Water in the atmosphere exists in three states: vapor, liquid, and solid. Water vapor is water in the gaseous state and is not visible. In the liquid state, it forms as rain, drizzle, fog, and as small water droplets which form clouds. In its solid state, it forms as snow, hail, ice pellets, ice-crystal clouds, and ice-crystal fog.
The oceans serve as the primary source of water vapor for the atmosphere. To a lesser extent, moisture sources also include lakes, rivers, swamps, moist soil, snow, ice fields, and vegetation. As moisture evaporates into its vapor state, the wind may carry moisture great distances before it changes states into liquid or solid precipitation.
Evaporation, condensation, sublimation, and deposition are changes of state. When water changes from a liquid to a gas, molecules escape from the liquid’s surface and enter the air as water vapor. Evaporation rate increases with temperature. This is a simplified explanation of evaporation. Condensation is the change of state from a gas to a liquid. Condensation takes place when a volume of air reaches its saturation point. Sublimation is the change of state directly from a solid to a gas. The reverse is called deposition which is the change directly from a gas to a solid. Two other familiar changes of state are melting and freezing.
Any change of state involves heat exchange where energy is absorbed or released. Figure 2-2 illustrates the heat exchanges between the different states of water. During evaporation, escaping water molecules absorb energy (heat) to break away from the attraction of the other molecules. This cools the remaining liquid since it has less heat. Heat required for evaporation is not lost, but remains hidden or latent in the water vapor. When the vapor condenses to liquid water or deposits directly to ice, this heat is released to the atmosphere and the surrounding air becomes warmer. As an example, the amount of heat given off in a thunderstorm during the process of precipitating one-half inch of rain over a square mile is over 71 trillion Joules. The energy released by the atomic bomb dropped on Hiroshima was almost 84 trillion Joules. Melting, freezing, and sublimation all involve the exchange of heat in a similar manner.
The capacity for a parcel of air to hold water vapor is related to the temperature, where as temperature is a measure of energy. Therefore, the warmer the air, the more energy is available for evaporation or sublimation. The cooler the air, the less energy is available for condensation or deposition, until the saturation point is reached and water droplets or ice crystals grow to the point they form visible clouds. According to Charles’ Law, when pressure is held constant, as temperature increases, so does the volume of air (Figure 2.3). In this case the warmer the air, the more water vapor it can hold before reaching saturation when the water condenses. For approximately every 20oF (11oC) increase in temperature, the capacity of a volume of air to hold water vapor doubles. Conversely, as a parcel cools, the temperature and dew point temperature eventually equalize, and the relative humidity becomes 100%. Further cooling allows some water vapor to visibly condense as fog, clouds, or precipitation. The relative humidity and dew point concepts are expanded during the next few paragraphs.