4.2 Measuring Water Vapor

4.2 Measuring Water Vapor

Humidity is a general word that refers to the amount of water vapor in the air. Humidity is so important in the atmosphere that numerous methods are available to quantify the amount of water vapor present. These measures of humidity include vapor pressure, absolute humidity, specific humidity, mixing ratio, relative humidity, and dew point.  Absolute humidity is not too useful nor widely used, so we will skip that one. The rest of these humidity terms are covered in this section. The two most important and widely used humidity measures are relative humidity and dew point.

4.2.1 Vapor Pressure

Vapor pressure is the portion of air pressure exerted by water vapor.  Vapor pressure (es) is measured in millibars like air pressure.  The vapor pressure of a volume depends on temperature.  Vapor pressure follows the Ideal Gas Law which directly relates pressure to temperature.  As temperature increases, the amount of water vapor in the air can increase.  Once the water vapor is in equilibrium with liquid water, saturation has been reached.  These conditions are similar to the hypothetical experiment conducted at the beginning of this module.  This graph has vapor pressure (es) on the y-axis and temperature on the x-axis

 

The air is unsaturated if the evaporation and condensation rates are not equal, and the air can hold more water vapor.  Saturation is an equilibrium state where the evaporation and condensation rates are balanced.  Supersaturation is a transient condition where excess water vapor is present in the volume because no liquid water or ice surface is available to promote condensation.  Supersaturation is unlikely to occur in our hypothetical experiment.  However, slight supersaturation conditions are possible in the atmosphere and will be examined in more detail.

Video: Mod 4.2.1 Vapor pressure (3:30 min.)

This short video talks about vapor pressure, humidity measurements in general and supersaturated conditions.

 

4.2.4 Dew Point

Dew point is the temperature at which saturation occurs.  Dew point is a widely used measurement expressed in degrees Celsius or Fahrenheit.  Dew point is not an actual measure of water vapor, but it does largely depend on the amount of water vapor present.  A high dew point number implies lots of moisture is in the air.  A lower dew point implies drier air.

Dew point temperature is always less than or equal to the air (ambient) temperature.  Why cannot the dew point be higher than the air temperature?  When the dew point equals the ambient temperature, the air is saturated and condensation can occur.  Condensation releases latent heat causing the air temperature to increase.  Thus, the ambient temperature will always be greater than or equal to the dew point.

The following illustration demonstrates the relationships between temperature, dew point, mixing ratio, saturated mixing ratio, and relative humidity.  The mixing ratio numbers were obtained from the table in the specific humidity and mixing ratio section.

Since dew point is the temperature at which saturation occurs when the dew point and temperature are the same the relative humidity is 100%.  Saturation is a relative humidity of 100% by definition.

Dew point and relative humidity calculators are available at the National Weather Service website.

4.2.5 Instruments for Measuring Humidity

As you have probably noticed, all the humidity measurements are closely related.  When one humidity value is known the others can be calculated.  Two types of instruments used to measure humidity are (1) a sling psychrometer and (2) electronic sensors.

The sling psychrometer is a manual method for remote locations.  Wild land firefighters use sling psychrometers to track weather conditions near wildfires.

A sling psychrometer has a wet bulb and a dry bulb thermometer.  The dry bulb is just a normal thermometer.  The wet bulb has a cotton wick attached to the end.  This wick is dipped in water and spun to get a temperature reading based on evaporating water into the air.  A handle to spin the thermometer frame (not shown) is attached to the hole at the end.  These two temperatures can easily be used to determine the dew point.

 

 

Electronic sensors tend to measure the relative humidity (RH), since an RH sensor is reliable and accurate.

These devices look like typical boxes containing electronics, such as this sensor from a company called Vaisala.

The widespread use of these electronic sensors makes RH a common parameter included in a weather observation.

 

 

References:

American Meteorological Society (AMS) Glossary of Meteorology found at https://glossary.ametsoc.org/wiki/Welcome used in definitions.

“Vapor Pressure at Saturation” by Roland Stull, LibreTexts is licensed under CC BY-NC-SA.  https://geo.libretexts.org/Bookshelves/Meteorology/Book%3A_Practical_Meteorology_(Stull)/04%3A_Water_Vapor/4.00%3A_Vapor_Pressure_at_Saturation  The graph originated from this web site.

Data in saturated mixing ratio table from http://www.atmo.arizona.edu/students/courselinks/fall12/atmo336/lectures/sec1/Saturation_Mixing_Ratio_Tables.htm  These tables are readily available or can be calculated from an equation.

Vaisala HMT330 sensor.jpg image from Vaisala’s web page at https://www.vaisala.com/en/products/instruments-sensors-and-other-measurement-devices/instruments-industrial-measurements/hmt330

 

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