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Heat Transfer Practically all of the energy that reaches the earth comes from the sun. Intercepted first by the atmosphere, a small part is directly absorbed, particularly by certain gases such as ozone and water vapor. Some energy is reflected back to space by clouds and the earth's surface. Most of the radiation, however, is absorbed by the surface. Energy is transferred between the earth's surface and the atmosphere in a variety of ways, including radiation, conduction, and convection. The graphic below uses a camp stove to summarize the various mechanisms of heat transfer. If you were standing next to the camp stove, you would be warmed by the radiation emitted by the gas flame. A portion of the radiant energy generated by the gas flame is absorbed by the frying pan and the pot of water. By the process of conduction, this energy is transferred through the pot and pan. If you reached for the metal handle of the frying pan without using a potholder, you would burn your fingers! As the temperature of the water at the bottom of the pot increases, this layer of water moves upward and is replaced by cool water descending from above. Thus convection currents that redistribute the newly acquired energy throughout the pot are established.

As in this simple example using a camp stove, the heating of the earth's atmosphere involves radiation, conduction, and convection, all occurring simultaneously. A basic tenet of meteorology is that the sun warms the ground and the ground warms the air. In this activity, we will focus on radiation, the process by which the sun warms the ground. Energy from the sun is the driving force behind weather and climate, and ultimately, life on earth. Radiation What do trees, snow, cars, horses, rocks, centipedes, oceans, the atmosphere, and you have in common? Each one is a source of radiation to some degree. Most of this radiation is invisible to humans but that does not make it any less real.

Radiation is the transfer of heat energy by electromagnetic wave motion. The transfer of energy from the sun across nearly empty space is accomplished primarily by radiation. Radiation occurs without the involvement of a physical substance as the medium. The sun emits many forms of electromagnetic radiation in varying quantities.

About 43% of the total radiant energy emitted from the sun is in the visible parts of the spectrum. The bulk of the remainder lies in the near-infrared (49%) and ultraviolet section (7%). Less than 1% of solar radiation is emitted as x-rays, gamma waves, and radio waves. A perfect radiating body emits energy in all possible wavelengths, but the wave energies are not emitted equally in all wavelengths; a spectrum will show a distinct maximum in energy at a particular wavelength depending upon the temperature of the radiating body. As the temperature increases, the maximum radiation occurs at shorter and shorter wavelengths. The hotter the radiating body, the shorter the wavelength of maximum radiation. For example, a very hot metal rod will emit visible radiation and produce a white glow. On cooling, it will emit more of its energy in longer wavelengths and will glow a reddish color. Eventually no light will be given off, but if you place your hand near the rod, the infrared radiation will be detectable as heat. The amount of energy absorbed by an object depends upon the following: · The object's absorptivity, which, in the visible range of wavelengths, is a function of its color · The intensity of the radiation striking the object Darker-colored objects absorb more visible radiation, whereas lighter-colored objects reflect more visible radiation. That's why we usually choose light-colored clothing on really hot days. Every surface on earth absorbs and reflects energy at varying degrees, based on its color and texture.

Conduction is one of the ways that energy is transferred from the earth's atmosphere to the air. Conduction is the process by which heat energy is transmitted through collisions between neighboring molecules.

Think of a frying pan set over an open camp stove. The fire's heat causes molecules in the pan to vibrate faster, making it hotter. These vibrating molecules collide with their neighboring molecules, making them also vibrate faster. This process continues until the entire pan has heated up due to the vibrating and colliding molecules. If you've ever touched the metal handle of a hot pan without a potholder, you have first-hand experience with heat conduction! Some solids, such as metals, are good heat conductors, while others, such as wood, are poor conductors. Air and water are relatively poor conductors and thus are called insulators. Not surprisingly, many pots and pans have insulated handles. What does conduction have to do with the atmosphere? Air is a poor conductor of heat energy but a good insulator. Imagine that you're a mountaineer, climbing Mt. Everest. Near the summit, you'll probably be wearing a thick down jacket and pants to guard against the extreme cold. It's not the down itself that keeps you warm. Rather, it's the enormous number of tiny air spaces trapped between the down filaments that retard the conduction of heat from your body out to the frigid environment. If air were not such a good insulator, no amount of down would allow survival under such conditions. Because air is such a poor energy conductor, large vertical temperature gradients can exist near the ground, particularly on clear and windless days. On such days, the land surface may experience a great deal of heating, as direct solar radiation is absorbed and converted to infrared radiation (heat energy). However, a series of thermometers mounted at different heights above the ground would reveal that air temperature falls off rapidly with height due to the poor conductivity of air. An interesting aside: Some species of ants inhabit hot desert areas and forage during the heat of the day. The temperature at ground level would kill them after a short time of constant exposure. They continue to function, though, by frequently climbing a few centimeters up sticks or bits of vegetation to rest in the much cooler air above. Heat moves in fluids through several processes, including convection. Convection is the transfer of heat by the actual movement of the heated material. Any substance that flows is considered a fluid. This includes such things as water, shampoo, sunscreen, and even honey. Although not necessarily obvious, even gases, such as air, can be classified as fluids. Consider what happens to the water in a pot as it is heated over an open camp stove.

The water at the bottom of the pot heats up first. This causes it to expand. Since the warmed water has a lower density than the water around it, it rises up through the cooler, dense water. At the top of the pot, the water cools, increasing its density, which causes it to sink back down to the bottom. This up and down movement eventually heats all of the water. The continual cycling of the fluid is called a convection current. Convection currents are found in many places and on many scales, from huge convection currents in the atmosphere, oceans, and even in the earth's interior to smaller convection currents found in a cup of hot cocoa or a fish tank. Meteorologists usually use "convection", referring to up and down motions of air. Heat gained by the lowest layer of the atmosphere from radiation or conduction is most often transferred by convection. Convective motions in the atmosphere are responsible for the redistribution of heat from the warm equatorial regions to higher latitudes and from the surface upward.


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