Its also about the convinience, size, look and utility, ease of use and not just only about the technicalities Sir...Che, it has to do with the RH value, not the temperature per se.
If the temperature is 55°C and the relative humidity is lower than 35% or so you will get good cooling. And if, like Delhi, RH values are at lower than 20% RH you will be amazed at what a desert cooler will do. Fifty five outside and you could be looking at 30 or lower inside!
The relative humidity in GCC Countries is never less than 35% anyway...
I stay half a kilometre away from the beach....the humidity is terrible in june-july...
Plus I keep my AC at 22 degrees in my home....30 degrees...naaaaaaaaaa!!!