Page 222 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

186 part II The Water, Weather, and Climate Systems
significant nonradiative heat transfer process dissipating positive net radiation at Earth’s surface.)
The process reverses when air cools and water va­ pour condenses back into the liquid state, forming mois­ ture droplets and thus liberating 585 cal for every gram of water as the latent heat of condensation. When you realize that a small, puffy, fair­weather cumulus cloud holds 500–1000 tonnes of moisture droplets, think of the tremendous latent heat released when water vapour condenses to droplets.
Satellites using infrared sensors now routinely monitor water vapour in the lower atmosphere. Water vapour absorbs long wavelengths (infrared), making it possible to distinguish areas of relatively high water va­ pour from areas of low water vapour (Figure 7.6). This technology is important to weather forecasting because it shows available moisture in the atmosphere and therefore the available latent heat energy and precipita­ tion potential.
Water vapour is also an important greenhouse gas. However, it is different from other greenhouse gases in that its concentration is tied to temperature. As global tem­ peratures rise, evaporation increases from lakes, oceans, soils, and plants, amplifying the concentration of water va­ pour in the atmosphere and strengthening the greenhouse effect over Earth. Scientists have already observed an in­ crease in average global water vapour in recent decades and project a 7% increase for every 1 C° of warming in the future. As water vapour increases, precipitation patterns
will change and the amount of rainfall will likely increase during the heaviest precipitation events.
Humidity
The amount of water vapour in the air is humidity. The capacity of air for water vapour is primarily a function of the temperatures of both the air and the water vapour, which are usually the same.
As discussed in Chapter 5, humidity and air tem­ peratures determine our sense of comfort. North Ameri­ cans spend billions of dollars a year to adjust the hu­ midity in buildings, either with air conditioners, which remove water vapour as they cool building interiors, or with air humidifiers, which add water vapour to lessen the drying effects of cold temperatures and dry cli­ mates. We also saw the relationship between heat and humidity, and its effects on humans, in our discussion of the humidex in Chapter 5.
Relative Humidity
The most common measure of humidity in weather re­ ports is relative humidity, a ratio (expressed as a percent­ age) of the amount of water vapour that is actually in the air compared to the maximum water vapour possible in the air at a given temperature.
Relative humidity varies because of water vapour or temperature changes in the air. The formula to calculate the relative humidity ratio and express it as a percentage
◀Figure 7.6 Global water vapour in the atmosphere. High vapour content is lighter and lower water-vapour content is darker in this February 19, 2013, composite image from the GOES (United States), Meteosat (european Space Agency), and MTSAT (Japan) satellites. [Satellite data courtesy
of Space Science and engineering Center, University of Wisconsin, Madison.]
Georeport 7.2 Katrina Had the Power
Meteorologists estimated that the moisture in Hurricane katrina (2005) weighed more than 27 trillion tonnes at its maxi- mum power and mass. With about 585 cal released for every gram as the latent heat of condensation, a weather event such as a hurricane involves a staggering amount of energy. Do the quick math (585 calories times 1000 g to a kg, times 1000 kg to a tonne,
times 27 trillion tonnes).