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Heat Transfer


The source of heat for our planet is the sun. The sun's energy moves through space, then through the earth's atmosphere and finally reaches the earth's surface. The sun's radiation warms the earth's atmosphere and surface and becomes heat energy. This heat energy is transferred through the atmosphere by one of three mechanisms:

1. Radiation
2. Conduction
3. Convection


Radiation

This type of heat transfer can be observed on sunny days. You face will feel warm when you are standing in the sun. The sunlight is absorbed by your face and warms you face, without warming the air around you. The energy from the sun that is absorbed by your face is called radiant energy or radiation. Radiation is the transfer of this heat energy by electromagnetic waves.


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Most of the electromagnetic radiation from the sun is in the form of visible light. Light is made up of waves of different frequencies. These frequencies are interpreted by our brain as colors. Infrared waves and ultraviolet waves are two types of waves from the sun that we cannot see.

Solar radiation mostly passes through the atmosphere and is absorbed by all objects, such as humans, trees, flowers, roads, etc. These objects will then warm up. Dark objects, such as asphalt roads, will absorb and warm faster than light colored objects, which reflect the radiation back to space.. All substances emit radiation, but this emitted radiation will be at a longer wavelength that our eyes cannot see. This emitted radiation, called infrared radiation, can be absorbed by the atmosphere. A substances's temperature will determine which wavelength of radiation the substance will emit and also the rate of emission. The higher the substances's temperature, the shorter the wavelength of the emitted radiation (think of how a burner on an electric stove turns from black to red as it heats up). Also, the higher the substance's temperature, the greater the emission rate of radiation.


Conduction

Conduction is the transfer of heat from one molecule to another within a substance. Remember that temperature is just the measure of the average kinetic energy or speed of the molecules in a substance. Imagine you are holding a metal pin between your fingers and you place this pin in a flame. The pin absorbs the energy from the flame and the molecules inside the pin begin to move faster (warmer temperature). These faster moving molecules cause adjoining molecules to move faster and will eventually cause the molecules in your fingers to move faster. The heat is now being transferred from the pin to your finger and your finger will heat up. This is an example of heat transfer through conduction. When heat is transferred through conduction, it flows from warmer to colder regions and will transfer more rapidly with greater temperature differences. The rate of heat transfer through conduction also depends on whether the substance is a good conductor. It turns out that air is an extremely poor conductor of heat. Therefore, conduction is only important in the atmosphere within the first several millimeters closest to the surface. How then does the air transfer energy from one region to another?


Convection

Convection is the transfer of heat through the movement of a fluid, such as water or air. This type of heat transfer can occur in liquids and gases because they move freely, making it possible to set up warm or cold currents. Convection occurs naturally in the atmosphere on a warm, sunny day. As the earth's surface absorbs sunlight, certain portions of the surface absorb more than other portions. The earth's surface and the air near the surface heats unevenly. The warmest air expands, becomes less dense than the surrounding cooler air, becomes buoyant and rises. These rising "bubbles" of warm air, called thermals, act to transfer heat up into the atmosphere. Cooler, heavier air then flows toward the surface to replace the warm air that just rose. When the cooler air reaches the surface, it is warmed and it too eventually rises as a thermal. This circulation is referred to as a convective circulation or thermal cell. These "bubbles" or thermals can result in cloud formation, which will be discussed more in the clouds section.



Convection transfers heat vertically into the atmosphere. In order for heat to be transferred to other regions, it must be transferred horizontally by the wind. The horizontal transfer of heat by the wind is called advection.


© 2006 Katie R. Roussy, University of Illinois at Urbana-Champaign
Images and photographs courtesy of the National Oceanic and Atmospheric Administration