Middle East Water Vapor

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Precipitable water is a quantity which meteorologists use to measure the water vapor content of the atmosphere. Imagine a column extending from the surface of the Earth to the top of the atmosphere. Imagine further that all of the water vapor in this column condenses and falls to the surface. The depth of this condensed water (in millimeters) is the "precipitable water." It is a measure of how much water vapor is available for conversion to precipitation.

The precipitable water data we present here is called NVAP, for the NASA Water Vapor Project (Randel et al., 1996). It consists of estimates made by weather balloons (mostly over land) and by satellite-borne instruments, which means that it is a truly global data set.

The map at the top shows the mean annual precipitable water for the period 1988-1992, and the plot below it shows the zonally averaged mean annual precipitable water. There are similarities between precipitable water and both cloud amount and precipitation, especially in the tropics. However, unlike precipitation and cloud amount, precipitable water shows only one peak. Precipitable water decreases monotonically away from the equatorial maximum to minima at the poles. The Tropics of Cancer and Capricorn are not local minima of water vapor; thus, the latitude band in which the Middle East lies is not especially deficient in water vapor.

The bottom graph shows that along the Tropics of Cancer and Capricorn, the precipitable water varies about the global mean value. As with precipitation and cloud amount, there is a tendency for the east sides of continents to be moist and the west sides to be dry. Again, the Middle East, lying at the west side of the Europe-Asia-Africa supercontinent, is dry, but not extremely so. Precipitation on the west sides of continents falls to nearly zero; precipitable water falls much less dramatically. Lack of water vapor does not appear to be the primary cause of the cause of the low precipitation in the Middle East--in spite of what we said above.

So what does cause the low precipitation in the Middle East? Click Next Page for further clues.

Annual Precipitable Water

Mean annual precipitable water (mm) constructed from the NVAP data set.

Compare with precipitation | Close-up of the Middle East | The terrain connection

Zonal Precipitable water

Zonally averaged annual precipitable water (mm) from the NVAP data set.

Compare with precipitation

Tropics Precipitable Water

Mean annual precipitable water along the Tropics of Cancer and Capricorn. The global mean (24.6 mm) is shown as a dashed line.

Compare with precipitation

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