Radiant Exchange Heat Transfer at the Speed of Light (3 x 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 (T T 2 4 ) Transfer Surf. 1 Surf. 2

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

Total radiant power - received outside earth’s atmosphere -on a plane - right angle to sun’s rays = 1360 W/m 2 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

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, CO 2, O 3, etc....) - absorbs infrared radiation

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

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

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

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.

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.

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

Most objects are not perfect radiators Many instruments - compensate for different emissivities Higher emissivity >> better chance getting accurate temperature 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

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 _____________________________________________ _

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] x sq. root [V(tg - ta)]} Tg = tg + 460tg = globe temperature (°F) V = air velocity (fpm)ta = air temperature (°F)

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