January 05, 2011

What is the Coldest Time of the Day?

It is a cold, clear winter night. No fronts or storms are coming in. What time will there be the coldest temperatures on average?

3 AM?
5 AM?
Ten minutes before sunrise?
At sunrise?
Some time during the 1/2 hr after sunrise.
None of the above.

The answer, believe it or not, is during the 1/2 hr after sunrise.

Here is an example during the past week (31 December) at the UW. Sunrise was at 7:57 AM, and the coldest temperature was few minutes later (click for larger image)

So why are the lowest temperatures so late? Its all about energy in and energy out. When more energy is leaving the surface then coming in, the temperature cools. More energy in than leaving, warming.

There are two main energy streams at the surface. First, at all times of the day the surface is emitting radiation, mainly in the infrared. The warmer the surface, the more infrared it emits. Then there is solar radiation coming in during the day. There can also be heat conducted from the underlying surface, but lets neglect that here (and the temperatures shown above were from a temperature sensor above the ground).

At night, after sunset, there is no solar heating, so the infrared cooling to space cools the surface . The cooling is often more rapid at first but continues through the night since the infrared loss to space continues. So the lowest temperature SHOULD wait at least until sunrise. However, cooling can continue for a short while AFTER sunrise. Why? Because initially the sun is very low in the sky and the solar radiation is very weak, weaker than the infrared loss to space. As long as this is true, the temperatures will continue to fall, EVEN THOUGH THE SUN IS UP!

Anyway, a nice weather factoid to amaze your friends and concern your enemies.

There was a nice rain shadow today for the Puget Sound lowlands (see radar image below ). The winds were relatively westerly so the rain shadow rotated over Seattle and the nearby lowlands. Those smug folks in Sequim and Port Townsend got rain today due to the rotated winds (they get the rainshadow during the more normal southerly or southwesterly flow).

By the way, I would not be surprised if some of the Sequim denizens were originally from southern CALIFORNIA ! In fact, I frequently get calls from Californians looking to retire in Washington, asking if Sequim is really about as dry as LA. After I tell them about the cacti and the irrigation festival, they are ready to pack their bags! When I describe the lavender farms and nearby Hurricane Ridge, they are rushing to call the moving van. But then I mention the winds....

Ah yes...the cold is on for this weekend. And I would be surprised if someone in the lowlands doesn't get a bit of snow. But no big snowstorm at this point. But more on that later.


  1. I have this ridiculous idea in my head that Sequim is the perfect place to move because of the lack of rain and the air quality is so good. My husband will be relieved to know that the idea of lots of wind has snapped me out of that crazy idea. I learned while living in San Franciso that I really don't like wind. Although, it does fascinate (as long as I'm inside). If you feel like posting more about wind I am all for it!

    1. Your definitely Right about the SF windy city too.

  2. Report from Sequim: we were begging for rain over here and got some during the early A.M. So yes indeed, we are pretty smug right now and more than happy to have you kick sand in our face, Cliff!

  3. After model riding for about a week now. I'm going with Wednesday next week for our big snow.

  4. I'm 2 miles southwest of Sequim and I don't think it rained more than an hour today at most, during the daytime. Much of the day it appeared to be hung up just to the west of me on radar. A shower blew through around 3:00pm then it was dry again.

    Also, I get MUCH less wind than Sequim proper at my location. I pity those poor folks living out in the fog and wind by the Strait.

  5. The half hour after the sunrise - right when I'm walking to work. Looking forward to spring!

  6. Ahh, thank you. This explains just why, when I get up before dawn and step outside to get the newspaper, it often feels slightly warmer then than it does when I go back outside an hour later to walk the dog. I attributed to my imagination or my carrying latent heat with me from having just arisen from beneath a down comforter or something!

  7. Thank you Cliff. That is fascinating. I have noticed on several occasions where I live how the temp drops during that time.

    BTW, I got your book for Christmas and I love it! Weather, especially NW weather, fascinates me. You did a great job. Boy, models are all over the place for next week aren't they. What do you think at this point?

  8. What you said is just a small part of the equation, I believe. When it's cloudy, you'll see only the effect you noted, similar to why the coldest day of the year is around Jan 16th or so rather that during the shortest day of the year.

    However, with sunrise on a clear day, something else much more pronounced happens -- the sunshine strikes dew, which then evaporates and draws a considerable amount of heat out of the surrounding area -- 540 calories to evaporate 1 gram of water, or 80 calories to liquify a gram of ice. Something like that.

    I notice that effect on the chart you posted, which shows that sudden drop at sunrise, and the 31st was a clear day. You would not get a sudden drop otherwise, but rather a slowed continuation of the 'leak' of heat out of the area as in the hour or so which preceded sunrise.

    I can't take credit for that observation -- my 8th grade science teacher told my class about being in the desert somewhere in New Mexico in the summer when the sunrise prompted a brief 10- to 15-degree temperature drop.

  9. JOSH-
    I really am wondering if the NWS and Cliff are looking at the same GFS I have been seeing the last several days.
    It looks like an EPIC snowstorm coming with arctic air in place and the a low rushing in on the southern periphery of the high over BC.

    Im starting to wonder if Im losing it, but it looks like you are seeing the same thing as I.

  10. http://www.youtube.com/watch?v=6rGp5f-q5dI

    Finally finished my video of the Nov. 22, 2010 drive home. I have wondered and continue to wonder if they Duwamish lead to extra icing in the curves...looking forward to next slug of white!!

  11. Josh and Michael,
    What about SW Washington- any ideas? An epic snowstorm would be wonderful...

  12. I'm sure we're seeing the same thing Michael. I bet Cliff is too. But with all the inconsistancies of the model runs I'm wondering if the models don't quite know how to handle this.

    As far as Cliff and the NWS. I'm sure they're just playing it safe.

    SW Wa, I really don't know. I am from Battle Ground. But I really can't be sure. I can bet on freezing rain one day. That east wind can be harsh sometimes.

  13. I expect some fraser winds as well.

  14. Hi Cliff,

    Thanks for the info. I'm curious why the graph shows the temperature dropping through the night until about 2 am, where it levels off at about 27 degrees until about 7:30 am. Why didn't the temp continue to drop during that 5+ hour time period?

  15. Michael and Josh - I have been watching this since late December. Yes, even then the GFS was showing the potential for some kind of ideal snow event setting up. I think the day will be Thursday. The 12z GFS/WRF dumps near a foot of snow on Seattle. For the 4pm Wednesday to 4pm Thursday time period alone:

    As for Cliff and the NWS, I'm sure they are playing it safe as Josh said, at least NWS. Remember, Cliff said this is not a forecasting service, so with everything else he has to do, he probably doesn't have much time to post about that. He may this weekend however when there are no classes for him to teach at UW. As for NWS, with that said that they are very likely playing it safe until they see more consistency in the model runs (as they mention in the 12z discussion), my experience shows that they usually play it too safe, to the point of not changing anything in their forecast for fear of being wrong if they do. They like as few changes in the forecast as possible to not confuse people, I guess. I disagree with that thinking, especially with something like this that would be a major event. It seems to me that people would like to be aware of its possibility more into the future, even if they have to back down later about it. I know I would.

  16. Now I know it is a ways out, almost a week, but man is next Wednesday through Friday looking interesting for Western Washington. Models have been having a hard time with consistency but as much as I want snow it is hard not to get a little excited.

  17. Now I know it is a ways out, almost a week, but man is next Wednesday through Friday looking interesting for Western Washington. Models have been having a hard time with consistency but as much as I want snow it is hard not to get a little excited.

  18. The GFS has been very consistent lately with the last 6 runs or so showing the huge snow event on tap.
    EVERY snow event is low confidence, but with cold air in place this is definitely decent chance of a large snow event next week.
    Just for kicks: the GFS also has another snowfall next sat before a warm finally occurs.
    This is going to be fun to watch.

  19. I use to go hunting with my father, we would leave camp before the sun was up (you could not shoot until the sun was up). Anyone who has been out in the mountains, in the fall or winter, and was out before the sun was up, could tell you the coldest part of the day....I always enjoy your weather blog. NOAA gives me the weather, you explain it….thanks!

  20. Hi Wx Enthusiast--forecasts out on day six or seven are surprisingly unskillful. Whether it is a chance for heavy snow or high wind, or a routine forecast for scattered showers. If you think about the number of things that have to come together to get snow, where snow is actually pretty rare, you can imagine how easily those things fade away over days and days of model simulations. Sometimes the progs flip flop out of a regime, and sometimes they just slowly eat away at a big event. I love big lowland snows as much as the next guy, but you're right, I think it is fair to say that Seattle meteorologists are sensitive to giving too many false alarms. It is actually quite an effort not to do too many false alarms!


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

Lightning Returns to the Pacific Northwest

 Lots of thunderstorms, some approaching severe levels, have hit eastern Oregon and Washington during the past day.....and there are severe ...