Why don't North American deserts cool off at night, like in the Sahara?

Question

The city of Phoenix (located in the Sonoran Desert) averages nightly lows of 82 degrees in July. Sometimes it doesnt even go below 90 at night.
Likewise for Las Vegas (located in the Mojave desert) Daily highs may reach 115, but nightly lows stay in the 80s.

In the Sahara, it could be 120 at high noon, and literally freezing at night.

So why aren't the American Southwest deserts like that?

Answer 1

Well, I googled about a hundred different sources and couldn't find an explanation. I did find that the freezing temps in the Sahara usually only occur during the winter, December to February, but the daytime temps were still higher year round in the Sahara. I did find one study that said that the reason for both heating and cooling depended on sand and rock, of which sand is a little faster to heat and cool than rock, so maybe it has to do with the higher percentage of rock in the North American desert. It might also have to do with the presence of surrounding mountains around the American deserts which would hold the heat rather than allowing it to be blown away as in the Saharan. Best bet would be to ask a librarian to help you find it or see if there is a college professor of geography or climatology nearby who could help you.

Answer 2

First of all, I would like to say a correction. A big high pressure system does not just sit in the middle of the country. The high pressure systems are pushed with cold fronts from the north. Then there is high pressure behind the fronts, which are low pressure. Even if there is a high pressure system, it couldn't bring in warm or hot air and distribute it around itself. So there being a high pressure system that brings warm air up from the south of the country is not true.
The low pressure over the Southwest brings in clouds. Clouds keep in the heat that is radiated back into space from the Earth. If there weren't any clouds, the ground will cool down. But if there is cloud cover, you wake up to a warm morning. But the southwest isn't always in cloud cover, so there is got to be more.
Another thing is wind. Wind takes the heat out of whatever is warm. That is why you feel colder when there is a breeze on you. Well, the wind is taking the heat from the ground, and releasing it into the air. But many things can slow or stop wind. Trees, houses, mountains, hills, etc. can all do this.
Well, that is all I know. Hope it helps.

Answer 3

First, I would love to see a reference for 120 at noon and freezing at night, I honestly don't believe the temperature range is that high. I have seen the temperature range from 100 degrees to freezing at night in the high deserts of California (for example the town of Campo, CA). This happens when it is very dry and the nights are clear, so that the ground radiates away the heat it gained during the day. Also, the location is a valley with nearby mountains, so that cold air can drain off the mountains at night into the valleys. If you get very large day/night temperature variations in the Sahaha it is probably for the same reasons.

Answer 4

the only reason i could think, is during the summer time, in the U.S. we go through a Monsoon season. basically, a big high pressure sets up over the central U.S. and that brings moisture, and warmer air from the south. Also, the orientation of the rocky mountains, will aid in keeping the area warmer, because as air comes down the mountain range, it adiabatically warms.

in the Sahara, there's not a mountain range on the west side. also they have a cold ocean current on that side. meaning, colder waters are being brought southward. as the air from those cooler waters are advected over land, it will aid in the rapid cooling of the land.. land already cools 3 times faster than water.