Humidity, the amount of water vapor in the air, depends on air temperature and the amount of liquid water available to evaporate into water vapor. As the air temperature rises, more heat energy is available for molecules of liquid water to change phase and become water vapor. This additional energy also allows existing water vapor in the air to maintain its gaseous state.

 

Robbie Hood prepares to show Argonaut Alumnus Liz Quintana a computer model combining satellite photos and other data from Hurricane Katrina. The model shows how sea-surface temperature in the Gulf of Mexico changed as the hurricane absorbed heat energy from the water.

Humidity is actually indicated in two different ways—as both absolute and relative measurements. Absolute humidity is a measure of the mass of water vapor in a volume of air. For example, an absolute humidity of 2 g/m3, means that each cubic meter of air contains two grams of water vapor particles mixed in among the other gases. Considering that a typical cubic meter of air has a mass of 1200 g, this example shows very little water vapor, or absolute humidity, as part of the total measure of atoms and molecules in the air.

 

Relative humidity is a ratio. It compares the amount of water vapor existing in a parcel of air with the total amount of water vapor the air could maintain as a gas at that temperature. Thus, when the relative humidity is 50 percent, or the weatherman says that the air is “50 percent saturated,” the air contains half the water that it is capable of maintaining in the vapor state at that current temperature.

 

Be careful however, when discussing relative humidity measurements. After all, these measurements are relative to their specific air masses with specific temperatures. Different air temperatures change the amount of water vapor the air can absorb. The potential amount of water that the air can absorb depends on the temperature of the air, and increases as the temperature of the air increases. Measurements of 50 percent relative humidity at two different air temperatures are very different if converted to absolute humidity measurements. The higher air temperature will have a higher absolute humidity measurement.

 

 

To avoid confusion, scientists sometimes prefer using a different measurement to indicate the amount of water in the air. The dew point is the temperature at which enough energy is removed or given up by the water vapor to cause condensation, or liquid water, to form.

 

Dew point is an important measurement because it tells scientists exactly how much the air temperature needs to cool to form water in some form of condensate, and possibly produce a storm. The closer the air temperature is to the dew point temperature, the more readily a precipitation event will happen.

 

As you can see, dew point and humidity are very different measurements for describing the amount of water in the atmosphere. Dew point is a temperature. It is measured in degrees and identifies the specific temperature at which water will form from water vapor. This gives more information to a meteorologist to make a weather forecast.

 

Relative humidity is a percentage. It indicates the amount of water vapor that is in the air, relative to the total amount of water vapor that the air could maintain when totally saturated. Unlike dew point, relative humidity does not identify the temperature at which clouds will form.

 

This map shows dew point temperatures across the continental U.S. on the afternoon of May 4, 2007. What do the comparative dew points tell you about how muggy the air might feel in different regions of the country? Extreme changes in dew point over a short distance, as seen over western Texas and eastern Colorado on this map, are an indicator of the potential for severe weather. As this boundary between air masses moved into Kansas later that day, it spawned a series of tornadoes throughout the state. One giant tornado destroyed 95% of the town of Greensburg, Kansas, leaving a path of damage 2.3 km (1.4 mi) wide. The next day, the National Weather Service estimated that tornado to be an upper F-4 to F-5, among the strongest tornadoes possible (see Fujita Scale on page 57).

Both measurements are valuable, and knowing when to use them is critical to weather forecasters. Relative humidity is most often used to indicate current weather conditions in a single location. Dew point is especially helpful when comparing two locations at the same time for which the air temperatures and water vapor amounts are certain to be different.

 

Dew point is also the most meaningful measure when talking about the comfort level of the air outside. Some people will begin to feel uncomfortable when the dew point temperature approaches 16°C (60°F). Most people will think the air feels very humid and oppressive when the dew point reaches 21°C (70°F) or higher. When you sweat in hot weather, what makes you feel cooler again? It’s the ability of the sweat to evaporate from your skin, that is, for the water on your skin to become water vapor. As dew point increases, your perspiration cannot evaporate as easily, leaving you feeling hot, sweaty, and “sticky.” Look at the dew point map above and consider how the air would feel in different regions of the country based on the comparative dew point temperatures.

 

With the ability to measure water vapor, interpret clouds, and understand the water cycle, scientists can combine this knowledge with satellite, airplane, and radar data to anticipate the threat of a monster storm. Through their research, Robbie Hood and other scientists contribute to saving lives and protecting the property of people who can be affected by hurricanes and other severe storms. Robbie uses her knowledge of the water cycle to study the meaning of the data she collects and understand the clues in the atmosphere.