introduction of pollutants and chemicals such as carbon dioxide into the atmosphere.

II

Climatic Zones

Climates are described by agreed-upon codes or by descriptive terms that are somewhat loosely defined but nevertheless useful. On a global scale, climate can be spoken of in terms of zones, or belts, that can be traced between the equator and the pole in each hemisphere. To understand them, the circulation of the upper atmosphere, or stratosphere, must be considered, as well as that of the lower atmosphere, or troposphere, where weather takes place. Upper atmospheric phenomena were little understood until the advent of such advanced technology as rocketry, high-altitude aircraft, and satellites.

Ideally, hot air can be thought of as rising by convection along the equator and sinking near the poles. Thus, the equatorial belt tends to be a region of low pressure and calms, interrupted by thunderstorms associated with towering cumulus clouds. Because of the calms, this belt is known as the doldrums. It shifts somewhat north of the equator in the northern summer and south in the southern summer. By contrast, air sinks in the polar regions. This leads to high atmospheric pressure, and dry, icy winds that tend to radiate outward from the poles.

Complicating this simplistic picture is the Earth's rotation, which deflects the northerly and southerly components of the atmosphere's circulation. Thus, the tropical and polar winds both tend to be easterlies (winds from the east), and two intermediate belts develop in each hemisphere. Around latitude 30° north and south is a zone of high pressure, where the upper air sinks and divides, sending air streams towards the equator. Steady north-east trade winds blow in the northern hemisphere, and south-east trade winds in the southern hemisphere. These high-pressure areas lead to arid areas on the continents but to moist air over the oceans, because of evaporation. If these trade winds meet an island or mainland coast, moist air is pushed up into cooler elevations, and heavy rainfall might occur.

Around latitude 50° to 60° north and south is a belt of low pressure characterized by the prevailing westerlies, which are deflected to the south-west in the northern hemisphere and to the north-west in the southern hemisphere. These are relatively mild, moist winds that tend to bring frequent cyclonic precipitation to all elevations along the west-facing side of continents. The precipitation is characterized by polar fronts, where cold air from the polar easterlies drives in under the warm, moist air of the westerlies, which, on cooling, drop their moisture. In winter this is the cause of most snowfall on continents.

III

Temperature and Precipitation Scales

Temperature is an important aspect of climate and can be used to grade climatic zones on a scale of five: (1) Tropical, with annual and monthly averages above 20° C (68° F); (2) Subtropical, with 4 to 11 months above 20° C, and the balance between 10° and 20° C (50° to 68° F); (3) Temperate, with 4 to 12 months at 10° to 20° C, and the rest cooler; (4) Cold, with 1 to 4 months at 10° to 20° C, and the rest cooler; and (5) Polar, with 12 months below 10° C.

Within each hemisphere, eight basic climatological zones can also be recognized in terms of precipitation: (1) Equatorial: rain in all seasons; (2) Tropical: summer rain with winters dry; (3) Semi-arid Tropical: slight summer rain; (4) Arid: dry in all seasons; (5) Dry Mediterranean: slight winter rain; (6) Mediterranean: winter rain, summers dry; (7) Temperate: precipitation in all seasons; (8) Polar: precipitation sparse in all seasons.

IV

Vegetation Classification

Both of the above meteorological parameters fail to meet the need for a true and universal climatic description. Vegetation, however, offers a useful guide, particularly in special cases, such as the selva, or equatorial rainforest belt, hot with tropical rain much of the year; the savannah, warm-hot, with strong seasonality; and the tundra, cold, with strong seasonality. It is a particularly helpful system for a person who wants to know the nature of an area and what it is like to live there. Because temperature relates to precipitation in terms of potential evaporation, a classification based on the latter two provides an excellent guide, with four fundamental divisions: hot-dry (arid), cold-dry (polar or glacial), hot-wet (selva), and moderate-warm to cool-humid (temperate).