If you click on the above link, it will show you the monthly average high and low temperatures over the United States (lower 48), for each month. Something interesting to note is that in winter, the range of temperatures is pretty big. You’ve got the coldest parts in North Dakota and Minnesota only getting into the teens but then dropping below zero on average in January, but meanwhile, the southern half of Florida is still getting into the seventies and only dropping into the fifties on average. And just overall, there are many more layers, a lot more variation across the map. For high temperatures in January, the map is divided up into 10s, 20s, 30s, 40s, 50s, 60s, and 70s.
Then if you look at the July or August map, the range is much smaller. It turns out, even the colder parts have “caught up” and make it into the eighties as their average high temperatures, and it turns out that the South’s summer isn’t that much hotter, getting into the nineties. The vast majority of the map is either eighties or nineties, with a few areas in the seventies or over a hundred.
Why are winter temperatures much more varied than summer temperatures in the United States? How come it doesn’t “scale” more evenly with the southern areas being that much warmer in the summer as they were in winter (aside from the Death Valley/Phoenix area)?
At a guess, and I’m not sure what the technical term is, it’s to do with proximity to the Equator as the planet orbits the sun. When the more northerly latitudes lean away from the sun, that’s winter; when they straighten up again, that’s the summer - only the northern latitudes move further in either direction, while those nearer the Equator are relatively less far away from the sun all the time.
Of course, that is also modified by things like where the jet stream in the atmosphere pushes warm and cold air, and where the major oceanic currents push warm and cold water - which is why the west of Europe and particularly the UK enjoys a much more temperate climate than places in Canada, eastern Europe and Russia that are on the same line of latitude.
Latitude is one factor. Another is distance from the Ocean, inland places typically have warm summers and cold winters. Altitude obviously. The local geography is another. A nearby mountain range can block or exacerbate effects.
Look at Mexico City. Much higher altitude then Denver was in summer, but hotter and less cold in winter The northern most tips of the Sahara are not much lower latitude than the US Eastern seaboard which sees snow for months.
In the winter it’s farther south and pushes storm systems across the country. So you get cyclic weather. In the summer it retreats to the north and large parts of the country (esp. the Southeast) can go a long time without any change in weather.
The more dynamic things are, the more variation you see.
Note that the jet stream itself can have several bends in it. These are greater in the winter. So there can be a sharp bend driving things north in the western US and then a bend driving things south in the eastern US. In the summer these bends are farther north and not so severe.
One effect of all this is the “Bermuda high” off the E. US coast. It sits there for a lot of the summer keeping things in place. But it gets pushed out of the way in the fall.
And the jet stream often pulls down Arctic air during the winter. Figure D on this page is a good illustration of what would probably be really cold weather in the middle and east of the country Homepage - NC Climate Education