ROANOKE, Va. – I'll be the first to admit that I regret not wearing long sleeves this morning. The 50s actually felt cool to me. It's probably because it's been exceptionally warm and humid most nights this summer. (The low has been 65°+ on 66 nights this summer - the second most on record so far.)
Up until Thursday, temperatures have dropped into the 50s only 7 times since June 1 in Roanoke. On average, we drop into the 50s 17 times through this date. You can see how this summer, then, has been an anomaly.
You can also see that night time temperatures in the 50s have become less and less frequent over the last 15-20 years.
This is most likely due to increased amount of water vapor on a large scale. Many climate scientists will tell you that the increase in water vapor could be linked to the earth's warming climate.
The more moisture there is, the less that the temperature can drop. Think about it this way. The dew point is the temperature the air would have to be to become wet. The air can't be wetter than wet, so the dew point is the threshold for temperatures to drop at night. If the dew point is in the upper 60s, that means there's more moisture in the air and that the temperature can't drop very much.
However, if dew points are lower like they are now - the air is drier and the temperature can drop more. That's why we're expecting Thursday night to be one of the coolest nights we've seen in about three months.