What is Dewpoint?

The dewpoint, or more precisely the dewpoint temperature, is the temperature at which the liquid and gaseous phases of a material present in a gas, such as water in air, are in equilibrium at a given gas pressure.

In other words, the dewpoint temperature, or dewpoint, is the temperature at which the liquid water, or dew, evaporates at the same rate at which it condenses.

Measurements of dewpoint and related humidity conditions represent a significant use of temperature sensors, usually integral to the device or instrument that reports the dewpoint temperature or humidity conditions.

Significance of Dewpoint Temperature

The significance touches each of us when one realizes that this important factor, the amount of moisture in a gas, impacts much more than Heating,Ventilation and Cooling (HVAC) considerations.

  • It is an vital factor in convective heat transfer, combustion of fossil fuels and combustion engineering, drying of paper, cardboard, plastics, wood, tobacco, leather, printed goods, textiles and grain.
  • It plays a major role in the efficient use of energy in many chemical manufacturing processes as well as the attainment of high product yield.
  • The effect of moisture in gases also plays a very significant role in corrosion phenomena which can result in damage and loss of not only unprotected metals, like iron and steel structural components, but also improperly treated or stored steel and other metal products.

Dew Forms – Fog “Appears” : Fog and Dew Disappear…it seems

The dewpoint temperature is most commonly observed in ambient air and is also called the saturation temperature of water vapor in air. If you lower the temperature, dew will form as fog or condense on a cooler surface faster than it evaporates.

This phenomenon is observed, for instance in very moist air when the dew appears as fine water droplets suspended in air (fog) and on cool beverage containers in hot weather conditions.

Conversely, if the temperature increases above the saturation temperature, dew will evaporate faster than it condenses. Fog in air and dew on surfaces will disappear under such conditions. This is commonly seen when dew on the ground vanishes as the air warms during the day after a cool night.

The dewpoint temperature depends on the air temperature, since hotter air can hold more water vapor per unit volume than can colder air.

Likewise, the value of the dewpoint at a given air temperature is also a function of air pressure. Air in Denver Colorado at 72 °F can not hold as much water vapor as in Philadelphia, Pennsylvania at 72°F simply because the air pressure is less (due to altitude effects) in Denver. Denver isn’t called the Mile-High City without reason and Philadelphia sits nearly at sea level.


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