Dear Tom,
What causes the sun to be warmer in Las Vegas than it is in Chicago or in Illinois?
Richard Parini, Joliet

Dear Richard,
The temperature of the sun is not a factor. I assume you are asking why air temperatures in Las Vegas are usually warmer than they are in Chicago. It has to do with the different climates of the two cities: Las Vegas has a much warmer climate.

Geography explains it. Las Vegas is farther south than Chicago and tempertures are usually higher as one moves south. However, the main reason is that Las Vegas is usually protected from outbreaks of frigid air masses by mountain ranges that block cold air from advancing across the city.

Air masses that originate over northern North America during the winter season are cold, and when they push south into the United States they bring their chill with them. Cold air is relatively heavy and ground-hugging. The Rocky Mountains, primarily, and other lesser mountain ranges, usually prevent Canadian and arctic air masses from reaching Las Vegas.

In addition, much higher elevations surround Las Vegas. Air masses that arrive in the city must descend into the city area, compressing and warming as they do so.