THE NATURAL ENVIRONMENT
Geography 101 Online
Humidity is a measure of the amount of water vapor in the atmosphere. It varies continuously from place to place, with altitude, and over time. On average, about 1% of the air is water vapor molecules.
Consider the NOAA water vapor image of the north Pacific (dark is dry, white is moist, red/blue indicates widespread rainfall). The image shows huge variability in humidly over the area and identifies some major features of the global climate. The band of clouds and rain near the equator (bottom) is the wet Intertropical Convergence Zone. The clear, dry area including Hawai'i is influenced by subtropical High pressure. The white spirals north of Hawai'i are midlatitude low pressure centers spinning in the westerly winds, carrying cool air and precipitation to America's west coast. The oval shape at the top right is hurricane Claudette (July 15, 2003) headed for the Texas coast.
One of the simplest and most widely used measures of water in the atmosphere is vapor pressure (VP). It refers to the atmospheric pressure exerted by water vapor molecules. For example, typically in Hawai'i, this value might be 25 millibars (mb). If the overall atmospheric pressure is 1015 mb, then 25 mb is due to water vapor and 990 mb is due to the other atmospheric gases. The amount of water vapor in the air varies from less than 1 mb up to about 70 mb for different places around the globe.
The maximum amount of water vapor that the air can hold is called the saturation vapor pressure (SVP). Saturation vapor pressure varies with temperature: the warmer the air, the more water vapor it can hold. This relationship is shown below.
The most widely reported measure of water in the atmosphere is relative humidity (RH). It is also, perhaps, the least understood. A simple formula for calculating relative humidity is: (NOTE: / means divide, * means multiply)
RH = VP / SVP *100
The key word is "relative." Vapor pressure gives you the absolute measure of how much water is in the atmosphere, regardless of air temperature. Relative humidity gives you the amount of water relative to the maximum amount air can hold (the SVP), which varies with temperature as shown above. This concept is simpler than it seems. Try a few simple calculations using the table below to guide you. Find the SVP values using the table above, and then calculate the relative humidity.
Study your completed table for a moment and think about what relative humidity means. For example, compare examples 1 and 4. Which has more water in the air (highest VP)? Which has a higher relative humidity? Be sure you understand why.
Relative humidity, then, depends on both the amount of water in the air and air temperature. If temperature increases, SPV increases, and relative humidity decreases. If the amount of water vapor increases and temperature remains the same, then relative humidity also increases. Compare some of the values in your completed table to make sure you understand this.
The graph shows how relative humidity changes throughout the day using data from three sites on the Big Island. At the windward Saddle Road and leeward Hualalai sites, the relative humidity is lowest during the daytime, and highest at night. While at first this seems counterintuitive, if you reflect on your own experience, it will make sense. When does dew form? Dew forms at night, when relative humidity at the surface increases to 100%, but seldom during daytime when relative humidity is lower.
Can you guess why the leeward site on Hualalai volcano has lower humidity values than the windward Saddle site? There are only two possible answers: either the leeward side has less water vapor in the air (lower VP) or it has higher air temperature (and, thus, higher SVP). In this case, higher air temperature at the dry leeward location explains the lower relative humidity; water vapor in the air is about the same.
The Mauna Kea site is quite interesting; it shows a rare opposite pattern. Can you guess why? In Hawai'i, almost all of the atmospheric water is below the trade wind inversion, the upper mountain slopes are usually extremely dry. During daytime, however, a sea breeze forms as the mountain heats up. This light breeze carries a thin layer of very moist air upward through the inversion along the mountain surface. Relative humidity is higher because of more water vapor in this thin layer of air near the surface. At night, when the land breeze forms, this thin, moist layer disappears and relative humidity drops again.
Just remember that relative humidity will change if the air temperature changes or if the amount of water vapor in the air changes.
When air cools, saturation vapor pressure decreases. If air cools to the point where SVP equals VP, then relative humidity becomes 100% and water condenses to form clouds and fog. The temperature at which relative humidity becomes 100% is called the dew point. In the table above, for example, notice that relative humidity for the third example is 100%. The dew point temperature, then, for this scenario, is 20 °C.
The dew point temperature for scenario 1 is 10 °C, because at that temperature the SVP would be the same as the VP,12, and relative humidity would be 100%. See if you can figure out what the dew point would be for scenario 9. How about scenario 8?
Dew point is also a way of reporting atmospheric water content, although not as commonly reported as relative humidity. If the dew point is high, then the amount of water in the air is also high. If you hear a weatherperson saying, "the dew point is declining," then you know that the amount of water in the air (VP) is also declining.
Kapiolani Community College Geography