Temperature, Heat, and the Atmosphere

Temperature can be defined as the degree of the hotness or coldness of a substance and reflects the average molecular activity of the substance. Heat energy represents the total molecular energy of a substance and is therefore dependent on both the number of molecules and the degree of molecular activity. If heat is applied to a substance and there is no change in physical structure (such as ice to water or water to water vapor), the molecular activity increases and the temperature rises. Conversely if heat is lost the temperature will drop.

Heat and temperature differ in that heat can be converted to other forms of energy and can be transferred from one substance to another, while temperature has neither capability. Temperature, however, determines the direction of net heat transfer from one substance to another. Heat always flows from the substance with the higher temperature to a substance of lower temperature, and stops flowing when the temperatures are equal. While the amount of heat received by the cooler substance is equal to that lost by the warmer substance, the temperature changes between the two substances are not necessarily equal.

Since different substances have different molecular structures, the same amount of heat applied to equal masses of different substances will cause one substance to get hotter than the other. In other words the two substances have different heat capacities. In the English system of measurement, one Btu (British thermal unit, a measure of heat capacity) is the amount of heat required to raise the temperature of 1 pound of water by 1 degree Fahrenheit. The specific heat of a substance is the ratio of its heat capacity to that of water (water therefore has a specific heat of 1.0). A substance with a specific heat of 0.5 would increase in temperature by 2 degrees F for each Btu of heat added.

With minor exceptions, solids and liquids expand when their molecular activity is increased by heating and contract when molecular activity decreases due to cooling. The amount of expansion/contraction is dependent upon the material. The expansion/contraction of a liquid is used in thermometers to measure changes in temperature. The response of gases to changes in temperature is more complex as temperature changes may change either the volume or pressure of the gas, or both. If volume is held constant, the pressure will increase as the temperature increases.

The atmosphere receives heat energy from the sun through radiation. This heat energy is converted into thermal energy. Since the atmosphere is not confined, atmospheric processes do not occur at constant volume. Either the pressure is constant or both pressure and volume change. If the pressure remains constant, the volume increases as temperature increases and the volume decreases as the temperature decreases. This change in volume brings about significant changes in the density (mass per unit volume) of the gas. Rising temperatures lead to decreased densities.

When the volume of a gas expands due to increased temperature, the gas must perform work on its environment and therefore expend some of its internal (molecular) energy. Decreasing the internal energy of the gas results in a cooling of the gas. Therefore expansion is a cooling process. Conversely, compression (decreasing the volume of the gas) is a warming process as the environment must do work on the gas to compress it, thereby transferring molecular energy to the gas. An increase in molecular energy of the gas leads to an increase in temperature of the gas.