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Global Distribution of Water Vapor

The total water vapor over both hemispheres has a distinctive annual cycle, with average TCWV significantly higher in summer than in winter, especially in the northern hemisphere. This is clearly shown in the summer and winter satellite images of the United States in Figures 16.1 and 16.2. Because the

(See color insert.) Enhanced total water vapor over the continental United States on a summer day (7 August 2011). NESDIS Operational Blended TPW Products

FIGURE 16.1 (See color insert.) Enhanced total water vapor over the continental United States on a summer day (7 August 2011). NESDIS Operational Blended TPW Products.

(See color insert.) Reduced total water vapor over the continental United States on a winter day (7 February 2012). NESDIS Operational Blended TPW Products

FIGURE 16.2 (See color insert.) Reduced total water vapor over the continental United States on a winter day (7 February 2012). NESDIS Operational Blended TPW Products.

hemispherical variations are asymmetrical, the global average TCWV also has an annual cycle, with one study giving 23 mm from November to January and 26.5 mm during July.1141 Spatial variations are substantial, with the polar regions and the Himalaya mountain range having a minimum TCWV less than 1mm and the equatorial zone having a maximum TCWV of 70 mm or more. Annual TCWV measured by a GPS receiver at Hawaii’s Mauna Loa Observatory, an alpine site (3.4 km above the mean sea level) surrounded by ocean, ranges from less than 1 to 15 mm, whereas TCWV at nearby sea level sites ranges from 20 to 50 mm.1151

The greatest abundance of water vapor is stored in a band that encircles Earth known as the Intertropical Convergence Zone (ITCZ). This especially moist region swings from generally north of the Equator in July to generally south of the Equator in January. The ITCZ is obvious in the satellite image in Figure 16.3.

(See color insert.) Expanded view of Figur

FIGURE 16.3 (See color insert.) Expanded view of Figure 16.2 showing global total water vapor. Note the significantly enhanced moisture over the tropics in the Intertropical Convergence Zone. Dark areas, mainly over land, indicate an absence of data (7 February 2012). NESDIS Operational Blended TPW Products.

Heating from extended hours of direct solar radiation transports considerable moist air in the ITCZ up into the middle and upper troposphere, where its water vapor condenses into heavy downpours. The atmospheric water vapor lost to precipitation is replenished by evaporation the next day.

 
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