I've always thought it was a humidity thing. I live in Fort Collins, Colorado which is at ~5100ft. The east we have the rolling plains with not much of anything blocking the weather going all the way out to Nebraska/Kansas and into Missouri. To the west we have the Rocky Mountains right just to the west of the town and heading up to 14k ft within 30 miles of the city (diagonal, not driving time).
So my hypothesis is that it's based on humidity. In the summer here in Fort Collins, it's gets a bit more humid (nothing a Texan or someone from the South would call humid... but for us dry Coloradoans, 50% relative humidity is pretty high) and when it is ~40-60% rel. humidity, the temperatures stay fairly regular. Nights are warmish, days are warmish. Then in the winter, it's super dry and the temperature is all over the place. Nights are super cold, days can be pretty pleasant. I'm not sure of cause and effect, but it does seem that higher humidity moderates the temperature range.
I grew up in the North Bay area, north of San Francisco in California, and in the summer there the daily temperature fluctuations were amazing. It would be foggy in the morning, and it would be 45F cold and damp, and then the fog would burn off by mid-day and then the temperature would rise to 90F, then by 5pm, you'd watch the fog literally creep down the hills and by 7pm, everything would be foggy and cold again. San Francisco can have some spectacularly bizarre temperature swings from the same effect in the summer. You wake up and it's 45F, and cloudy, and by the afternoon it's in the mid-80's and everyone is sunbathing in Mission Delores park.