| Altitude of sun. .. . Relative lengths of path through the | o° | 5° | 10° | 20° | 30° | 40° | 50° | 60° | 70° | 80° | 90° | atmosphere. .. ... . | 44.7 | io 8 | 5.7 | 2.92 | 2.00 | 1.56 | 1.31 | 1.15 | 1.06 | 1.02 | 1.00 | Intensity of radiation on a surface nor- mal to the rays | 0.0 | 0.15 | 0.31 | 0.51 | o 62 | o 68 | 0.72 | 0.75 | 0.76 | 0.77 | 0.78 | Intensity of radiation on a horizontal surface. .. .. ... . | 0.0 | 0.01 | 0.05 | 0.17 | 0.31 | 0.44 | 0.55 | 0.65 | 0.72 | 0.76 | 0.78 Values of Daily Insolation at the Upper Limit of the Earth's Atmosphere and at Sea-Level. The following table gives, according to W. Zenker, the relative thickness of the atmosphere at different altitudes of the sun, and also the amount of transmitted insolation: fact, in the higher latitudes, the former sometimes follow the meridians more closely than they do the parallels of latitude. Hence it has been suggested that the zones be limited by isotherms rather than by parallels of latitude, and that a closer approach be thus made to the actual conditions of climate. Supan 1 (see fig. 2) has suggested limiting the hot belt, which corresponds to, but is slightly greater than, the old torrid zone, by the two mean annual isotherms of 68°-a temperature which approximately coincides with the polar limit of the trade-winds and with the polar limit of palms. The hot belt widens somewhat over the Relative Distances traversed by Solar Rays through the Atmosphere, and Intensities of Radiation per Unit Areas. Physical Climate.-The distribution of insolation explains many of the large facts of temperature distribution, for example, the decrease of temperature from equator to poles; the double maximum of temperature on and near the equator; the increasing seasonal contrasts with increasing latitude, &c. But the regular distribution of solar climate between equator and poles which would exist on a homogeneous earth, whereby similar conditions prevail along each latitude circle, is very much modified by the unequal distribution of land and water; by differences of altitude; by air and ocean currents, by varying conditions of cloudiness, and so on. Hence the climates met with along the same latitude circle are no longer alike. Solar climate is greatly modified by atmospheric conditions and by the surface features of the earth. The uniform arrangement of solar climatic belts, arranged latitudinally, is interfered with, and what is known as physical climate results. According to the dominant control we have solar, continental and marine, and mountain climates. In the first-named, latitude is the essential; in the second and third, the influence of land or water; in the fourth, the effect of altitude. Classification of the Zones by Latitude Circles.-It is customary to classify climates roughly into certain broad belts. These are the climatic zones. The five zones with which we are most familiar are the so-called torrid, the two temperate, and the two frigid zones. The torrid, or better, the tropical zone, naming it by its boundaries, is limited on the north and south by the two tropics of Cancer and Capricorn, the equator dividing the zone into two equal parts. The temperate zones are limited towards the equator by the tropics, and towards the poles by the Arctic and Antarctic circles. The two polar zones are caps covering both polar regions, and bounded on the side towards the equator by the Arctic and Antarctic circles. These five zones are classified on purely astronomical grounds. They are really zones of solar climate. The tropical zone has the least annual variation of insolation. It has the maximum annual amount of insolation. Its annual range of temperature is very slight. It is the summer zone. Beyond the tropics the contrasts between the seasons rapidly become more marked. The polar zones have the greatest variation in insolation between summer and winter. They also have the minimum amount of insolation for the whole year. They may well be called the winter zones, for their summer is so short and cool that the heat is insufficient for most forms of vegetation, especially for trees. The temperate zones are intermediate between the tropical and the polar in the matter of annual amount and of annual variation of insolation. Temperate conditions do not characterize these zones as a whole. They are rather the seasonal belts of the world. | | | | | | | ?.? ?? ?. ? ? -.? ? , ?? | ?. ? | ? ? lD o ? Wa | ?gt ??? | | `"'u? | °...... ; ?` ? | | | ?? e | | ? ?? | | | ? TEMPERAT??` ?? | | | | ? ? | | . e ? ? -?, | | ? ?. `E , ?." ? | ? ?? . | ?, i: | | t | 'OT a? . | ? ? , _ I .?,? ? | ??: ? ,o`? ? ?? ? ? . e? | ?? ? ? ?? | ??. |
| | | | | | .-. 1 I soth ? | ? V | ? | | | | T '? BELT | | | | | | | | ? | ? | | Isothe? | ? o ` for War est '" | | | | | SOU | H COLD C ' P | | | | Temperature Zones.-The classification of the zones on the basis of the distribution of sunshine serves very well for purposes of simple description, but a glance at any isothermal chart shows that the isotherms do not coincide with the latitude lines. In continents, chiefly because of the mobility of the ocean waters, whereby there is a tendency towards an equalization of the temperature between equator and poles in the oceans, while the stable lands acquire a temperature suitable to their own latitude. Furthermore, the unsymmetrical distribution of land in the low latitudes of the northern and southern hemispheres results in an unsymmetrical position of the hot belt with reference to the equator, the belt extending farther north than south of the equator. The polar limits of the temperate zones are fixed by the isotherm of so° for the warmest month. Summer heat is more important for vegetation than winter cold, and where the warmest month has a temperature below 50°, cereals and forest trees do not grow, and man has to adjust himself to the peculiar climatic conditions in a very special way. The two polar caps are not symmetrical as From Grundzuge der physischen Erdkunde, by permission of Veit & Co. FIG. 2.-Supan's Temperature Zones. regards the latitudes which they occupy. The presence of extended land masses in the high northern latitudes carries the temperature of 50 in the warmest month farther poleward there than is the case in the corresponding latitudes occupied by the oceans of the southern hemisphere, which warm less easily and are constantly in motion. Hence the southern cold cap, which has its equatorial limits at about lat. 50° S., is of much greater extent than the northern polar cap. The northern temperate belt, in which the great land areas lie, is much broader than the southern, especially over the continents. These temperature zones emphasize the natural conditions of climate more than is the case in any subdivision by latitude circles, and they bear a fairly close resemblance to the old zonal classification of the Greeks. Classification of the Zones by Wind Belts.-The heat zones however, emphasize the temperature to the exclusion of such 1 A. Supan, Grundziige der physischen Erdkunde (Leipzig, 1806). 88-89. Also Atlas of Meteorology, P1.1. important elements as wind and rainfall. So distinctive are the larger climatic features of the great wind belts of the world, that a classification of climates according to wind systems has been suggested.' As the rain-belts of the world are closely associated with these wind systems, a classification of the zones by winds also emphasizes the conditions of rainfall. In such a scheme the tropical zone is bounded on the north and south by the margins of the trade-wind belts, and is therefore larger than the classic torrid zone. This trade-wind zone is somewhat wider on the eastern side of the oceans, and properly includes within its limits the equable marine climates of the eastern margins of the ocean basins, even as far north as latitude 30 or 3 5°. Most of the eastern coasts of China and of the United States are thus left in the more rigorous and more variable conditions of the north temperate zone. Through the middle of the trade-wind zone extends the sub-equatorial belt, with its migrating calms, rains and monsoons. On the polar margins of the trade-wind zone lie the sub-tropical belts, of alternating trades and westerlies. The temperate zones embrace the latitudes of the stormy westerly winds, having on their equatorward margins the subtropical belts, and being somewhat narrower than the classic temperate zones. Towards the poles there is no obvious limit to the temperate zones, for the prevailing westerlies extend beyond the polar circles. These circles may, however, serve fairly well as boundaries, because of their importance from the point of view of insolation. The polar zones in the wind classification, therefore, remain just as in the older scheme. 1 Need of a Classification of Climates
2 Marine or Oceanic Climate
3 Desert Climate
4 Mountain and Plateau Climate
5 Supan's Climatic Provinces
6 Temperature
7 The Seasons
8 Pressure
9 Winds and Rainfall
10 Land and Sea Breezes
11 Thunderstorms
12 Cloudiness
13 Intensity of Sky-Light and Twilight
14 Climatic Subdivisions
15 4
16 5
17 Temperature
18 Pressure and Winds
19 Rainfall
20 Humidity and Cloudiness
21 Seasons
22 Weather
23 Climatic Subdivisions
24 Sub-tropical Belts: Mediterranean Climates
25 Continental Interiors
26 East Coasts
27 Mountain Climates
28 Pressure and Winds
29 Humidity, Cloudiness and Fog
30 Cyclones and Weather
31 Twilight and Optical Phenomena
32 Physiological Effects
33 What Meteorological Records show
34 Value of Evidence concerning Changes of Climate
35 Periodic Oscillations of Climate: Sun-spot Period
36 Briuckner's 35 - Year Cycle
37 Conclusion
Need of a Classification of Climates A broad division of the earth's surface into zones is necessary as a first step in any systematic study of climate, but it is not satisfactory when a more detailed discussion is undertaken. The reaction of the physical features of the earth's surface upon the atmosphere complicates the climatic conditions found in each of the zones, and makes further subdivision desirable. The usual method is to separate the continental (near sea-level) and the marine. An extreme variety of the continental is the desert; a modified form, the littoral; while altitude is so important a control that mountain and plateau climates are always grouped by themselves. Marine or Oceanic Climate Land and water differ greatly in their behaviour regarding absorption and radiation. The former warms and cools readily, and to a considerable degree; the latter, slowly and but little. The slow changes in temperature of the ocean waters involve a retardation in the times of occurrence of the maxima and minima, and a marine climate, therefore, has a cool spring and a warm autumn, the seasonal changes being but slight. Characteristic, also, of marine climates is a prevailingly higher relative humidity, a larger amount of cloudiness, and a heavier rainfall than is found over continental interiors. All of these features have their explanation in the abundant evaporation from the ocean surfaces. In the middle latitudes the oceans have distinctly rainy winters, while over the continental interiors the colder months have a minimum of precipitation. Ocean air is cleaner and purer than land air, and is generally in more active motion. Continental Climate. - Continental climate is severe. The annual temperature ranges increase, as a whole, with increasing distance from the oceans. The coldest and warmest months are usually January and July, the times of maximum and minimum temperatures being less retarded than in the case of marine climates. The greater seasonal contrasts in temperature over the continents than over the oceans are furthered by the less cloudiness over the former. Diurnal and annual changes of nearly all the elements of climate are greater over continents than over oceans; and this holds true of irregular as well as of regular variations. Fig. 3 illustrates the annual march of temperature in marine and continental climates. Bagdad, in Asia Minor (Bd.), and Funchal on the island of Madeira (M.) are representative continental and marine stations for a low latitude. Nerchinsk in eastern Siberia (N.) and Valentia in south-western Ireland (V.) are good examples of continental ' W.M.Davis, Elementary Meteorology (Boston, and marine climates of higher latitudes in the northern hemisphere. The data for these and the following curves were taken from Hann's Lehrbuch der Meteorologie (1900. Owing to the distance from the chief source of supply of water vapour - the oceans - the air over the larger land areas is naturally drier and dustier than that over the oceans. Yet even in the arid continental interiors in summer the absolute vapour content is surprisingly large, and in the hottest months the percentages of relative humidity may reach 20% or 30%. At the low temperatures which prevail in the winter of the higher latitudes the absolute humidity is very low, but, owing to the cold, the air is often damp. Cloudiness, as a rule, decreases inland, and with this lower relative humidity, more abundant sunshine and higher temperature, the evaporating power of a continental climate is much greater than that of the more humid, cloudier and cooler M. A. M. J. J. A. S. 0. N. D. J. marine climate. Both amount and F. frequency of rainfall, as a rule, decrease 86° inland, but the con ditions are very largely controlled by local topography 68° and by the prevailing winds. Winds average somewhat 5 °°lower in velocity, and calms are more frequent, over continents than over oceans. The seasonal changes of pressure over the former give rise to systems of inflowing and outfl ow in g, so-called continental, winds, ..40 sometimes so well developed as to become true monsoons. The extreme tem perature changes which occur over the continents are the more easily borne J. F M. A. M J. J. A. S. 0. N. D. J. because of the dryFIG. 3. - Annual March of Air Temperature. ness of the air; beInfluence of Land and Water. (After Angot.) the minimum M, Madeira. V, Valentia. cause temperatures of Bd, Bagdad. N, Nerchinsk. winter occur when there is little or no wind, and because during the warmer hours of the summer there is the most airmovement. Desert Climate An extreme type of continental climate is found in deserts. Desert air is notably free from microorganisms. The large diurnal temperature ranges of inland regions, which are most marked where there is little or no vegetation, give rise to active convectional currents during the warmer hours of the day. Hence high winds are common by day, while the nights are apt to be calm and relatively cool. Travelling by day is unpleasant under such conditions. Diurnal cumulus clouds, often absent because of the excessive dryness of the air, are replaced by clouds of blowing dust and sand. Many geological phenomena, and special physiographic types of varied kinds, are associated with the peculiar conditions of desert climate. The excessive diurnal ranges of temperature cause rocks to split and break up. Wind-driven sand erodes and polishes the rocks. When the separate fragments become small enough they, in their turn, are transported by the winds and further eroded by friction during their journey. Curious conditions of drainage result from the deficiency in rainfall. Rivers " wither " away, or end in sinks or brackish lakes. 1$ 94)9 PP. 334-335. V C. | | PLATE I. | | 160 1 | r | | 160 120 HO 40 0 40 90 t20 t p _ t I 1 I 1 I 1 | | .' - oF ' | | - | !. _ _ | -, ? ?-, c?o ? ' _ " _ r80 ? ° | | | | | ..n?? - ?w?.? | | | | | ??,. +?` .a p? o ? ?o ? ?? + GC;°° ? ?? Arc C1rcl? ' | | | | | | ? r _ Z ? - 4 | | | | ' ` 70? ? ? , ?, | | | | | | | '1/4% | | | | | | , ? >L | | | | | -. . | 4 Sainte 10 0. | 40 | | | | Q o | | P.4 ?,. ? ? * ? a1 | | | | | rup ,. T,c ??y? y, ' /!' ? ??.,,*:,/, | | | | | | //% ???? ?_:,??,?4yy/?, / ? . *!.d/? YYi ?? '};
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Antarctic -? MEAN ANNUAL TEMPERATURE - ' - ? ? r"' | | | ? | | | Temperature in degrees Fahrenheit and Cen ti grade | | | 160 | | | 160 120 80 40 I 1 I 1 I 1 i 1 1 0 - 0 80 120
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| 7 39?? ?ao | | `Zg.9p ? | | | 7369 mm | | | | | 7 4?4,,, | | | 9 700 601n 1 4. | | | ? . ip 7. m | .... | | | 401n | | | ,.m 746?: ?. - | 744? 2m | | | | | _ ltntarnUe- Slrcle-. _ _ _ _ _ ? _: '- -. _ ___. _ .------ - --- - - - - -- - - - - - - _'cti. - - - - - - -; MEAN ANNUAL PRESSURE - - - - _ __ _ - - -. -?-- - -' -' | | | | | | ?=?-. - Pressure in i s and millimetres - - ?- - " ' | | | - | | | -.. _.. . - :, _ , ?-:: I _. . . -. .. -: < | | | 160 | . | | t 160 120 BO 40 0 ? 40 so .20 | | | 160 140 | 40 e0 120 | | | 40 _. 1 | | eo | . ': ': : ro ?? .? | | | ec | | ?° | `ti ,,,,sr,,,,,, '? ,' ? oo ?;bd ? | | ?? K W ^ ? '?'? '- o | ? o 4 - yr | | | .r G,? Q ? C`J? 4'Ca�. . .. c?o? ? ? n???il?????? | | | ,? n ?, `?.??'?Q.. .?. ??? | | | | ? ? | | | | | :.1.1 ................. .? s t ??.9? ? 0r Gbnco' ? / ? ?Hi%?? ?? // ^? ??? .1, ._??:?: | ? ??? | ?' ? ? | i :? :? | | ?/?y ? /?? ? ? ? ? t 0 ? .. y. ? ?? i??/ ,!/ ? /?/y/ % ? y??t14` .. ... '? ? | | | 1 ? ?%.? // ? ? I ii? " y ' ? 4,1 ? ? / ??? ??;. ? /.? r ,?j??s ? ? i ? a ? | | .n | , ?ar `? % i??/ ? I???'1%. f ? r ,, 4, l ? ?/??// ? ? ? 'n ? ? ? ,.. ..?. / ,... ?: ... 2. ,; .....I /??a? .. ` ??1;? '??t | ? | | ?, ??/ .,., c', n `, 10' L `, ,. ? ..:... .., ri r ci / ? ? SS | | r 00 B' r' ?% | | | . 4 | ?? - - | | r.? ?ag; | r. . ? " | | ? 111" | OOr.. | , | | i. 4?44 C. | | | | | | | | IIC | | | | | | . | +-- ... - ? `j - CF{,h.`?1??C ntx ,; !' ,'r .' ;-I.' ' T EM PERATURE ::: ? | Antarctrc Cvde , ? | | ? TEMPERATURE | | | _ ' JULY. ? | | | ...'6 JANUARY | | | '?' -? ' ? . -j. _:. -.:--.'I ..?t... _. .:'.... . ` -?-..' - | | | 'I? r z , K0 120 a 0 n0 0 0 | | | i-' -r 160 160 120 40 B 4 | | | IW r60 110.40 n0 e 120 1 1 i_ _.1 | | | 160 ¦f10 BO W a0 120 0 ¦ r 1 | | BO | B | B | | | ' | /5Bm | -2.,7,_ ' o :f,?, | | ? ? | a.Y/y. 0? | | | ?G`.' . QD J?? | | ? I b®O 6 | | ?`; di?' ? ? ?l ??/ /// / r a? | | | | | | ?. ? o ` .s rc1 - ? ? | | | III(? "?y?H //// //rl/?j? ? ? ? ? ?', . ? ?y ? ? ? ?' /?? ???.? ? ?i i? | | | P i .t.? | | ,.? ff >?. - ?
| y` / , ? ? ? yr? ? ? E 4?1 s ?y? y?? /// ??? ' %? '?,?,vYY/?y/??/?.?? ?//, ?? 1 ? | | ? f in 4 ,,, ? | 1 | ? r5,m^' ? I % r ?????. ?, ?? | ? ? ? | 1 ?? ' ?i ' f? P ? ? ,f6fy ? ,???III ???``` | .y. y s ??? %/ ?? ????. ~ ????' ?? ? 4 ?/ ?? ?? ? i? E ?? 6 | | | aB | ? i r s:?? i n ' / 1 d? ? M ??? | | | ? aNry?? .v9li/h? ????????
.zminll /'.r/? Ie ? l / ? ??%,?? a;?, r .? ?/?% 91 OOin t, 4.4 ! ? 41,?/?. ?' l -h1..??? , ? ? ?? ' .r ?: rop12 of-- mm ,i? ?? ??b ? | | | ?LY ? ? r r???. ? ?. i ?.' ?'{. ' '? /// .?l" ??' 'j(? p 1 ? ' I?/?? ? ? ?? ? ? ?/ ? I. ? I ?? ? ?% / _ a= C?ecer. ,, ... 1 - ?. ? | | | ? s ?! l .? ?? | ?' , ???1?1? | | | .r ? `^ ti? J 1414401r- ?? ? ,?: | | 0 | ?? '/ i., i ?. ?? ^? " | D | | ® 4 '? ? ? ? 1111-1111m1 ? j111` : ? ? ".----".----. f. ?. _ ? | | | | ti. ?. ? | ? ?`wl? l | ? | l' | ? .c:? ¦ ` ??? ,?? ? | s | ? ?', t ? ? ? .';? ?° IB, ? ? m ` ? "?' . | | ? | f' ? ?? ???. `` ` | ? | . | | :? ? i; | ? 1 ' ??;,. ?'. ei l N- :. _i ???` /?. OP /? Q,? I ? ?. i ..? ? `? ? ? ?? ?/ ? /% ? ?/ `??%`? | ? | / ? "I l ? | | | l r ? I/L f ? / ??I ? ? ? 1 / ??/ j IO + ? f? ???? ` ?pI/? ? 9?S?;Y,?? ???h/.:./?' ? ¢ ? r???? .' o - I ? hA9? ? //?/?,? ?????S?L???'1? i, %, R | | | ab | ? ? ? | ?'. r | | I? | | ? ? ? ?m .? | | | or | | | | | i +L 111011". 'R ? - ' ? ? | Z'?y | 60 | _ _ ... ,'n:: .'. _ _ _ - _ - -.___.. ., _ .... _ R_. _. .. _. _ _ _.. _ _ _. _.. . _ _ _ _ _ _ | | | | ../. .?. ..: ...: :.::..' ?::. .?. . . . .":.'::.. _ - ..: _
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.: . .':.: . . RntarctcCrc ? - ' .'ti:._F...-__ - - .?,... - - =' ' - ?:%?: :r ='P REBWINDS - -?-?`ic - - _ - JULY ` ?- | | | ._.... ..c..?: _::?::- ___ ------
_ - :; -. - - : | -- - ' --?- ---- :_ -:_- . .- - - = ? ' PRESSURE & W I NDS : | | '?- --- - R -- t.-.. --: ...... ...........:.......... .. .. --- - ........ .-- | | | - `r:- __......_ ' | | | | | | m0 r60 e0 4D 40 6 | 30° 200 10° 0° 100 20° 300 N Bd M _B:d M Bd fah Centigrade Pressure in inches t millimetres Seasonal Distribution Of Temperature And Pressure. Desert plants protect themselves against the attacks of animals by means of thorns, and against evaporation by means of hard surfaces and by a diminished leaf surface. The life of man in the desert is likewise strikingly controlled by the climatic peculiarities of strong sunshine, of heat, and of dust. Coast or Littoral Climate. - Between the pure marine and the pure continental types the coasts furnish almost every grade of transition. Prevailing winds are here important controls. When these blow from the ocean, the climates are marine in character, but when they are off-shore, a somewhat modified type of continental climate prevails, even up to the immediate sea-coast. Hence the former have a smaller range of temperature; their summers are more moderate and their winters milder; extreme temperatures are rare; the air is damp, and there is much cloud. All these marine features diminish with increasing distance from the ocean, especially when there are mountain ranges near the coast. In the tropics, windward coasts are usually well supplied with rainfall, and the temperatures are modified by sea breezes. Leeward coasts in the trade-wind belts offer special conditions. Here the deserts often reach the sea, as on the western coasts of South America, Africa and Australia. Cold ocean currents, with prevailing winds along-shore rather than on-shore, are here hostile to rainfall, although the lower air is often damp, and fog and cloud are not uncommon. Monsoon Climate. - Exceptions to the general rule of rainier eastern coasts in trade-wind latitudes are found in the monsoon regions, as in India, for example, where the western coast of the peninsula is abundantly watered by the wet south-west monsoon. As monsoons often sweep over large districts, not only coast but interior, a separate group of monsoon climates is desirable. In India there are really three seasons - one cold, during the winter monsoon; one hot, in the transition season; and one wet, during the summer monsoon. Little precipitation occurs in winter, and that chiefly in the northern provinces. In low latitudes, monsoon and non-monsoon climates differ but little, for summer monsoons and regular trade-winds may both give rains, and wind direction has slight effect upon temperature. The winter monsoon is off-shore and the summer monsoon on-shore under typical conditions, as in India. But exceptional cases are found where the opposite is true. In higher latitudes the seasonal changes of the winds, although not truly monsoonal, involve differences in temperature and in other climatic elements. The only well-developed monsoons on the coast of the continents of higher latitudes are those of eastern Asia. These are off-shore during the winter, giving dry, clear and cold weather; while the on-shore movement in summer gives cool, damp and cloudy weather. Mountain and Plateau Climate Both by reason of their actual height and because of their obstructive effects, mountains influence climate similarly in all the zones. Mountains as contrasted with lowlands are characterized by a decrease in pressure, temperature and absolute humidity; an increased intensity of insolation and radiation; usually Copyright Statement
These files are public domain. Bibliography Information
Chisholm, Hugh, General Editor. Entry for 'Climate and Climatology'. 1911 Encyclopedia Britanica. https://www.studylight.org/encyclopedias/eng/bri/c/climate-and-climatology.html. 1910.
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