It is Darkest Before the Dawn, But When are Surface Air Temperatures the Lowest

Here is an interesting question.   During a clear night and day like we have just had, when is the coldest temperature observed?

1.  10 PM
2.  4 AM
3.  6 AM
4.  Right before Sunrise
5.  Just After Sunrise
6.  9 AM

Did you write it down?

The answer is generally just after sunrise, although temperatures are pretty similar in the prior hour or so.

Today's sunrise was at 7:40 AM.  Let's check out the temperatures at some local stations.   A good place to go for higher time resolution is the WSU AgWeather Site (every 15 minutes).  Starting with Seattle, the surface air temperature (red line) is lowest around 6:30 AM, but doesn't really rise until after 8:30 AM

Coupeville on Whidbey Island is coolest just after sunrise

 Woodinville... just before sunrise

Or the Weatherunderground site near SeaTac Airport (KWASEATT1683), with the lowest temperature right after sunrise

You will also notice that the highest temperatures this time of the year are quite early....around 2 PM, in comparison to 5-6 PM during summer.  And most of the daily cooling occurs between roughly 3:30 and 8 PM.

So why are the lowest temperatures often at or just after sunrise?   With no solar radiation at night, the earth cools by emitting infrared radiation to space.  That in turn cools the adjacent air layers. But how can the temperatures continue to cool or stay the same for a while when the sun comes up?

Good question.  Because it takes a while for the incoming solar radiation to be larger than the outgoing infrared radiation.  Eventually, the solar radiation wins, but that can take 15-30 minutes to happen.

Tomorrow, Wednesday, morning should be similar to today...so watch your thermometer.

from Cliff Mass Weather and Climate Blog https://ift.tt/2E5Rprt