Recognizing Heat Stress • Normal Animal Values and Indexes • Heat Exchange in Livestock
Heat is transferred between an animal and the environment by radiation, conduction, convention and evaporation processes (Figure above). The magnitude of heat transfer by the first three avenues is highly dependent on the temperaturedifferences between the exchange objects in the environment. Conduction occurs when two stationary objects are in contact with each other. Other determinants of the magnitude of conductive heat transfer include the surface area of contact, thickness of the conducting materials, and thermal conductivity of the conducting objects (i.e., an intrinsic property). For example, a metal surface with high thermal conductivity will conduct heat rapidly from the skin where the thermal receptors are located. In contrast, a wood surface with lower thermal conductivity would conduct heat from the skin at a slower rate. As a result, a metal surface will feel cooler than a wooden one even at the same temperature.
Convective heat transfer occurs when objects of different temperatures flow by each other. Similarly to conduction, area of contact and the thermal gradient are important determinants, however thickness of the exchange materials is not. Convection occurs in two mechanisms: passive or forced. Passive exchange occurs when air near the skin surface microenvironment is heated. This results in small currents of air that may result in significant heat loss under resting conditions. Forced convection occurs when external energy is used to move one material over another. Forced convection is very important for internal heat movement, such blood flow and air movement in the respiratory system. Likewise, external movement of gas and fluid over the animal surface is an effective means of heat dissipation from the body provided that the temperature of the gas or fluid is below skin temperature.
Radiant energy exchange relies on several factors that are the same for conductive and convective exchanges. These include the temperature of the objects, the thermal gradient, and the exchange surface areas. There are, however, several differences between radiant exchange and the other avenues. Radiant exchange is the only heat loss process that occurs at the speed of light and in a vacuum. Any object with a temperature above absolute zero emits electromagnetic radiation. The color of an object is an important factor during exposure to direct sunlight, with dark objects absorbing more energy in the visible portion of the spectrum than light objects. However, infrared exchange is the primary avenue in the absence of sunlight, and the color of an object is not a factor (Heppner, 1970). A dark-colored object will absorb and emit radiation equal to a light-colored object in the absence of visible solar radiation. The more important factor for radiant exchange, on average, is the nature of the surface (i.e., dull or shiny; rough/smooth hair or feather coat).
Evaporation is the fourth avenue for heat loss from an animal. Adams (2001) noted that “evaporation of a liquid is not an avenue of heat exchange (gain or loss); it is only a way to lose heat.” Thermal energy is put into a liquid to produce a phase change to a gaseous state. When water evaporates from a surface, there is a loss of approximately 580 cal g−1 at a skin temperature of 30◦C (Morimoto, 2001). The important point is that the water must evaporate from the surface (e.g., skin, respiratory mucosa) for heat loss to occur. If it does not change to a gaseous state due to high humidity, or if it drips off the animal, there is essentially no heat dissipation.