Dew point

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Dew Point

The dew point is that temperature below which the water vapor in a body of air cannot all remain vapor. When a body of air is cooled to its dew point or below, some fraction of its water vapor shifts from gaseous to liquid phase to form fog or cloud droplets. If a smooth surface is available, vapor condenses directly onto it as drops of water (dew). The dew point of a body of air depends on its water vapor content and pressure. Increasing the fraction of water vapor in air (i.e., its relative humidity) raises its dew point; the water molecules are more crowded in humid air and thus more likely to coalesce into a liquid even at a relatively warm temperature. Decreasing the pressure of air lowers its dew point; lowering pressure (at constant temperature) increases the average distance between molecules and makes water vapor less likely to coalesce.

If the dew point of a body of air is below 32°F (0°C), its water vapor will precipitate not as liquid water but as ice. In this case, the dew point is termed the frost point.

Air at ground level often deposits dew on objects at night as it cools. In this case, the dew point of the air remains approximately constant while its temperature drops. When the dew point is reached, dew forms. Ground mist and fog may also form under these conditions.

The dew point can be measured using an instrument called a dew-point hygrometer. Invented in 1751, this consists essentially of a glass with a thermometer inserted. The glass is filled with ice water and stirred. As the temperature of the glass drops, the air in contact with it is chilled; when it reaches its dew point, water condenses on the glass. The temperature at which condensation occurs gives the dew point of the surrounding air.

See also Atmosphere, composition and structure; Atmospheric pressure; Atmospheric temperature; Clouds; Evaporation; Evapotranspiration; Precipitation; Weather forecasting.

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Dew point

The dew point is that temperature below which the water vapor in a body of air cannot all remain vapor. When a body of air is cooled to its dew point or below, some fraction of its water vapor shifts from gaseous to liquid phase to form fog or cloud droplets. If a smooth surface is available, vapor condenses directly onto it as drops of water (dew). The dew point of a body of air depends on its water vapor content and pressure . Increasing the fraction of water vapor in air (i.e., its relative humidity ) raises its dew point; the water molecules are more crowded in humid air and thus more likely to coalesce into a liquid even at a relatively warm temperature. Decreasing the pressure of air lowers its dew point; lowering pressure (at constant temperature) increases the average distance between molecules and makes water vapor less likely to coalesce.

If the dew point of a body of air is below 32°F (0°C), its water vapor will precipitate not as liquid water but as ice . In this case, the dew point is termed the frost point.

Air at ground level often deposits dew on objects at night as it cools. In this case, the dew point of the air remains approximately constant while its temperature drops. When the dew point is reached, dew forms. Ground mist and fog may also form under these conditions.

The dew point can be measured using a dew-point hygrometer. This instrument, invented in 1751, consists essentially of a glass with a thermometer inserted. The glass is filled with ice water and stirred. As the temperature of the glass drops, the air in contact with it is chilled; when it reaches its dew point, water condenses on the glass. The temperature at which condensation occurs is recorded as the dew point of the surrounding air.

See also Atmosphere, composition and structure; Atmospheric pressure; Atmospheric temperature; Clouds; Evaporation; Evapotranspiration; Precipitation; Weather forecasting.

views updated

Dew point

The dew point is that temperature below which the water vapor in a body of air cannot all remain vapor. When a body of air is cooled to its dew point or below, some fraction of its water vapor shifts from gaseous to liquid phase to form fog or cloud droplets. If a smooth surface is available, vapor condenses directly onto it as drops of water (dew).

The dew point of a body of air depends on its water vapor content and pressure. Increasing the fraction of water vapor in air (i.e., its relative humidity ) raises its dew point; the water molecules are more crowded in humid air and thus more likely to coalesce into a liquid even at a relatively warm temperature. Decreasing the pressure of air lowers its dew point; lowering pressure (at constant temperature) increases the average distance between molecules and makes water vapor less likely to coalesce.

Air at ground level often deposits dew on objects at night as it cools. In this case, the dew point of the air remains approximately constant while its temperature drops. When the dew point is reached, dew forms. Ground mist and fog may also form under these conditions.

The dew point can be measured using a dew-point hygrometer. This instrument, invented in 1751, consists essentially of a glass with a thermometer inserted. The glass is filled with ice water and stirred. As the temperature of the glass drops, the air in contact with it is chilled; when it reaches its dew point, water condenses on the glass. The temperature at which condensation occurs is recorded as the dew point of the surrounding air.

If the dew point of a body of air is below 32°F (0°C), its water vapor will precipitate not as liquid water but as ice. In this case, the dew point is termed the frost point.

See also Atmospheric inversion layers; Atmospheric lapse rate; Cloud seeding; Clouds and cloud types; Evaporation; Precipitation; Weather forecasting methods; Weather forecasting

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Dew point

An expression of humidity defined as the temperature to which air must be cooled to cause condensation of its water vapor content (dew formation) without the adding or subtracting of water vapor or changing its pressure. At this point the air is saturated and relative humidity becomes 100%. When the dew point temperature is below freezing it is also referred to as the frost point. The dewpoint is a conservative expression of humidity because it changes very little across a wide range of temperature and pressure, unlike relative humidity which changes with both. Dew points, however, are affected by water vapor content in the air. High dew points indicate large amounts of water vapor in the air and low dew points indicate small amounts. Scientists measure dew points in several ways: with a dew point hygrometer; from known temperature and relative humidity values; or from the difference between dry and wet bulb temperatures using tables. They use this measurement to predict fog, frost, dew, and overnight minimum temperature.

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dew point • n. the atmospheric temperature (varying according to pressure and humidity) below which water droplets begin to condense and dew can form.

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dew point Temperature at which a vapour begins to condense, for example when water vapour in the air condenses into cloud as the air becomes saturated with vapour.

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dew point: see dew.