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8.2: Water Moisture in the Air

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    Water Moisture in the Air (Humidity)

    The amount of water vapor in the air can range from trace amounts up to about 4% by volume. Humidity is a term used to describe the relative amount of water vapor dissolved in the air. In general terms, humid refers to moist air and arid refers to dry air conditions.

    Warm air can hold more moisture than cold air. As warm, moist air cools, the relative humidity increases. When air has reached the maximum amount or water it can hold it is called saturated - this occurs when clouds form and moisture condenses to for water droplets. As the air continues to cool, microscopic water droplets grow in size and it may start raining! The atmospheric temperature below which water droplets begin to condense is called the dew point. The dew point is the temperature at which moist air reaches 100% saturation. Dew can form on objects and consists of tiny drops of water that form on cool surfaces at night when the atmospheric vapor condenses (such as on grass) (Figure 8.6).

    Weather reports frequently report the relative humidity. Relative humidity of 100% indicates that the dew point is equal to the current temperature when the air is holding the maximum amount of water vapor (it is saturated), and water vapor will begin to condense into water droplets, forming fog (clouds). Moist air is less dense than dry air. This explains why moist are rises to form clouds.

    The amount water moisture that air can hold depends on factors including air temperature, air pressure, and the amount and kinds of particulate matter dispersed in the air (see cloud condensing nuclei [CCNs] discussed below). For example, warm, humid air near sea level may be clear clear during warm daylight hours, but as the air temperature drops at night the relative humidity will increase until it reaches the dew point and fog begins to form. Fog is a thick cloud of tiny water droplets suspended in the atmosphere at or near the earth's surface (restricting visibility) (Figure 8.7). As the air continues to cool, the condensing droplets water form mist. When the mist droplets grow large enough to be influenced by gravity, it will fall as precipitation.

    The dew point is called the frost point when the temperature is below the temperature that water freezes. Below freezing, water moisture in the air will sublimate directly into ice, forming frost, snow, or hail (Figure 8.8).

    A hygrometer is a device that can measure the humidity in the air. There are many kinds of hygrometers and different ways to measure humidity in the air. Modern hygrometers use sensors that can directly measure the electrical, optical, thermal, and other means to accurately measure water content in the air. Hygrometers are part of any meteorological station, and can be measured with regional radar and satellite sensor data. Hygrometers are important devices for measure moisture in soil.

    National Weather Service (NOAA) - Relative Humidity Calculator (enter data for current temperature and dew point to determine relative humidity).

    Dew forming on leaves. Fog layer beneath the Golden Gate Bridge, California. Large frost crystals on frozen ground.
    Figure 8.6. As the atmosphere cools, the moisture content dissolved in the air will condense to form water droplets, such as illustrated by dew on leaves. As the air continues to cool, more and more moisture will be released and water droplets Figure 8.7. A Fog Bank Fog under the Golden Gate Bridge, CA. where water moisture condenses in the air near the cool ocean surface. The warm moist air above the bridge remains clear because it is above the dew point. Figure 8.8. Frost forms when air moisture directly sublimates from the air onto cold surfaces. These spectacular frost ice crystals formed on frozen ground in South Dakota on a day when the high temperature only reached -11º Fahrenheit.

    The Water Cycle

    The water cycle involves all processes by which water circulates between the Earth's oceans, atmosphere, and land. It involving precipitation as rain, snow, hail, drainage in streams and rivers, and return to the atmosphere by evaporation and transpiration. The weight of the atmosphere provides the pressure needed to keep water liquid on the surface of the planet. Planets and moons with thin or no atmosphere may have water as ice, but there will be no permanent bodies of liquid water. Ice will sublimate directly to water vapor in a vacuum.

    The Water Cycle is also called the Hydrologic Cycle (USGS)

    Water Cycle (NASA version)
    Figure 8.9. The Water Cycle

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