Weather Theory

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