Radiant heat transfer occurs at the speed of light through electromagnetic radiation between surfaces without a medium. It follows the Stefan-Boltzmann law which states that the net radiant heat transfer between two surfaces is proportional to the fourth power of their absolute temperatures. Animal coats and skin absorb, reflect, and transmit radiation differently depending on factors like color, density, and surface irregularities which can influence heat absorption and dissipation. The greenhouse effect occurs as certain gases in the atmosphere absorb and reradiate infrared radiation from the sun, moderating global temperatures.

1. Radiant Exchange
Heat Transfer at the Speed of Light (3 x 1010 cm/sec)
No medium required - can occur in vacuum
Not dependent on air temperature
Net transfer - Stefan-Boltzmann Law
Radiant Heat = SB Constant x Emiss. x Emiss. x (T14 - T24)
Transfer Surf. 1 Surf. 2

2. Significance of this transfer
Man - shorts - sitting quietly
~ 50 - 70% heat loss (30 W/m2) - via radiant exchange
Animal - bright sun - solar radiation (intercepted) =
much larger than MR

3. Total radiant power - received outside earth’s atmosphere
-on a plane - right angle to sun’s rays = 1360 W/m2
Atmosphere scatters light
Blue (shorter wavelength) more than red (longer wavelength)
>> blue sky
Sun - orange or red because blues & violets have been
scattered out + at sunset & sunrise - greater amount
atmosphere for light to pass through.
UV radiation diminished by:
1. Ozone absorption - stratosphere
2. scattering

4. Solar radiation - received by earth’s surface dependent on:
1. Sun’s elevation above horizon
2. Light scattering by atmosphere (including effects -
water droplets & ice particles - clouds
3. Absorbance - atmospheric gases (water vapor, CO2,
O3, etc....) - absorbs infrared radiation

5. Infrared radiation (sun) - almost entirely absorbed by
atmosphere
Visible & near-infrared (sun) pass through >> earth’s surface
- then trapped - reradiated as infrared from surface - but
cannot entirely leave
This = GREENHOUSE EFFECT by atmosphere >>
moderating effect on daily temperature swings of earth’s
surface.
Clear, dry atmosphere - night - rapid radiant cooling
Clear sky - night - serves as radiant heat sink

6. Low-temperature infrared radiation does not penetrate
water or tissues with water.
+ There is no effect on heat transfer within body
Color affects visible radiation absorption
Black absorbs more radiation - visible spectrum
White reflects more radiation visible spectrum
1/2 solar radiation reaching earth - in visible region

7. Would expect animals with dark coats or skin - to have
heat stress problems.
+ animals with light coats or skin to have few heat-
related problems.
NOT ALWAYS TRUE - polar animals
Fur or plumage coats - absorption site = coat surface
Smooth or even surface exposed to solar radiation -
heat absorbed dependent on color.
Irregular coat - light color - beam reflected into
coat and absorbed near skin.
Dark color >> little reflectance - less penetration

8. Combine this with the effect of windspeed.
Temperature of superficial layers of insulation much
higher for dark plumage.
BUT - high wind speeds - heat absorbed - dark plumage -
much less - due - dissipation via convection.
Light plumage - less effect - wind speed - due to greater
penetration.

9. Coat density - important -
Sheep example
Awassi sheep - loose coat -
Deep penetration >> high skin temperature
Also - affected by wind speed
Merino sheep - dense coat little penetration
Skin temperature not as high
BUT - fleece temperature - very high
Large infrared heat loss
Large reduction - heat flow with increased fleece length
Ogaden sheep (Persian) - smooth white coats
Decreased heat load due to high reflectance of
solar radiation.

10. EMISSIVITY
Measurement of an objects ability to emit radiation at a
given temperature
Blackbody Emissivity = 1.0
Also an ideal absorber
• Emissivity + Reflectivity + Transmittance = 1.0
Reflectivity = measurement of an object's ability to reflect
radiation
Transmittance = measurement of an object's ability to
pass or transmit radiation
• Ideal surface for infrared measurements is a perfect
radiator with an emissivity = 1

11. Most objects are not perfect radiators
Many instruments - compensate for different emissivities
Higher emissivity >> better chance getting accurate
Low emissivity objects = polished, shiny surfaces
• Most organic substances have emissivity = 0.95
Transmission - not an important consideration - except
in case of plastics and glass

12. BLACK GLOBE THERMOMETER
1) Practical / Inexpensive means - isolating mean radiant
temperature from other factors in - thermal
Environment
2) Indication of combined effects of radiant energy, air
temperature, and air velocity
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13. MEAN RADIANT TEMPERATURE
Temperature of a uniform "black" enclosure in which an
object would exchange same amount of energy as in
actual environment.
MRT = 100 {[Tg / 100]4 + 1.028 x sq. root [V(tg - ta)]}.25-460
Tg = tg + 460 tg = globe temperature (°F)
V = air velocity (fpm) ta = air temperature (°F)

14. RADIANT HEAT LOAD
Total radiation received by an object from all surroundings
RHL = S x Ts4
Ts = MRT + 460
S = Stefan-Boltsman Constant = 0.173 x 10-8