Israel Science and Technology Directory

The text and the images below have been adapted by permission from a document written by Stanley Q. Kidder, G. Garrett Campbell, David L. Randel, and Thomas H. Vonder Haar from the Cooperative Institute for Research in the Atmosphere of the Colorado State University, Fort Collins, CO 80523, USA.

• Why is it so dry in the Middle East?
• Why is the Sahara so dry?
• Precipitation along the tropics of Cancer and Capricorn
• Middle East clouds
• Middle East water vapor
• Net radiation in the Middle East
• Vegetation in the Middle East
• Seasonality

Why is it so dry in The Middle East?

The map below shows the mean annual precipitation for the whole Earth. As can be seen on the map the yellow and brown colored regions mark the driest regions in the Sahara Desert in North Africa and the Middle East located east of Sahara.

Why is the Sahara so dry?

Precipitation on Earth varies with latitude (in the north-south direction), with longitude (in the east-west direction) and with time. The image below shows average precipitation all the way around the Earth at each latitude. This is called a "zonal" average (Zones are areas of constant latitude).

The graph shows that precipitation falls in preferred zones: The greatest precipitation falls in the deep tropics, within about 15° of the equator, where intense solar radiation causes rising air, clouds, and precipitation. Near the Tropics of Cancer and Capricorn, which bound the sun on its annual journey, are areas of low precipitation -low, in comparison with the global average precipitation. Here the air which rose near the equator sinks, which evaporates clouds and suppresses precipitation. Poleward of the subtropics are the mid-latitude precipitation maxima caused by eastward-moving storm systems. Finally, the polar regions are very dry, but, due to low temperatures, evaporation is also low, and the precipitation which does fall stays as ice (at least in Antarctica). The Sahara and the Middle East are dry in part because they straddle the the Tropic of Cancer. But the Tropic of Cancer isn't dry everywhere.

Precipitation along the tropics of Cancer and Capricorn

In the tropics and subtropics (between about 30° North and 30° South) there is a distinct tendency for the eastern sides of continents to be wet and the western sides to be dry. This can be clearly seen in the graphs below and also in the global map. There are several reasons for this. Among them: The primary source of water vapor which eventually falls as rain is evaporation from the warm tropical and subtropical oceans. In the tropics and subtropics, the wind usually blows from the east (the Trade Winds), which means that moist ocean air impinges on the east side of continents. On the east side of the continent, heating by the land or flow over elevated terrain causes the air to rise, forming clouds and precipitation. Air flowing over the west side of continents and over the adjoining eastern oceans comes from the east side of the continent; it has been depleted of much of its water vapor and therefore of its precipitation potential. This process is evident even for islands such as Taiwan and Madagascar.

There are interesting variations on this basic mechanism caused by atmospheric circulation. One example of this is the precipitation maximum at 90° East. Here southerly winds from the Bay of Bengal impinge on the Himalayas, causing heavy rain.

Asia, Africa, and Europe form the world's largest land mass. Because the Sahara and Middle East are in the subtropics on the western side of this supercontinent, they are exceedingly dry.

Middle East clouds

Clouds tell a similar story to precipitation in the Middle East. The figure below shows the mean annual cloud amount for the period 1984-1990 as determined by the International Satellite Cloud Climatology Project (ISCCP, Rossow and Schiffer, 1991).

On average, about 62% of the Earth is cloud covered. The zonal average cloud amount (figure below) shows that there is a cloud maximum in the tropics associated with the ITCZ, and there are substantial mid-latitude maxima, which rain far less frequently than do tropical clouds. In fact, the mid-latitude oceans are quite cloudy, in excess of 80% cloudy in many locations.

Along the Tropics of Cancer and Capricorn (figure below) the cloud amount decreases from the eastern to the western sides of continents just as does precipitation. The oceans, however, are quite cloudy. In fact the only places where the cloud amount is substantially below the global average are over land. Along the equator, however, the least cloudy places are over the ocean.

The Middle East, being on the western side of a super-continent in the heart of the subtropics is among the most cloud-free places on Earth.

Middle East water vapor

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 below 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 graph below 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.

Net radiation in the Middle East

The Earth receives solar radiation from the sun. About 30% is reflected back to space; the rest is absorbed. Absorbed solar radiation heats the Earth, just as deposits increase your bank balance. The Earth also emits infrared radiation to space. This emitted radiation cools the Earth, just as writing a check decreases your bank balance. Net radiation is simply the difference between the absorbed solar radiation and the emitted infrared radiation. Where the net radiation is positive, the Earth tends to warm; where it is negative, the Earth tends to cool. The bank analogy is that if you write checks for less than you deposit, your bank balance will increase; if you write checks for more than you deposit, your bank balance will decrease.

Using satellites, we can measure the amount of solar radiation which the Earth absorbs and the amount of infrared radiation which the Earth emits. Subtracting the latter from the former yields the net radiation. The figure below shows the net radiation for the four-year period June 1985 through May 1989 as measured by three satellites: the Earth Radiation Budget Satellite, NOAA 9, and NOAA 10 (from the Earth Radiation Budget Experiment; Barkstrom, 1984).

The figure below shows the zonal average net radiation. Not surprisingly, the tropics and subtropics are warming radiatively (positive net radiation) due to the intense solar radiation. The mid-latitudes and poles are cooling radiatively. The net radiation varies substantially in the north-south direction, but not in the east-west direction--except for North Africa and the Middle East.

The figure below shows the variation of net radiation along the Tropics of Cancer and Capricorn. The Sahara and the Arabian Peninsula show large, negative values of net radiation, which indicates that these hot regions are always cooling radiatively. They emit more infrared radiation to space than the sunlight that they absorb. If we estimate (conservatively) that the average net radiation in this region is -10 W/m? (watts per square meter), we can calculate the rate at which the atmosphere is cooling: about 30°C (54°F) per year. Needless to say, the Sahara is not cooling at this rate. (If it were, we would soon have a Saharan ice cap!)

To balance this radiative loss, the atmosphere must import energy from outside the Middle East. This is primarily accomplished by importing high-energy air at upper levels in the atmosphere and exporting low-energy air at low levels. To conserve mass, there must be sinking motion over the entire area on average. Since clouds and precipitation form in rising air, this radiative imbalance in effect suppresses precipitation.

Vegetation in the Middle East

Vegetation cover can be measured by satellites. The reflectance of chlorophyll is much higher in the near infrared than in the visible portion of the spectrum. The Landsat satellites and the NOAA weather satellites make measurements in both spectral regions. By combining these measurements, in what is called the Normalized Difference Vegetation Index (NDVI), the vegetation cover can be inferred.

The image below shows the maximum NDVI in the Middle East in 1990. The greens of the color scale are designed to indicate vegetation cover. Apart from mountainous regions in the north and south, significant vegetation grows only in the Nile Valley, near the Jordan River Valley, along coasts, and at a few places irrigated by well water in the Arabian Peninsula. An obvious reason for the sparse vegetation is the meager rainfall, but there is another reason.

Seasonality

The image below shows the percentage of the annual precipitation which falls in the rainiest three-month period. The range of this parameter is 25% (for locations in which every month receives the same amount of precipitation) to 100% (for locations in which all precipitation falls in one three-month season). The higher this number, the more effort must be put into storing water for the dry season. Areas which are green to red, including most of the Middle East, have highly seasonal rainfall. Of note are the red areas, including the Sahel and the Great Indian Desert, which have extremely seasonal precipitation.

For agriculture, a problem related to seasonality is how much rain falls in the growing season. The figure below shows the fraction of precipitation which falls in the cold season, that is in the nongrowing season. For the Northern Hemisphere, the cold season is defined as October through March. For the Southern Hemisphere it is defined as April through September. In the tropics, this parameter is not useful because temperatures are always high enough to grow crops. Outside the tropics, however, if rain falls in the cold season, it may be useless for watering warm-season crops unless extensive storage projects have been constructed.

Most of the Middle East outside of the tropics has the dry summer climate typical of the Mediterranean. It shares this climate also with western Australia, western portions of South America and southern Africa, and especially with the west coast of the United States.

Conclusions

Accurate observations, both remotely sensed observations and in situ observations, are essential if water problems are to be understood and dealt with. The dryness of the Middle East is part of a global pattern of climate. Only by observing and understanding the global climate can problems associated with the local climate be wisely approached.

In regions which have climates similar to those of the Middle East, the answer since Roman times to water shortages has been water projects. The Romans built aqueducts throughout the dry parts of their empire. Today, California is extensively supplied with water from distant sources. Water projects are common in the Middle East also. The Asswan High Dam on the Nile in Egypt, the Israeli National Water Carrier, and the Ataturk Dam on the Euphrates River in Turkey are but a few examples. What these Web pages make clear to us, at least, is that the climate of the Middle East is unrelentingly dry. Only extraordinary cooperative efforts to increase water supply and decrease water demand will be able to cope with the rapidly increasing population of the region. We hope that the long history of conflict in the area can be overcome to achieve a stable water supply. And we will even go as far as stating that the sharing of data and ideas might be a key step toward this much needed cooperation.