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Temperature Variations with Topography
Temperature variations are also induced by water and terrain. Water absorbs and radiates heat energy with less temperature change than does land. Large, deep water bodies tend to minimize temperature changes, while large land masses induce major temperature changes. Wet soil, such as that found in swamps and marshes, is almost as effective as water in suppressing temperature changes. Thick vegetation tends to control temperature changes since it contains some water and also insulates against heat transfer between the ground and the atmosphere. Arid, barren surfaces generate the greatest temperature changes.
These topographical influences are both diurnal and seasonal. For example, the difference between a daily maximum and minimum may be 10° or less over water, near a shore line, or over a swamp or marsh, while a difference of 50° or more is common over rocky or sandy deserts.
Abrupt temperature differences develop along lake and ocean shores. These variations generate pressure differences and local air flows or winds, which may become a consideration in the balloon pilot’s study of the air mass.
Prevailing wind is also a factor in temperature control. In an area where prevailing winds are from large water bodies, temperature changes are rather small. Most islands enjoy fairly constant temperatures. On the other hand, temperature changes are more pronounced where prevailing wind is from dry, barren regions.
Temperature Variation with Altitude
Temperature normally decreases with increasing altitude throughout the troposphere. This decrease of temperature with altitude is defined as lapse rate. The standard lapse rate seldom exists. In fact, temperature sometimes increases with height. An increase in temperature with altitude is defined as an inversion.
An inversion often develops near the ground on clear, cool nights when wind is light. The ground radiates and cools much faster than the overlying air. Air in contact with the ground becomes cold, while the temperature a few hundred feet above changes very little. Thus, temperature increases with height. Inversions may also occur at any altitude when conditions are favorable. For example, a current of warm air aloft overrunning cold air near the surface produces an inversion aloft.
Low level inversions, which are usually of most interest to the balloon pilot, generally dissipate as the air mixes with insolation. Heat Transfer
The heat source for this planet is the sun; energy from the sun is transferred through space and the Earth’s atmosphere to the Earth’s surface. As this energy warms the Earth’s surface and atmosphere, some of it is or becomes heat energy. Heat is transferred into the atmosphere in three ways: radiation, conduction, and convection.Radiation
Radiation is the transfer of heat by electromagnetic waves. No medium of transfer is required between the radiator and the body being irradiated (receiving the radiation). Heat waves, a form of this electromagnetic energy, may be reflected. In meteorology, the principal reflectors are the Earth’s surface, water vapor in the air, and particulate matter in the atmosphere.Conduction
Conduction is the transfer of heat energy from one substance to another or within a substance. As with electricity, some materials are good conductors while others are poor conductors. Poor conductors are considered to be insulators. Air is one of the poorest conductors of heat in comparison to silver, one of the best conductors. Silver will pass 20,000 times more heat than an equal mass of air across a similar temperature difference during a fixed period of time. Conduction in the atmosphere is considered to be a significant
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method of heat exchange only at the Earth’s surface, where the lowest few centimeters of the atmosphere are actually in contact with the ground or water.Convection
Convection is the transfer of heat energy in a fluid. This type of heating is most commonly seen in the kitchen when a liquid boils. This type of heat transfer occurs in the atmosphere when the ground is heated by the sun. The warm ground heats the air above it by radiation and conduction, causing the warm air to rise. Meanwhile, the dense cooler air aloft moves in to take the warm ground air’s place to be heated.
Heat can be transferred by convection in either a vertical or a horizontal direction. In meteorology, “advection” is the term used for the horizontal transport of heat by wind. It is important to differentiate between the vertical and horizontal paths of convection. In the atmosphere, the amount of heat transferred horizontally over the surface of the Earth by advection is about 1,000 times greater than that transferred by convection.
The Adiabatic Process
The adiabatic process is the change of the temperature of air without transferring heat. In an adiabatic process, compression results in warming, and expansion results in cooling. The adiabatic process takes place in all upward and downward moving air. When air rises into an area of lower pressure, it expands to a larger volume. As the molecules of air expand, the temperature of the air lowers. As a result, when a parcel of air rises, pressure decreases, volume increases, and temperature decreases. When air descends, the opposite is true.
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Balloon Flying Handbook(40)
