January 29, 2009

Coldest Temps

The answer to my question in the last blog: the coldest temperatures are generally in the hour AFTER sunrise--so this time of the year around 8 AM. Temperature is controlled mainly by two factors: solar radiation during the day that warms, and infrared radiation emitted by the surface that cools (day and night). When the sun sets there is only infrared cooling and thus fairly rapid cooling sets in, cooling continues albeit at a lesser pace all night, when the sun just rises its feeble warmth is still dwarfed by the infrared cooling, so net cooling occurs and temp can still fall. Not until about an hour after sunrise is the sun's warmth large enough to balance the infrared cooling and temperature starts to rise.
I have attached the plot of temperature on the roof of the atmospheric sciences bldg at the UW that illustrates many of these points...the plot on the bottom is the solar radiation and air temperature is the third plot from the top.
Weatherwise, a weak front will approach western Washington late tomorrow afternoon, and showers should be felt tomorrow evening. The mountains will get some light snow--but not enough to really help. A convergence zone should form Saturday am and with cold air moving in behind the front, there could be some wet snowflakes in the CZ, particularly above 300 ft. More on this tomorrow.


  1. This comment has been removed by the author.

  2. Guess I`ll be the first to post in this new blog tonight.

    I see that Cliff has the same thought as me in regards to CZ, snow. 300 to say 400ft+ seems to be the 'magic' snow line with the cool NW flow and -8, -9c temps at 850mb moving in early Sat.

    In fact, if we all take a look at the Seattle profiler for tomorrow evening, you see that up around 1000m(or little over 3000ft) That NWLY flow is already working it`s way into our region. So should be a cool showery day on Saturday with maybe a little snow mixed in.



    Made a few spelling errors on first post I deleted my frist post and am re-posting again. :o)

  3. I think you mean that temp is the third from the top. Is there a correction for the time to convert to our time zone?

  4. Here`s the 850mb temps and CZ showing at 10am Sat for anyone who would like to check them out and see what it shows for them..

    10am Sat both maps.


  5. Does the cold temperature rule you mentioned apply in summer as well?

  6. Obviously the sun comes up quite a bit earlier in the summer than it does in the winter so that would make it earlier. However I would assume that the cool temperature would occur only shortly after sunrise in the summer because once the sun rises we get more direct sunlight much more quickly than in winter.

  7. Cliff,
    Pray tell me:
    Why have the local television weather-person's adopted the lable "BREEZY" in substitution for "light winds"?
    To me, it seems politically correct-as if we could offend the pneuma by describing light winds vs. strong winds.
    Thank you.

  8. Just a quick note on the Sand Point Profiler... those are observations not model output. Plotted are current and past winds/temps.


  9. So....when are we going to get some really interesting weather around here? Snow, fog, and rain are getting boring...how about some wind??? :)

  10. Cliff - any idea when the last freeze of the seattle weather season might occur? We want to deploy some mason bee blocks that we have, but are supposed to wait until another freeze is unlikely... how is this determined?

    Thanks, Matt

  11. Matt: Check the climate data graphs at NWS



    So it looks like mid-March to me for low chance of freezing). Or early April for almost no chance of freezing i.e almost the start of spring!

    But the long term forcast with the weak La Nina now in place is saying lower temps for the PNW in then next three months.


    So April, perhaps.

    BTW, is this data in tabular form with variances rather than extreme values?


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