December 04, 2018

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

Or how about Albro Place/Airport Way in south Seattle near Boeing Field during the past two days? (the blue line is the time of sunrise).  On December 3rd the lowest temperatures were right after sunrise.
 Just before sunrise on December 4.
Plots courtesy of Jeff Baars, UW

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 watch your thermometer.


  1. After standing in a wet trench far too many times, fixing a broken water main. I can say it always felt the coldest just before sunrise....I have a question for you, why do most water mains fail at night especially just after the crews bedtime?

    1. Murphy's Law.. If the failure was due to freezing, it would make sense at night as that is the time temps are colder and most likely to be below freezing. While Cliff indicated the coldest temps are just after sunrise, most cooling occurs after sunset, so your most likely to first experience freezing temps in the evening.

      However most water mains should be installed below the frost line, and I don't suspect freezing is the cause of most failures. For that I think you already answered your question, "after crews have allready gone to bed". System pressure would be highest when demand was the least. So because crews went to bed (along with everyone else) the highest system pressure would be at night and more prone to failure then.

  2. Used to notice or experience this sense of coldest at or around dawn when I was a paperboy.

  3. Off Topic question - do you know what happened to the PROBCAST website. It's been down for awhile.

  4. Right on 99.9 percent of the time. I've been recording lows for a few years, and unless "something wicked this way comes" (a warm front is moving in, or some-such) that's how it goes. Very cold in the mountains right now - we've begun having whole days under 32, but it's still humid and the hoarfrost is gorgeous. When it's below freezing all the time, low may hit at 4 AM... Weather, all weather!

    Our online Wunderground instrument:

  5. Except it really isn't darkest before dawn, is it?

  6. What effect does the geography have? At any point on the Earth, there is another point that is located to the east. At any point on the surface of the Earth, the amount of solar radiation would be different just one hundred miles to the west, right? If the solar radiation is heating the ground at one point and not heating the ground at another point nearby to the west then how much effect does that have?

  7. 22.4F 1.5mi SW of KBLI at 6:50AM 12/5

  8. I know this is a weird observation. I do not know why I believe I see more full moons at night. Why do I see them? Because of the lack of cloud cover.

    Which makes me wonder, does the moon influence precipitation or cloud cover?

    1. A full moon by definition is when the moon is exactly "opposite" the sun in the sky. So a full moon always rises just after sunset, and sets around sunrise. You only see full moons at night.

  9. Stefan... My theory is that we always look up and notice full Moons. We only get to see them when the clouds are broken or gone.

    Given the very small amount of energy that reaches the cloud layer from sunlight reflected from the Moon, I think this has to be a human perception thing. Our brains are wired to perceive (or create) patterns, even when they don't exist.

  10. I wonder and would love to know since predicting frost delays is hard and effects my job as a golf pro. In borderline scenarios on temperature and frost...when the sun first comes up it often goes from no frost to frost. In my non scientific observation, there is something about the sun coming up that pushes the coldest air to the ground and all of sudden we go from no frost to frost.

  11. I live on the east slope of Capitol Hill and my kitchen window looks across Madison Valley to the hill that makes up the Madison Park neighborhood. Often after clear nights at the break of sunrise, looking towards Madison Park (and Lake Washington beyond – which I can’t see) I’ll see a thin layer of fog forming. Then, within about 30 minutes after sunrise, that thin layer of fog has turned into low cloud cover encompassing the east side of Capitol Hill and eastern Seattle. It looks like any other cloudy day in the dead of winter. On these days, the overcast skies usually burn off by 11:00 AM in the summer and 2 PM or so on cold winter days. I’ll check the Space Needle web cam and (though delayed by 30 minutes or so) one can often see clear skies all around except for that portion of the city. I’m sure it must be lake-effect fog. And the temperature must drop enough for moisture in the air to condense. But we often see the sun rise for 10-15 minutes before that greater fog layer forms. I’ve always assumed it must be because the coldest temperatures occur a little after sunrise on many days. I’m still not sure it’s related to this blog post, but something tells me it must be.

  12. 21.4F 1.5mi SW of KBLI at 6:14AM 12/6

  13. I know Cliff has suggested in the past and here that after sunrise we are still in a cooling period due to more radiation leaving the planet than coming in...while that is true...I still think there is an unexplained phenonmenom of why we go from zero or little frost to massive frost once the sun pops up over the horizon.

  14. I am not sure how it relates, but watching the fog form on Sinclair Inlet it often is clear an hour or two before sunrise, and is at its thickest and hour or two after sunrise.

  15. As I understand, I think sunrise and sunset are marked when the Sun's elevation is at or about -0.8°. (The Sun takes up 0.5° in the sky; the atmosphere is able to refract sunlight even when the Sun is no longer in direct view; sunrise and sunset are marked when the *top* of the Sun is at the horizon.)

    So, my thinking is that the lowest temps typically occur after sunrise at least partly because there is virtually no solar radiation reaching the surface until the Sun has reached 0° or a positive elevation in the sky.

    The overall explanation could be that solar radiation can be measured as a sine function with respect to solar elevation. It increases rapidly from 0°, being 50% as strong as zenith at just 30° up. It's already 70% as strong as zenith at 45° up and 90% as strong at 65° up (about where the Sun is at solar noon around the summer solstice in Seattle).


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