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Fig. 2. The earth's motions in relation to the sun, to show how inclination of the axis of rotation (N-S) causes alternation of summer and winter in the higher latitudes. Diagrammatic: neither scale nor perspective is true.
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The greatest amount of heat falls on the tropics. The tropics are by definition the zone on which the sun's rays fall perpendicularly. The limits of the zone are therefore the northernmost and southernmost latitudes at which the sun is directly overhead, at the extremes of the earth's annual cycle. These latitudes are 23° 27' north and south of the equator.
In general, the tropics are hot throughout the year: maximum temperatures, at least in the wet tropics, are lower than those reached on hot summer days in many parts of the north-temperate zone, but the tropical heat is maintained within a few degrees at all seasons and by night and day.
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North and south of the tropics the heat received from the sun diminishes approximately with the curve of the earth, and becomes increasingly seasonal.
In the temperate zones annual cycles of summer and winter appear, and the winters increase in severity with distance from the tropics; and still farther around the curve of the earth, across the Arctic and Antarctic Circles (which are about 23° 30' from the poles), there are periods in summer when the sun never sets, and in winter when it never rises.
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Across the Arctic and Antarctic Circles (which are about 23° 30' from the poles), there are periods in summer when the sun never sets, and in winter when it never rises. The periods increase in length toward the poles, becoming six months of day and six of night at the poles themselves. Alternation of seasons and severity of winters in the north may limit animal distributions more than mean temperature does.
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On mountains, temperature falls with altitude, not because less heat is received from the sun (the sun's rays burn intensely at high altitudes) but because the air becomes thinner and therefore colder. The decrease of temperature with altitude can be compared with the decrease northward
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If conditions of snow and ice at 16,000 feet on mountains near the equator (e.g. on the Carstensz Mountains of New Guinea-Umbgrove 1949, PI. IX) are comparable to those at sea level at 80° North, then it might be said that 200 feet of altitude has the same effect on climate as 1° of latitude northward.
There is some truth in this comparison, but it is a partial truth and a misleading one.
NB. Each degree of latitude is approximately 69 miles (111 kilometers) apart.
The range varies (due to the earth's slightly ellipsoid shape) from68.703miles (110.567 km) at the equator to 69.407 (111.699 km) at the poles. This is convenient because each minute (1/60th of a degree) is approximately one [nautical] mile.
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Degrees of latitude are parallel so the distance between each degree remains almost constant but since degrees of longitude are farthest apart at the equator and converge at the poles, their distance varies greatly.
Each degree of latitude is approximately 69 miles (111 kilometers) apart. The range varies (due to the earth's slightly ellipsoid shape) from 68.703 miles (110.567 km) at the equator to 69.407 (111.699 km) at the poles. This is convenient because each minute (1/60th of a degree) is approximately one [nautical] mile.
A degree of longitude is widest at the equator at 69.172 miles (111.321) and gradually shrinks to zero at the poles.At 40° north or south the distance between a degree of longitude is 53 miles (85 km)
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Degrees of latitude are parallel so the distance between each degree remains almost constant but since degrees of longitude are farthest apart at the equator and converge at the poles, their distance varies greatly.
Each degree of latitude is approximately 69 miles (111 kilometers) apart. The range varies (due to the earth's slightly ellipsoid shape) from 68.703 miles (110.567 km) at the equator to 69.407 (111.699 km) at the poles. This is convenient because each minute (1/60th of a degree) is approximately one [nautical] mile.
A degree of longitude is widest at the equator at 69.172 miles (111.321) and gradually shrinks to zero at the poles.At 40° north or south the distance between a degree of longitude is 53 miles (85 km)
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The effect of moving 1° north would be different at different places between the equator and the pole. The effect would be very small in the tropics and would gradually increase across the temperate zone, as the surface of the earth curved more and more away from the sun. But this is a minor criticism.
The main criticism is that the decrease of temperature upward and the decrease northward are inherently different and have very different effects on life.
Bio-geographers knew this but have not sufficiently emphasized it. They customarily call the successive altitudinal zones on tropical mountains
"subtropical," "temperate," and "arctic."
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Altitudinal climates in the tropics are not the same as the northern climates for which they have been named. Climate in the north-temperate zone is cool and very seasonal; climate at high altitudes in the tropics is cool but nearly uniform throughout the year. In the north, a mean temperature of 50° F. may include extremes of from 100° to zero. High on a tropical mountain the mean may be the same, but the temperature may never fall to freezing. That is one of the reasons why palms that thrive in the "temperate" zone on tropical mountains cannot exist in most parts of the North Temperate Zone.
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Rainfall, too, tends to occur in zones. Rain depends on wind, which picks up water vapor from oceans or other wet surfaces and carries it to where it condenses and falls. The basic zonation of rainfall is therefore determined by the earth's pattern of atmospheric pressure and wind
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There are zones of high pressure just outside the tropics in both the northern and southern hemispheres; winds blow away from the high-pressure zones, taking water with them; and that leaves deserts.
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There are zones of high pressure just outside the tropics in both the northern and southern hemispheres; winds blow away from the high-pressure zones, taking water with them; and that leaves deserts. Most of the great deserts of the world, in North Africa, southwestern Asia, and southwestern North America, and in Southwest Africa, Australia, and southwestern South America, lie in the high-pressure zones near or across the edges of the tropics. The deserts of east-central Asia (the Gobi, etc.) are exceptions; they owe their dryness to (among other things) distance from the sea.
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Most of the great deserts of the world, in North Africa, southwestern Asia, and southwestern North America, and in Southwest Africa, Australia, and southwestern South America, lie in the high-pressure zones near or across the edges of the tropics.
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Winds (trade winds) tend to blow into the tropics from the subtropical high- pressure zones, bringing water with them; and that makes tropical rain and rain forest. And winds tend to blow north and south from the high-pressure zones into cool temperate regions, meeting other more variable winds and bringing variable rain.
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This basic zonation of rainfall is modified, distorted, and interrupted in many ways. The size and form of land masses affect rainfall locally. On any given piece of land, rain is usually heaviest near the sea, on the windward side, and on mountains. Where the land is irregular and winds come mostly from one direction, rainfall may vary greatly in short distances.
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Such abrupt differences of vegetation, reflecting local differences of rainfall, are common in the tropics. In many places in the tropics rainfall is seasonal, and the annual dry seasons may be so pronounced that forests lose their leaves, most insects disappear, and some birds migrate to feeding grounds hundreds of miles away.
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The effect of oceans on land climates involves both temperature and rainfall.
In general, oceans moderate extremes of temperature on nearby land. In special cases ocean currents carry warm (or cool) climates far beyond expected limits.
Oceans make rainfall, too. The water they give up by evaporation falls as rain wherever winds take it, especially where the winds first come against land and mountains.
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The factors that make climate a partly zonal, partly local, very complex thing have existed ever since the surface of the earth has consisted of land and water. The earth has always been a revolving sphere with an equator which receives much heat from the sun and poles which receive little, so climates must always have been somewhat zonal, although the zonal climates may sometimes have been less differentiated than now. Moreover, the earth's axis has probably always been inclined so that, as the earth passes around the sun, northern and southern lands have always had alternately warm and cold seasons which increase the effect of zonation of climate on the distribution of life. The details of the relation of land to water and wind must have differed very much at different times in the past, as different pieces of land have changed their shapes and heights, but they must always have made a diversity of local climates. Every existing continent is partly wet and partly dry in a complicated pattern, and every continent has probably always been wet in some parts and dry in others.
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