Vertical Distribution of Water Vapor
While water vapor is present from the surface to the stratosphere, more than half is usually stored in the first few kilometers nearest to the surface. If Earth could be reduced to the size of a basketball, most of the planet’s water vapor would be found in a film of air around the ball having an average thickness of several sheets of office paper. The amount of water vapor in a given parcel of air can be expressed as the ratio of the mass of the water vapor to that of an equivalent volume of completely dry air. This mixing ratio declines exponentially with elevation1161 until the water vapor mixing ratio in the stratosphere between 16 and 28 km declines to about 1/1000 of that at sea level.1171
Residence Time in the Atmosphere
Because annual precipitation matches the sum of evaporation and the other processes that inject water vapor into the atmosphere, the mean residence time of a water molecule in the atmosphere can be estimated by dividing the average of global TCWV by the average global precipitation of 2.5 mm/day.1181 A global TCWV of 24.46 mm191 gives a typical residence time of nearly 10 days. Some water vapor may return to a liquid state almost immediately, whereas vapor that reaches the stratosphere may remain there considerably longer than 10 days. Over geological time a given molecule of water spends minuscule time in the atmosphere. It is much more likely to be found immersed in an ocean, stored in an aquifer or locked in a glacier.
Measuring Atmospheric Water Vapor
The measurement of water vapor on a global scale is of increasing interest in view of its significance as a greenhouse gas and its role in climate models. TCWV is measured from the surface by various kinds of sun photometers that monitor changes in near-infrared sunlight caused by the absorption of water vapor119,201 and by microwave radiometers that receive frequencies emitted by water molecules.1211 TCWV can also be measured by an inexpensive infrared thermometer pointed at a cloud-free zenith sky.1221 Water vapor delays the microwave signals from GPS satellites, and this provides a high-quality method for inferring TCWV.1231 The U.S. National Oceanic and Atmospheric Administration’s (NOAA) Ground-Based GPS-IPW Network processes data from a vast array of more than 550 GPS receivers across the United States and a number of other countries and islands. The network provides automatic TCWV measurements that are posted online every half hour.1151 This GPS network is arguably the most precise source of site-specific TCWV data, and its expansion around the world would greatly improve the understanding of water vapor and its cycles and trends over land.
Since 1930 the vertical distribution of water vapor has been measured by humidity sensors flown from upper air sounding balloons.1241 Integrating the balloon humidity measurements provides the TCWV. Studies of the global distribution of water vapor based only on upper air sounding balloon measurements were biased because most sounding balloons are launched from land, which comprises only 29% of Earth’s surface. Additional biases were caused by different humidity sensor designs and inconsistent performance, especially at reduced temperatures.
Earth satellites equipped with water vapor sensing instruments have dramatically improved the understanding of the global distribution of water vapor. Most satellite instruments monitor the infrared or microwave wavelengths emitted by water vapor.1251 Selective monitoring of these wavelengths permits TCWV and the water vapor at various levels in the atmosphere to be measured day and night. Some satellite instruments monitor TCWV by measuring changes in the water vapor absorbing near-infrared sunlight reflected from both oceans and land.1261 Near real-time water vapor imagery from various satellite sensors is available online, including the National Environmental Satellite, Data, and Information Service (NESDIS) Operational Blended TPW Products.1271 As shown in Figures 16.1 and 16.2, NESDIS imagery is particularly informative over the continental United States due to the inclusion of TCWV measurements by NOAA’s GPS network.