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DISCOMFORT INDICES DI AND THI

DISCOMFORT INDICES DI AND THI. L. De Wet. INTRODUCTION . Compare Durban and Bloemfontein weather Indices quantify effects of environment on human beings T or solar radiation = absolute measure of heat or energy Standard measurements = describe environment and can be compared

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DISCOMFORT INDICES DI AND THI

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  1. DISCOMFORT INDICESDI AND THI L. De Wet

  2. INTRODUCTION • Compare Durban and Bloemfontein weather • Indices quantify effects of environment on human beings • T or solar radiation = absolute measure of heat or energy • Standard measurements = describe environment and can be compared • But what is the effect on humans? • So various factors can be combined to provide an index to measure the effect

  3. IDEAL INDEX • Requirements: • All environments, physiological factors and coverings • One value to describe environment and body • But not possible – so need the following: • Simple and accurate • 3 parameters: environment, reaction and effect • Environment – Ta, Rh, wind speed & radiation • Reaction – dependant on species, metabolic rates, cover type & length, age, sex, nutrition, diseases, etc. • Effects – production, growth, reproduction, physiology, pathology, etc.

  4. INDICES • Many indices for humans • Example: windchill index – necessary when temperatures are low  unpleasant and/harmful to humans • At higher temperatures the wind has a cooling effect by removing hot air parcels near skin surface and by promoting evaporation of perspiration

  5. DISCOMFORT INDEX (DI) Temperature (Ta) and relative humidity (Rh) Body Ta = 37 oC Mechanism to control Ta: Heatblood pressureperspiration evaporation (if air is not saturated)

  6. DI • If moisture level is high, discomfort • DI equation: • DI = (2*Ta)+[Ta*(RH/100)] +24 • Where Ta = Tmx • Rh = Rhmn • DI < 80 IS FAVOURABLE

  7. DI val ue (no units) Level of discomfort 80 - 90 Relatively uncomfortable 90 - 100 Very uncomfortable 100 - 110 Extremely uncomfortable 110 and higher Dangerous DI Scale for DI values

  8. DI Example, if DI = 100

  9. TEMPERATURE HUMIDITY INDEX (THI) • FOR ANIMALS (E.g. Dairy cattle) • Animals cannot cool off as easily as humans, so provision must be made for them • Equation: • THI = Ta+0.36Tdp+41.2 • Where Ta = Tmx or Ta • And Tdp = dew point temperature THI < 70 IS FAVOURABLE

  10. THI • Dairy cattle • Normal THI for dairy cattle is < 70 • Body Ta ↑ when Ta ↑ • THI 70-72 = critical • Milk production and feeding ↓ when THI > 72 • ↓ ↓ between 76 and 78

  11. THI

  12. THI • Types of shade: • Food & H20 always available • Trees • Grass roofs • White painted corrugated roof • Management • Example • Holstein • Thermo neutral zone for milk production • -5oC – 21 oC

  13. THI • If cattle eat less ↓ milk production & reproduction • If environmental T > 21 oC – 25 oC, production ↓ (because food intake ↓) • Critical Ta is 21 oC for milk production for Holsteins, and Jerseys & Brown Swiss cows = 24 oC and lower • Critical body temperature (Tc) = 37.5 oC – 38.5 oC • Recovery time = 7 – 8 days • Milk production ↓ 15% if THI > 72 • SA is safe in winter • Summer – large areas have THI> 72, so farmers must provide additional cooling for cattle for ↑ production

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