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. The rate at which temperature decreases  with an increase in altitude is referred to as its lapse rate.  As air ascends through the atmosphere, the average rate of  temperature change is 2 °C (3.5 °F) per 1,000 feet.  The atmosphere, by nature, contains moisture in the form  of water vapor. The amount of moisture present in the  atmosphere is dependent upon the temperature of the air.  Every 20 °F increase in temperature doubles the amount of  moisture the air can hold. Conversely, a decrease of 20 °F  cuts the capacity in half.  Since water vapor is lighter than air, moisture decreases air  density, causing it to rise. Conversely, as moisture decreases,  air becomes denser and tends to sink. Since moist air cools  at a slower rate, it is generally less stable than dry air since  the moist air must rise higher before its temperature cools  to that of the surrounding air. The dry adiabatic lapse rate  (unsaturated air) is 3 °C (5.4 °F) per 1,000 feet. The moist  adiabatic lapse rate varies from 1.1 °C to 2.8 °C (2 °F to  5 °F) per 1,000 feet.  Water is present in the atmosphere in three states: liquid,  solid, and gaseous. All three forms can readily change to  another, and all are present within the temperature ranges of  the atmosphere. As water changes from one state to another,  an exchange of heat takes place. These changes occur through  the processes of evaporation, sublimation, condensation,  deposition, melting, or freezing. However, water vapor  is added into the atmosphere only by the processes of  evaporation and sublimation.  The combination of moisture and temperature determine the  stability of the air and the resulting weather. Cool, dry air  is very stable and resists vertical movement, which leads to  good and generally clear weather. The greatest instability  occurs when the air is moist and warm, as it is in the tropical  regions in the summer. Typically, thunderstorms appear on  a daily basis in these regions due to the instability of the  surrounding air.  Evaporation is the changing of liquid water to water vapor.  As water vapor forms, it absorbs heat from the nearest  available source. This heat exchange is known as the latent  heat of evaporation. A good example is the evaporation of  human perspiration. The net effect is a cooling sensation  as heat is extracted from the body. Similarly, sublimation  is the changing of ice directly to water vapor, completely  bypassing the liquid stage. Though dry ice is not made of  water, but rather carbon dioxide, it demonstrates the principle  of sublimation when a solid turns directly into vapor.  As air rises and expands in the atmosphere, the temperature  decreases. There is an atmospheric anomaly that can occur;  however, that changes this typical pattern of atmospheric  behavior. When the temperature of the air rises with altitude, a  temperature inversion exists. Inversion layers are commonly  shallow layers of smooth, stable air close to the ground. The  temperature of the air increases with altitude to a certain  point, which is the top of the inversion. The air at the top  of the layer acts as a lid, keeping weather and pollutants  trapped below. If the relative humidity of the air is high, it  can contribute to the formation of clouds, fog, haze, or smoke  resulting in diminished visibility in the inversion layer.  Humidity refers to the amount of water vapor present in the  atmosphere at a given time. Relative humidity is the actual  amount of moisture in the air compared to the total amount of  moisture the air could hold at that temperature. For example,  if the current relative humidity is 65 percent, the air is  holding 65 percent of the total amount of moisture that it is  capable of holding at that temperature and pressure. While  much of the western United States rarely sees days of high  humidity, relative humidity readings of 75 to 90 percent are  not uncommon in the southern United States during warmer  months.  [Figure 12-20]  The relationship between dew point and temperature defines  the concept of relative humidity. The dew point, given in  degrees, is the temperature at which the air can hold no  more moisture. When the temperature of the air is reduced  to the dew point, the air is completely saturated and moisture  begins to condense out of the air in the form of fog, dew,  frost, clouds, rain, or snow.  Surface-based temperature inversions occur on clear, cool  nights when the air close to the ground is cooled by the  lowering temperature of the ground. The air within a few  hundred feet of the surface becomes cooler than the air above  it. Frontal inversions occur when warm air spreads over a  layer of cooler air, or cooler air is forced under a layer of  warmer air.  Relative Humidity  Inversion  Temperature/Dew Point Relationship  12-13  Moisture and Temperature 