Clive M. Countryman

“ Radiation and Weather Heat reaches the earth through radiation One way radiation affects fire behavior and fire control is through its effect on weather. Almost all the heat at the earth's surface and in its atmosphere comes from the sun as radiant energy. Because of the sun's high temperature, most solar radiation is in the short wavelengths of thermal radiation— much of it is visible light. A small part of the shortwave radiation is absorbed by the atmosphere, mostly in the upper layers, but the majority passes through the air without warming it much. ”

“ Radiation is energy in the form of electromagnetic waves In Part 1, we learned that heat is a form of energy called thermal energy. Often we sense radiation as heat too, such as when standing in front of a fire in a fireplace or near a black-topped road on a hot summer day. But radiation is not heat—it is an entirely different form of energy. Radiant energy exists as electromagnetic waves, similar in form to the waves of alternating current electrical energy. ”

“ Fuel density affects thermal conductivity In our discussion of the nature of heat (Part 1) , we saw that the heat capacity of wildland fuels increases with their density, or weight per unit of volume. The ability of wildland fuels to conduct heat is also closely related to fuel density; the greater the density, the higher the thermal conductivity. Thus, heat conduction in heavy fuels such as oak and maple can be considered analogous to the flow of water in a smooth pipe—these fuels impede the conduction of heat much less than do low-density fuels. ”

“ But because radiation intensity increases with temperature, the warmer object therefore loses heat, and its temperature decreases—part of its heat is transferred to the other object. Conversely, the cooler object receives more radiation than it is emitting and its amount of heat and temperature increases. Eventually, both objects attain the same temperature—their radiant energy gain and loss just balances. Thus, if a substance receives more radiation than it is losing, it gains heat and becomes warmer, but if it is radiating more energy than it is receiving, it loses heat and becomes cooler. ”

“ Often information on the amount of heat needed to produce a given temperature change in some volume of a material is needed. In wildland fire, for example, we are interested in the amount of heat needed to raise the temperature of a layer or volume of the fuels, because this helps determine the characteristics of a firebrand that can ignite a particular fuel. The amount of heat required to raise the temperature of a unit volume of a substance by one degree is its heat capacity. ”

“ When heated, the fuels first produce water vapor and other gases that are mostly noncombustible. Not until the fuel temperature reaches 400° F or more does significant amounts of combustible gas begin to appear. At about this temperature too, the chemical reactions themselves begin to generate heat; then if heat losses are small, the pyrolysis may become self-sustaining, the temperature continuing to rise without an outside heat source. These heat-producing reactions at relatively low temperatures are important in the spontaneous combustion of fuels. ”

“ Solar radiation provides the energy for photosynthesis, hence vegetation provides a storage reservoir for the sun's energy. This energy, often accumulated over years, is released quickly when the vegetation burns. In the absence of precipitation, the moisture content of dead fuel is affected to some extent by the fuel temperature, but the effect of relative humidity is much stronger. And for a given air mass, relative humidity is controlled almost entirely by air temperature, which in turn is closely related to solar radiation intensity. ”

“ The heat of vaporization of water is important in natural processes also. Many living creatures, including man, depend on the cooling effect of water evaporating from the skin or lungs to regulate their internal body temperature. The evaporation of water into vapor and the condensation of the vapor back into water in the form of clouds or precipitation has a major effect on our weather and climate. Because of the large volumes of water and vapor involved, enormous amounts of thermal energy can be released or stored. ”

“ Heating by radiation occurs at the surface, so even a thin layer of dirt over fuels that cannot be moved, such as stumps or large logs, is often sufficient to prevent them from igniting. Locating the cleared control line just over the crest of a ridge on the slope away from the fire reduces the amount of fire the fuel can see, decreases the time it is exposed to radiation, and increases the distance of the fuel from flames inclined over the ridge. Lopping dead fuel so that it lies closer to the ground can also reduce the fire area to which it is exposed and the amount of heat it receives. ”

“ We can halt fire spread by reducing or preventing heat transfer by convection or radiation. But to extinguish a fire completely, we must eliminate heat transfer by conduction.
Small fuels are most important Anyone who has started a fire is well aware that small or thin fuels ignite more readily than do larger fuels, and such "kindling" is usually needed to start the larger fuels burning at all. The reason lies in the rapidity with which small fuels can be heated and ignited. All of the heat that gets into a piece of fuel must go through the surface. ”

“ All substances radiate energy when their temperature is above absolute zero. Radiant energy is not the same as thermal energy or heat, but radiant energy can be converted to heat and heat can be converted to radiant energy. Heat transfer by radiation is accomplished through this interconvertibility of radiant energy and heat. Radiation is the only means of heat transfer that does not require an intervening substance between the heat source and the receiving substance.
Radiant energy varies greatly in wavelength. ”

“ For example, radiant energy from the sun is transformed by the process of photosynthesis to stored chemical energy in vegetation. When the vegetation is burned, the chemical energy is transformed to thermal energy, radiant energy, and the kinetic energy in the rising air in the convection column over the fire. The kinetic energy in falling water can be used to generate electrical energy, which in turn can be changed back to thermal and radiant energy by an electric heater, or to kinetic energy again through the use of an electric motor. ”

“ Dark-colored materials are usually better emitters and absorbers of radiation than are light-colored materials, and opaque materials are better than transparent ones. A substance capable of emitting and absorbing all thermal radiation is called a black body.
Not all thermal radiation reaching a substance is absorbed. Part of the radiation may be transmitted unchanged through the substance, and some may be reflected. Reflection does not change the wavelength of the radiation, only its direction of travel. Only absorbed radiation is converted to heat. ”

“ The combustible gases do not contain enough oxygen to burn when emitted from the fuel, and must first mix with the surrounding air before a flammable mixture is produced. If the pyrolysis is slow, not much gas is generated and the flames are short and intermittent. But when large amounts of fuel are burning rapidly, the volume of gas is large and some must move a considerable distance from the fuel before the mixture becomes flammable. ”

“ If you are caught in a wildland fire situation from which escape is not possible, your survival may depend on protection from intense radiation while the main flame wave passes. Possible temporary shelters include road cuts and cut banks, boulders, large logs, depressions in the ground, motor vehicles, and buildings in good conditions. Because fire and radiation intensity usually increase with fuel loading, refuge should be sought in areas where the fuel is as sparse as can be found. ”

“ Enough oxygen for combustion is almost always available in our wildland areas, and fuel is usually abundant. However, we cannot have a fire until the third ingredient, heat, is added—usually in the form of a firebrand of some sort. But the mere presence of a heat source does not necessarily mean that a fire will start. For a flaming or hot firebrand to start a fire, some of its heat must be transferred in some way to the fuel. And if the fire is to continue to burn and to grow, heat must be transferred to the unburned fuel around the fire. Hence, heat transfer is essential for wildland fire. ”

“ Radiation strikes the receiving object from all of the points—in greatest intensity from points perpendicular and closest to the object, and in lesser intensity from the other points. Because so many points are producing radiant energy, the decrease in intensity with distance from a flame source is less than that from a line source and much less than that from a point source. The number of points from which radiation can be received also increases rapidly with the area of the radiating surface. ”

“ Wildland fuels do not burn directly If we look closely at a piece of burning wood, such as a lighted match, we see that the flames are not attached directly to the wood surface, but are separated from it by a thin layer of vapor or gas. This happens because solid organic materials do not burn in flaming combustion directly, but must first be decomposed or pyrolyzed by heat and chemical reactions into various gases—some combustible and some not. ”

“ The peculiarities of heat transfer by radiation are also useful in reducing the chance that a fire will ignite fuel across the control line. Because radiation travels only along straight lines and its intensity decreases rapidly with distance, the likelihood of ignition of highly susceptible fuels can be reduced by moving them away from the edge of the control line or to a place where they will be shielded from direct radiation by nonflammable material or less flammable fuels. ”

“ Radiant energy travels outward in all directions from the emitting substance until it encounters something capable of absorbing it. The absorbed radiation increases the molecular activity in the substance, thereby increasing its temperature and the amount of heat it contains—this is why we frequently sense radiation as heat. The heated substance radiates energy too, and this energy can be absorbed by other substances and converted to heat. ”

“ Many of our wildland fuel beds in their natural state are made up of a mixture of fuel sizes. These sizes may range from a small fraction of an inch to 4 or 5 inches in diameter in brush fuels, or to several feet in diameter in timber types. But because of their relatively slow burning rate, the large fuels usually contribute little to heat production in the early stages of a fire. For example, in dead and fairly dry large fuels, the pyrolysis characteristically penetrates at the rate of 1 to 2 inches per hour into the fuel. A 2-inch limb may require up to 2 hours to burn completely. ”

“ All of the radiant energy arranged in the order of wavelength forms the electromagnetic spectrum of radiant energy.
Most radiant energy is invisible, and its wavelength and often its presence can be detected only with special instrumentation. Visible light occupies a narrow band near the middle of the spectrum, and its wavelengths can be readily distinguished because light waves of differing lengths are visible as different colors. Radiation from the sun includes all visible wavelengths. ”