Read this: The permanent dissolution of clouds allows unhindered light and thermal radiation. The stability of the atmosphere above the desert prevents any convective overturning, thus making rainfall virtually non-existent. As a consequence, the weather tends to be sunny, dry and stable with a minimal risk of rainfall. Subsiding, diverging, dry air masses associated with subtropical high-pressure systems are extremely unfavorable for the development of convectional showers. The subtropical ridge is the predominate factor that explains the hot desert climate (Köppen climate classification BWh) of this vast region. The lowering of air is the strongest and the most effective over the eastern part of the Great Desert, in the Libyan Desert which is the sunniest, the driest and the most nearly rainless place on the planet rivaling the Atacama Desert, lying in Chile and Peru.
Now answer this question, if there is an answer (If it cannot be answered, return "unanswerable"): What combines with converging dry air masses to make conditions for rain unfavorable?
unanswerable