• The unending circulation of Earth's water supply is called the hydrologic cycle (or water cycle). The cycle illustrates the continuous movement of water from the oceans to the atmosphere, from the atmosphere to the land, and from the land back to the sea. The quantitative view of the hydrologic cycle is referred to as the water balance.

  

• Water vapor, an odorless, colorless gas, can change from one state of matter (solid, liquid, or gas) to another at the temperatures and pressures experienced on Earth. The heat energy involved in the change of state of water is often measured in calories. The processes that change the state of matter of water include evaporation (liquid to gas), condensation (gas to liquid), melting (solid to liquid), freezing (liquid to solid), sublimation (solid to gas), and deposition (gas to solid). During each change, latent (hidden or stored) heat energy is either absorbed or released.

  

• Humidity is the general term used to describe the amount of water vapor in the air. The methods used to express humidity quantitatively include (1) absolute humidity, the mass of water vapor in a given volume of air, (2) mixing ratio, the mass of water vapor in a unit mass of dry air, (3) vapor pressure, that part of the total atmospheric pressure attributable to its water vapor content, (4) relative humidity, the ratio, most often expressed as percent, of the air's actual water vapor content compared with the amount of water vapor required for saturation at that temperature, and (5) dew point, the temperature to which a parcel of air would need to be cooled to reach saturation. When air is saturated, the pressure exerted by the water vapor, called the saturation vapor pressure, produces a balance between the number of water molecules leaving the surface of the water and the number returning. Because the saturation vapor pressure is temperature-dependent, at higher temperatures more water vapor is required for saturation to occur.

• Relative humidity can be changed in two ways, (1) by changing the amount of moisture in the air, or (2) by changing the air's temperature. Adding moisture to the air while keeping the temperature constant increases the relative humidity. Removing moisture lowers the relative humidity. When the water vapor content of air remains at a constant level, a decrease in air temperature results in an increase in relative humidity and an increase in temperature causes a decrease in relative humidity. In nature, there are three major ways that air temperatures change to cause corresponding changes in relative humidity; (1) daily (daylight versus nighttime) changes in temperature, (2) temperature changes that result as air moves horizontally from one location to another, and (3) changes caused as air moves vertically in the atmosphere.

  

• An important concept related to relative humidity is the dew-point temperature (or simply dew point), which is the temperature to which a parcel of air would need to be cooled to reach saturation. Unlike relative humidity, which is a measure of how near the air is to being saturated, dew-point temperature is a measure of the air's actual moisture content. High dew-point temperatures equate to moist air and low dew-point temperatures indicate dry air. Because the dew-point temperature is a good measure of the amount of water vapor in the air, it is the measure of atmospheric moisture that appears on daily weather maps.

• A variety of instruments, called hygrometers, can be used to measure relative humidity. One of the simplest hygrometers, a psychrometer, consists of two identical thermometers mounted side by side. One thermometer, called the wet bulb thermometer, has a thin muslin wick tied around the bulb. After spinning or fanning air past the instrument and noting the difference between the dry- and wet-bulb readings (known as the depression of the wet bulb), tables are consulted to determine the relative humidity. A second instrument, the hair hygrometer, can be read directly without using tables.

  

• When air is allowed to expand, it cools. When air is compressed, it warms. Temperature changes produced in this manner, in which heat is neither added nor subtracted, are called adiabatic temperature changes. The rate of cooling or warming of vertically moving unsaturated ("dry") air is 10°C for every 1000 meters (5.5°F per 1000 feet), the dry adiabatic rate. At the lifting condensation level (the altitude where the parcel of air has reached saturation and cloud formation begins), latent heat is released and the rate of cooling is reduced. The slower rate of cooling, called the wet adiabatic rate of cooling ("wet" because the air is saturated) varies from 5°C per 1000 meters for air with a high moisture content to 9°C per 1000 meters for air with a low moisture content.

  

• When air rises, it expands and cools adiabatically. If air is lifted sufficiently high, it will eventually cool to its dew-point temperature, and clouds will develop. Four mechanisms that cause air to rise are (1) orographic lifting, where air is forced to rise over a mountainous barrier, (2) frontal wedging, where warmer, less dense air is forced over cooler, denser air along a front, (3) convergence, a pileup of horizontal air flow resulting in an upward flow, and (4) localized convective lifting, where unequal surface heating causes localized pockets of air to rise because of their buoyancy.

  

• When air rises, it cools and can eventually produce clouds. Stable air resists vertical movement, whereas unstable air rises. The stability of air is determined by knowing the environmental lapse rate, the temperature at various heights. The three fundamental conditions of the atmosphere are (1) absolute stability, when the environmental lapse rate is less than the wet adiabatic rate, (2) absolute instability, when the environmental lapse rate is greater than the dry adiabatic rate, and (3) conditional instability, when moist air has an environmental lapse rate between the dry and wet adiabatic rates. In general, when stable air is forced aloft, the associated clouds have little vertical thickness, and precipitation, if any, is light. In contrast, clouds associated with unstable air are towering and frequently accompanied by heavy rain.

• Any factor that causes air near the surface to become warmed in relation to the air aloft increases the air's stability. The opposite is also true; any factor that causes the surface air to be chilled results in the air becoming more stable. Most processes that alter stability result from temperature changes caused by horizontal or vertical air movement, although daily temperature changes are important too. Changes in stability occur as air moves horizontally over a surface having a markedly different temperature than the air. Furthermore, subsidence (a general downward airflow) generally stabilizes the air, while upward air movement enhances instability.

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