The Sun Belt is a region of the Unite States generally considered to stretch across the South and Southwest (the geographic southern United States). Another rough boundary of the region is the 37th to 38th parallels, north latitude — southward. The Sun Belt has seen substantial population growth in recent decades, partly fueled by a surge in retiring baby boomers who migrate domestically, as well as the influx of immigrants, both legal and illegal. Also, over the past several decades, air conditioning has made it easier for people to deal with the oppressive heat that grips the region during the summertime.