**1.3 Ultraviolet radiation**

198 Solar Radiation

detrimental to ectotherms relying on external heat sources, because any such barrier would slow the rate of heat transfer into the body (Speakman, 2004). Endotherms such as birds and mammals have varying degrees of insulation made from feathers, fur, and/or fat deposits allowing them to retain their internal heat and thus rely less upon external

The ratio of surface area to the volume of an organism is also an important component of heat transfer through conduction. Animals not only gain or lose heat via conduction with surfaces and objects in the environment they come in contact with, but also from the core of the body outwards toward the skin surface. Larger animals create thermal inertia, requiring less energy to balance heat loss and so have a low thermal conductance. Smaller animals lose heat rapidly and need more energy per gram of tissue to maintain heat balance with their higher thermal conductivity. Behavioral adjustments that change the amount of exposed surface area as well as physiological responses can increase or decrease an animals' thermal conductivity in either short periods of time or for seasonal

Rather than two solid objects adjacent with one another, convection requires one fluid coming in contact with another fluid (water or air) of a different temperature. Rather than heat transferring at the molecular level or through electromagnetic waves, heat is dispersed via the bulk motion of fluids (Cengel, 2003). Once again the temperature gradient between the two surfaces influences the rate of heat transfer. Conduction and convection along the human body is illustrated in Figure 2. Water conducts heat 23-25 times greater than air, making water an efficient medium for heat loss exhibited by both animals and humans moving towards sources of water during periods of intense solar radiation and high ambient temperatures (TA). Wind is also an efficient mode of convection. When cool air flows over the warmer skin surface of an animal, this may allow that organism to remain active longer on warm, sunny days. Convection even occurs within the body. Blood transfers heat throughout the body, bringing warmth from the core to the extremities, or bringing cooled blood from the surface back to overheated

Evaporation uses the energy required to convert liquid water to gas, allowing organisms to transfer heat even if the TA is greater than TB. This is often the only mechanism available to organisms during prolonged exposure to solar radiation that efficiently dumps excess heat in an effort to maintain a safe TB. Both environmental as well as metabolic water (Figure 2) can be used to transfer heat from an organism's body to the surrounding air (Gupta, 2011). Though highly effective, if the air is saturated with moisture already, such as during periods of high relative humidity or in constantly humid environments, evaporation as a thermoregulatory mechanism is rendered almost useless because net evaporation ceases when the air can no longer absorb additional moisture

heat sources.

modifications.

**1.2.3 Convection** 

organs or muscle tissue.

**1.2.4 Evaporation** 

(Speakman, 2004).

The emitted solar spectrum contains ultraviolet (UV) radiation that is separated into different wavelength bands: UVA (315-400 nm), UVB (280-315 nm), and UVC (200-280 nm). UVC and 70-90% of UVB wavelengths are typically blocked by the ozone layer (Rafanelli et al., 2010; Schaumburg et al., 2010). However, due to ozone deterioration there is an increase in UVB levels, especially over Antarctica. Certain environments, such as polar areas, are more susceptible to varying levels of UV radiation (Rafanelli et al., 2010). Organisms across all taxa are directly or indirectly affected by UV radiation. Such responses are discussed in more detail later in this chapter.
