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If you look at this wind map (for July, but following the link you can also view other months):

you will see that Egypt gets its air from the Mediterranean sea. During other months it could get less moist air, but still from the same direction.

If you look at the topography of Egypt:

(taken from Wikipedia), you will see that it doesn't have any mountain ranges that could intercept the precipitation. But it still receives very little rain.

I don't quite understand why. Is the Mediterranean air not moist enough?

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If you take a look at the atmospheric circulation pattern, the Hadley Cells in particular, they tell the story. The northern edge of Africa is on the descending edge of a Hadley Cell, which means

Having lost most of its water vapor to condensation and precipitation in the upward branch of the Hadley cell circulation, the descending air is dry. As the air descends, low relative humidities are produced as the air is warmed adiabatically by compression from the overlying air, producing a region of higher pressure.

The main air mass being dry already, and the High pressure system generated by the descending air mass along the 30 degree lines work to make the physics required for precipitation just not want to happen in these regions. You have to get a combination of enough humidity to reach saturation, and then cool the air to reach its dew point. The desert region formed by the lack of atmospheric moisture(remember Hadley cell) can't add any humidity of its own through evaporation or transpiration, so the global patterns dominate, even being right on the oceans edge.

Note that Hadley cell edges at the 30 degree south also cut through the Namib,Kalahari, and in South America the Atacama deserts, with their own ocean exposure.

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  • $\begingroup$ If you look at the wind map (using the link in the main question), you will see that in July, the northern edge of the Hadley cell lies to the north of Egypt. Actually it's in Europe. Because the Hadley cell follows the sun. 30° is just a convenient, but very approximate and often wrong value to put in Wikipedia. You can also see that for some reason Kalahari is not at all like Sahara, don't get confused by the word "desert". I can agree that your reasoning can hold in winter, but not in summer. Because in summer there is no air descending over Egypt. $\endgroup$ – user6130990 Aug 6 '16 at 16:18
  • $\begingroup$ And there is no "main air mass" when the winds reach 6 m/s. At that speed wind can pass from the ocean to Sudan in two days' time. And at Alexandria, the relative humidity is around 70%. Which is not that dry. $\endgroup$ – user6130990 Aug 6 '16 at 16:32
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    $\begingroup$ @ user6130990 Yes, I considered not listing the Kalahari here,since it is a different type area, but the regional association to the dry falling air mass seems still relevant . Check the section on precipitation-conditions to reach dew point. You still have to change the temperature or pressure of the humid air to get rainfall. Being summer, and subtropical, the temperature wants to stay hot. $\endgroup$ – user2448131 Aug 6 '16 at 17:56
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I suppose that the water gets lost to some underground aquifer. And then evaporates during the dry months.

http://www.mwri.gov.eg/PDF_files/Ground_Water.pdf

Otherwise it wouldn't be possible to explain why the water that rains down in the Qattara stripe doesn't evaporate straight away to cause rains further south, and so on all the way towards Sudan. It obviously can't drain towards the Nile, because the Nile basin in Egypt is very narrow.

(Combined graphics; basin outline: Nile Basin Initiative, precipitation map: Ministry of water resources and irrigation).

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  • $\begingroup$ Not all rain that falls gets re-evaporated. Some stays in the ground, perhaps refilling aquifers, some gets used by plants or collected and used by humans. Also, as temperature rises the atmosphere can hold more water. You should compare a rainfall map to a temperature map with highs and lows. Hotter air retains more water. 100% humidity air, when the temperature rises 5 or 10 degrees, is now 60%-80% humidity, after 1 rain fall, maybe 20%-30%. It's not that the Air over the Mediterranean isn't wet enough, it's not hot enough to carry enough water (if I was to oversimplify). $\endgroup$ – userLTK Aug 7 '16 at 23:46

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