Heat transfer

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HEAT TRANSFER

The atmosphere is constantly gaining and losing heat. Heat is constantly being transported from one part of the world to another by wind movements. It is due to the inequalities in gain and loss of heat that the air is almost constantly in motion. The motions and heat transformations are directly expressed by wind and weather.

Methods

In meteorology, one is concerned with four methods of heat transfer. These methods are conduction, convection, advection, and radiation. Heat is transferred from Earth directly to the atmosphere by radiation, conduction, and advection. Heat is transferred within the atmos-phere by radiation, conduction, and convection. Advection, a form of convection, is used in a special manner in meteorology. It is discussed as a separate method of heat transfer. As radiation was discussed earlier in the unit, this section covers conduction, convection, and advection.

CONDUCTION.— Conduction is the transfer of heat from warmer to colder matter by contact. Although of secondary importance in heating the atmosphere, it is a means by which air close to the surface of Earth heats during the day and cools during the night.

CONVECTION.— Convection is the method of heat transfer in a fluid resulting in the transport and mixing of the properties of that fluid. Visualize a pot of boiling water. The water at the bottom of the pot is heated by conduction. It becomes less dense and rises. Cooler and denser water from the sides and the top of the pot rushes in and replaces the rising water. In time, the water is thoroughly mixed. As long as heat is applied to the pot, the water continues to transfer heat by convection. The transfer of heat by convection in this case applies only to what is happening to the water in the pot. In meteorology, the term convection is normally applied to vertical transport. Convection occurs regularly in the atmosphere and is responsible for the development of air turbulence. Cumuliform clouds, showers and thunderstorms occur when sufficient moisture is present and strong vertical convection occurs. Vertical transfer of heat in the atmosphere (convection) works in a similar manner. Warmer, less dense air rises and is replaced by descending cooler, denser air, which in turn, acquires heat.

Specific Heat

The specific heat of a substance shows how many calories of heat it takes to raise the temperature of 1 gram of that substance 1°C. Since it takes 1 calorie to raise the temperature of 1 gram of water 1°C, the specific heat of water is 1. The specific heat of a substance plays a tremendous role in meteorology because it is tied directly to temperature changes. For instance, the specific heat of earth in general is 0.33. This means it takes only 0.33 calorie to raise the temperature of 1 gram of earth 1°C. Stated another way, earth heats and cools three times as fast as water. Therefore, assuming the same amount of energy (calories) is available, water heats (and cools) at a slower rate than land does. The slower rate of heating and cooling of water is the reason temperature extremes occur over land areas while temperatures over water areas are more consistent.

The specific heat of various land surfaces is also different, though the difference between one land surface and another is not as great as between land and water. Dry sand or bare rock has the lowest specific heat. Forest areas have the highest specific heat. This difference in specific heat is another cause for differences in temperature for areas with different types of surfaces even when they are only a few miles apart; this difference is important in understanding the horizontal transport of heat (advection) on a smaller scale.

ADVECTION.— Advection is really a form of convection, but in meteorology it means the transfer of heat or other properties HORIZONTALLY. Convection is the term reserved for the VERTICAL transport of heat. In this manual the words convection and advection are used to mean the vertical and horizontal transfer of atmospheric properties, respectively.

Horizontal transfer of heat is achieved by motion of the air from one latitude and/or longitude to another. It is of major importance in the exchange of air between polar and equatorial regions. Since large masses of air are constantly on the move somewhere on Earth’s surface and aloft, advection is responsible for transporting more heat from place to place than any other physical motion. Transfer of heat by advection is achieved not only by the transport of warm air, but also by the transport of water vapor that releases heat when condensation occurs.

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