ARID CLIMATIC ZONES

WHAT IS ARID REGION?

A REGION IS ARID WHEN IT IS CHARACTERIZED BY A SEVERE LACK OF AVAILABLE WATER, TO THE EXTENT OF

HINDERING OR PREVENTING THE GROWTH AND DEVELOPMENT OF PLANT AND ANIMAL LIFE. ENVIRONMENTS

SUBJECT TO ARID CLIMATES TEND TO LACK VEGETATION AND ARE CALLED XERIC OR DESERTIC.

MOST "ARID" CLIMATES SURROUND THE EQUATOR; THESE PLACES INCLUDE MOST OF AFRICA AND

PARTS OF SOUTH AMERICA, CENTRAL AMERICA AND AUSTRALIA.

1. Meaning of aridityArid environments are extremely diverse in terms of their land forms, soils, fauna, flora, water balances, and human activities.Because of this diversity, no practical definition of arid environments can be derived. However, the one binding element to all arid regions is aridity.Aridity is usually expressed as a function of rainfall and temperature. A useful "representation" of aridity is the following climatic aridity index: p/ETPwhereP = precipitationETP = potential evapotranspiration, calculated by method of Penman, taking into account atmospheric humidity, solar radiation, and wind.Three arid zones can be delineated by this index: namely, hyper-arid, arid and semi-arid. Of the total land area of the world, the hyper-arid zone covers 4.2 percent, the arid zone 14.6 percent, and the semiarid zone 12.2 percent. Therefore, almost one-third of the total area of the world is arid land.The hyper-arid zone (arid index 0.03) comprises dryland areas without vegetation, with the exception of a few scattered shrubs. True nomadic pastoralism is frequently practiced. Annual rainfall is low, rarely exceeding 100 millimeters. The rains are infrequent and irregular, sometimes with no rain during long periods of several years.The arid zone (arid index 0.03-0.20) is characterized by pastoralism and no farming except with irrigation. For the most part, the native vegetation is sparse, being comprised of annual and perennial grasses and other herbaceous vegetation, and shrubs and small trees. There is high rainfall variability, with annual amounts ranging between 100 and 300 millimeters.The semi-arid zone (arid index 0.20-0.50) can support rain-fed agriculture with more or less sustained levels of production. Sedentary livestock production also occurs. Native vegetation is represented by a variety of species, such as grasses and grass-like plants, fortes and half-shrubs, and shrubs and trees. Annual precipitation varies from 300-600 to 700-800 millimeters, with summer rains, and from 200-250 to 450-500 millimeters with winter rains.Arid conditions also are found in the sub-humid zone (arid index 0.50-0.75). The term "arid zone" is used here to collectively represent the hyper-arid, arid, semi-arid, and sub-humid zones.

2. CAUSES OF ARIDITY:-ARIDITY RESULTS FROM THE PRESENCE OF DRY, DESCENDING AIR. THEREFORE, ARIDITY IS FOUND MOSTLY IN PLACES WHERE ANTICYCLONIC CONDITIONS ARE PERSISTENT, AS IS THE CASE IN THE REGIONS LYING UNDER THE ANTICYCLONES OF THE SUBTROPICS.THE INFLUENCE OF SUBTROPICAL ANTICYCLONES ON RAINFALL INCREASES WITH THE PRESENCE OF COOL SURFACES. ARID CONDITIONS ALSO OCCUR IN THE LEE OF MAJOR MOUNTAIN RANGES THAT DISRUPT THE STRUCTURE OF CYCLONES PASSING OVER THEM, CREATING "RAIN SHADOW" EFFECTS. RAINFALL IS ALSO HINDERED BY THE PRESENCE OF GREATLY HEATED LAND SURFACES; AS A CONSEQUENCE, LARGE AREAS OF DRY CLIMATE EXIST FAR FROM THE SEA.

3. ARID ZONE CLIMATE:-THE ARID ZONE IS CHARACTERIZED BY EXCESSIVE HEAT AND INADEQUATE, VARIABLE PRECIPITATION; HOWEVER, CONTRASTS IN CLIMATE OCCUR. IN GENERAL, THESE CLIMATIC CONTRASTS RESULT FROM DIFFERENCES IN TEMPERATURE, THE SEASON IN WHICH RAIN FALLS, AND IN THE DEGREE OF ARIDITY. THREE MAJOR TYPES OF CLIMATE ARE DISTINGUISHED WHEN DESCRIBING THE ARID ZONE: THE MEDITERRANEAN CLIMATE, THE TROPICAL CLIMATE AND THE CONTINENTAL CLIMATE.IN THE MEDITERRANEAN CLIMATE, THE RAINY SEASON IS DURING AUTUMN AND WINTER. SUMMERS ARE HOT WITH NO RAINS; WINTER TEMPERATURES ARE MILD. FIGURE 1.1 ILLUSTRATES THE MEDITERRANEAN CLIMATE, WITH A WET SEASON STARTING IN OCTOBER AND ENDING IN APRIL OR MAY, FOLLOWED BY 5 MONTHS OF DRY SEASON.IN THE TROPICAL CLIMATE, RAINFALL OCCURS DURING THE SUMMER. THE GREATER THE DISTANCE FROM THE EQUATOR, THE SHORTER THE RAINY SEASON. WINTERS ARE LONG AND DRY. IN SENNAR, SUDAN, AN AREA THAT IS TYPICAL OF THE TROPICAL CLIMATE, THE WET SEASON EXTENDS FROM THE MIDDLE OF JUNE UNTIL THE END OF SEPTEMBER, FOLLOWED BY A DRY SEASON OF ALMOST 9 MONTHS.IN THE CONTINENTAL CLIMATE, THE RAINFALL IS DISTRIBUTED EVENLY THROUGHOUT THE YEAR, ALTHOUGH THERE IS A TENDENCY TOWARD GREATER SUMMER PRECIPITATION. IN ALICE SPRINGS, AUSTRALIA, EACH MONTHLY PRECIPITATION IS LESS THAN TWICE THE CORRESPONDING MEAN MONTHLY TEMPERATURE; HENCE, THE DRY SEASON EXTENDS OVER THE WHOLE YEAR.

Characteristics of Hot Arid Zone:

The salient characteristics of hot arid zones are following:

(1) ARIDITY INDEX IS HIGH (MORE THAN 70%).

(2) HIGH RANGE OF TEMPERATURE AND SOLAR RADIATION; ERRATIC AND LOW RAINFALL; LOW ATMOSPHERIC HUMIDITY AND HIGH WIND VELOCITY IN SUMMER.

(3) FAMINE OF FOOD, FODDER AND WATER I.E., “TRIKAL” IS A PERMANENT GUEST IN EVERY 3 OR 5 YEARS.

(4) SOIL IS SANDY WITH LOW WATER HOLDING CAPACITY; LOW ORGANIC CONTENT, DEFICIT IN NITROGEN AND PHOSPHORUS CONTENTS.

(5) VEGETATION IS SPARSE AND LIMITED WITH LOW BIOMASS PRODUCTION.

(6) VEGETATION IS XEROPHYTIC.

(7) HEAVY PRESSURE OF MEN AND ANIMALS (CUTTING, FELLING, LOOPING, GRAZING, BROWSING ETC.) ON VEGETATION.

(8) HUMAN POPULATION IS THIN EXCEPT IN INDIAN ARID ZONE; WHICH IS DENSELY POPULATED ARID ZONE OF THE WORLD

HOT AND ARID REGIONS

• IN THESE REGIONS, THE CLIMATE IS EXTREME, THE TEMPERATURE RANGES FROM 50 C°MINIMUM, OR, MORE OR LESS.

• CLOUDLESS SKY, LOW HUMIDITY,AND HIGH INCIDENCE OF SUNS GLARE ARE THE MAIN FEATURES.

• THE SUNNY AREAS ARE HOT AND DRY IN THE DAY TIME AND COOL TO COLD AT NIGHT.

• AS FAR AS POSSIBLE, THE BUILDINGS SHOULD BE PROTECTED FROM DAY TIME HEAT AND GLARE DURING SUMMER AND AT SAME TIME, THE RATE OF HEAT LOSS AT NIGHT DURING WINTER SHOULD BE REDUCED.

• IN THESE REGIONS THE BUILDING SHOULD BE ORIENTED FOR SUN, NOT FOR WIND AS IN HUMID REGIONS.

INDIAN ARID CLIMATIC ZONESINDIAN CLIMATIC ZONES MAP

DESIGN FOR HOT-ARID ZONES

THE MAIN POINTS:-

*PROVIDE MAXIMUM SHADING OF DIRECT AND REFLECTED SUN RADIATION IN SEASON.

*BALANCE EXTREMES OF SUMMERS AND WINTERS BY MOVABLE PARTS.

*PROVIDE VENTILATION BY REGULATED AIR MOVEMENT AND SMALL OPENINGS.

*AVOID LARGE EXPOSED EXTERIOR SURFACES.

*USE REFLECTIVE OUTER SURFACES

*BALANCE THE EXTREMES OF DAY AND NIGHT TEMPERATURES BY ADEQUATE THERMAL STORAGE MASS.

*REDUCE INTERNAL HEAT PRODUCTION AND CONDUCTION GAIN IN HOT SEASONS.

*PROMOTE EVAPORATION AND HEAT LOSS BY RADIATION.

*INCREASE AIR CIRCULATION IN HUMID MARITIME REGIONS

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