Classification of Tropical Climates- MODULE 1Climatic Zones

• Infinite variety of climates▫ Interaction of solar radiation with atmosphere and gravitational forces▫ Distribution of land and sea masses

• Certain zones of approximately uniform climates can be distinguished• Boundaries cannot be accurately mapped• One zone merges gradually into the next

Classification• Classification suggested by G A Atkinson in 1953• Basis of classification – Temperature and Humidity – dominant influence on comfort

Tropical climate-warm humid equatorial (warm humid island or trade wind climate), Hot-dry desert (hot-dry maritime desert climate), composite or monsoon (tropical upland climate)Comparison

1. Warm Humid Climate2. Subgroup : Warm-humid island climate

Warm-humid climate • Near the equator- 15 deg N and S

▫ Lagos (Nigeria)▫ Dar-es-Salam (Tanzania)▫ Mombasa (Kenya)▫ Colombo (Sri Lanka)▫ Singapore▫ Jakarta (Indonesia)▫ Quito (Ecuador)▫ Pernambuco (Brazil)

• Very little seasonal variation• Periods with more or less rain• Gusty winds and electric stormsDBT

▫ Mean Max : 27-32 ºC▫ Mean Min : 21-27 ºC▫ Diurnal and Annual ranges are narrow

• Humidity▫ Remains high – about 75 %▫ May vary from 55% to almost 100%

• Vapor Pressure : 2500 -3000 N/sqm • Precipitation

▫ High Throughout the year▫ More intense for several consecutive months▫ Annual rainfall : 2000-5000 mm▫ May exceed 500 mm in wettest month▫ Sever storms upto 100mm/hr for short periods

• Sky Condition▫ Fairly cloudy 60-90%▫ Bright – luminance of 7000 cd/sqm ▫ Slightly more when sun illuminates white cumulus clouds▫ When heavily overcast, sky is dull (850 cd/sqm or less)

• Solar Radiation▫ Partly reflected and partly scattered by the clouds or high vapour content

▫ Radiation reaching ground is diffuse but strong▫ Causes painful sky glare▫ Reduces outgoing radiation from earth and sea▫ Accumulated heat is not readily dissipated

• Wind▫ Typically low velocities▫ Frequent calm periods▫ Strong winds can occur during rain squalls▫ Gusts of 30 m/s have been reported▫ Usually one or two dominant directions

• Vegetation▫ Grows quickly due to frequent rains and high temperatures▫ Difficult to control▫ Subsoil water table is usually high ▫ Ground may be water logged▫ Little light is reflected from the ground

• Special characteristics▫ High humidity accelerates mould and algal growth, rusting and rotting.▫ Organic buliding materials tend to decay rapidly.▫ Mosquitoes and insects▫ Thunder storms with lightning▫

Warm-humid island climate

• Islands within the equitorial belt and in the trade wind zone▫ The Caribbeans ▫ The Philippines

• Seasonal Variations are negligible• DBT

• Mean Max : 29 - 32ºC• Mean Min : 18 - 24 ºC• Diurnal range : less than 8ºC• Annual Range : 14ºC

• Humidity• Varies between 55% and 100%• Vapor pressure : 1750 – 2500 N/sqm

• Precipitation• 1250 – 1800 mm per annum• 200-250 mm in wettest month• Driving rain almost horizontal in windward coasts

• Solar Radiation• Strong and mainly direct• Varies with cloud cover

• Sky Conditions

• Normally clear or filled with broken clouds of high brightness• Dark and dull during storms• Clear blue skies of low luminance (1700-2500 cd/sqm)

• Winds• Predominant trade wind (6-7 m/s)• Higher during cyclones

• Vegetation• Less luxuriant and of much lighter green color• Sunlight reflected from coral and sand – very bright• Dry soil with low water table

• Tropical Cyclones – 45-70 m/s• High salt content in atmosphere – corrosion in coastal areas

3. Hot-dry desert climate4. Subgroup : Hot-dry maritime desert climate

Hot-dry desert climate• Two belts : 15º–30º N and S

▫ Assuan (Egypt)▫ Baghdad (Iraq)▫ Alice springs (Australia)▫ Phoenix (Arizona, USA)

• Two marked seasons▫ A hot period▫ A cooler period

• DBT▫ Rises quickly after sunrise▫ Mean max :

Hot period : 43-49ºC Cool period : 27-32ºC

▫ Mean Min : Hot : 24-30ºC Cool: 10-18ºC

▫ Extreme Maximum 58ºC – Libya 1922

▫ Very high diurnal Range : 17-22ºC • Humidity

▫ Varies from 10-55%▫ Wet bulb depression is very large due to rapid evaporation▫ Vapor pressure : 750-1500 N/sqm

• Precipitation▫ Slight and variable : 50-155 mm▫ Flash-storms may occur over limited areas with upto 50mm in few hours▫ No rain for several years in some regions

• Sky Conditions▫ Normally clear▫ Very few clouds – low AH▫ Dark blue sky : 1700 – 2500 cd/sqm ▫ Dust or sand storms : 850 cd/sqm ▫ End of hot period- dust suspended in air creates white haze- 3500-10000

cd/sqm- Produces diffuse light and painful glare• Solar Radiation

▫ Direct and strong▫ Absence of cloud – easy release of heat stored in day, as long wave radiation in

night▫ Diffuse radiation only present during dust haze

• Winds▫ Usually local winds▫ Heating of air over hot ground causes temperature inversion▫ Lower warm air mass breaks through higher cooler air – local whirlwinds▫ Winds are hot, carrying dust and sand – often develop into dust storm

• Vegetation▫ Sparse and difficult to maintain because of lack of rain and low Humidity▫ Soil is dusty and very dry▫ Strong sunlight, highly reflective light coloured ground – creates 20k – 25k

cd/sqm

▫ Soil dry quickly after rain▫ Generally fertile if irrigated▫ Water table very low

• Special Characteristics▫ Dust and sandstorms may be frequent▫ High diurnal variation may cause materials to crack and break up

Hot-dry maritime desert climate

• Same latitude as hot-dry desert climate but near sea• Amongst the most unfavorable climates

▫ Kuwait▫ Karachi

• Two seasons – Hot and somewhat cooler one• DBT

• Mean Max : 38ºC / 21-26ºC• Mean Min : 24-30 ºC / 10-18 ºC• Diurnal range : 9-12 ºC (larger during cold season)

• Humidity• Steadily high - 50% and 90%• Vapor pressure : 1500 – 2500 N/sqm • Strong evaporation from sea – moisture not precipitated, remains suspended in

air creating uncomfortable conditions• Precipitation – Very low• Sky condition – little more cloudy than Hot dry, in form of thin translucent haze, causes

glare• Solar Radiation

• Strong with higher diffuse component• Due to thin clouds and suspended moisture

• Winds• Mostly local, coastal winds• Caused by unequal heating and cooling of land and sea• From sea to land during day and reverse during night

• Vegetation• Sparse, not more than some dry grass• Ground and rocks are brown or red• Dry and dusty throughout the year• Intense ground glare

• Special Characteristics• Dust and sand-storms• Salt laden atmosphere accelerates corrosion

5. Composite or Monsoon climate6. Subgroup : Tropical Upland climate

Composite or Monsoon climate

• Large land masses near the tropics▫ Sufficiently far from equator▫ Lahore▫ Mandalay▫ New Delhi

• Two marked seasons▫ Two-thirds of year is hot dry period▫ Other is warm-humid▫ Further north and south Cool-dry also

• DBT

Seasons Hot-dry Warm-humid Cool-dry

Mean-max 32-43 27-32 <27

Mean-min 21-27 24-27 4-10 Diurnal range 11-22 3-6 11-22

• Humidity▫ Low throughout the dry periods (20-55%)▫ Vapor pressure – 1300 -1600 N/sqm ▫ Rises in wet period (55-95%)▫ Vapor pressure – 2000-2500 N/sqm

• Precipitation▫ Monsoon rains intense and prolonged▫ 25-38 mm can fall in an hour▫ Annual – 500-1300 mm▫ 200-250 mm in wettest month

• Sky conditions▫ Vary markedly with seasons – heavily overcast and dull during monsoons – clear

with dark blue color in dry seasons▫ Towards end of hot-dry season sky becomes brighter with frequent dust haze

• Special characteristics▫ Changes in relative humidity cause rapid weakening of building materials▫ Dust and sand-storms, termites

• Solar Radiation▫ Alternates between warm-humid and hot-dry conditions

• Winds▫ Hot and dusty during dry period▫ Directional changes in prevailing winds at beginning of warm humid season

brings rain clouds and humid air from sea▫ Monsoon winds are fairly strong and steady

• Vegetation▫ Sparse with brown and red barren ground, changes rapidly with rain▫ landscape becomes green and fertile in a few days – plants grow quickly▫ Vegetation covers ground when cool but diminishes as temperature rises▫ Soil is damp during rains but dries out quickly▫ Risk of soil erosion during monsoons▫ Strong ground glare during dry season ▫

Tropical upland climate

• Mountainous regions and plateau more than 900 -1200 m above sea-level experience such climate

▫ Bogota, Mexico city, Nairobi• Seasonal Variations are small near equator, but further away, seasons follow those of

nearby low lands• DBT

▫ Decreases with altitude▫ Mean Max – 24-30 ºC▫ Mean min – 10-13 ºC▫ May fall below 4ºC at some locations – ground frost can occur▫ Large diurnal range▫ Annual range depends on lattitude – Slight variation at equator – at tropics, 11-

20ºC

• Humidity▫ RH- 45-99% and Vapor pressure 800-1600 N/sqm

• Precipitation ▫ Variable but rarely less than 1000mm▫ Often falls in heavy concentrated showers of upto 80mm per hour

• Sky Condition▫ Normally clear or partly cloudy (40%)▫ During monsoons sky is overcast – clouds are heavy and low

• Solar Radiation▫ Strong and direct during clear periods – stronger than at sea level▫ UV radiation is stronger▫ Becomes more diffuse as cloud cover increases

• Winds▫ Variable – predominantly trade winds, but may be deflected by topography▫ Rarely exceeds 15 m/s

• Vegetation▫ Green but not luxuriant during wet seasons , but wither when dry▫ Soil is damp in the rains but dries quickly▫ Heavy dew at night

• Special Characteristics▫ Heavy dew at night▫ Strong radiation loss during dry season – leads to formation of radiation fog▫ Thunder storms with air to ground electric discharges▫ Hail may occur