Wednesday, November 23, 2011

Inversion Collapses: Cold Snap Ends

Temperatures aloft warmed dramatically over the past 36 hours, and persistent cloud cover and a moderate pressure gradient have taken their toll on the inversion. Here is a plot of the lowest 1000 meters from the morning sounding from today and Monday.

At 2am today (Nov 23) the temperature at the Fairbanks Airport rose to 2 above. This effectively ends the cold snap. The numerical models have not distinguished themselves in glory with the end of this event, and that is directly related to the inability of the models to effectively resolve the processes in the very near surface (boundary layer). That said, the airmass remains chilly and if and when clouds break temperatures will again fall in the valley. However, the unprecedented nature of this cold snap is over. I'll have a wrap-up in the next couple days.


  1. I don't understand why clear skies are supposed to allow for temperature to remain cold and why cloudy skies would cause warming. This would be counter-intuitive really, as far as how weather works in lower latitudes. I would think that clouds cover would block solar radiation and prevent temperatures from rising. Therefore, the summers in cloudy climates are much cooler, like those on Aleutians, than summers in sun-exposed areas. I guess it suppose to work differently in the winter, but how lol? (confused) Thanks

  2. Trung,

    For the next couple months, when noon sun angles are less than 5 degrees above the horizon in Fairbanks, there is effectively no warming from solar radiation. Of course, the amount of solar heating has been small and declining for weeks, but now we're down to functionally none. Perhaps you've heard the saying the "the lowest temperatures occur about an hour after sunrise". The reason this is a useful generalization (at mid-latitudes) is that at very low sun angles (five degrees or is a useful approximation), there is no significant heating.

    Think of clouds serving as a blanket; they keep heat from escaping from the surface to outer space. Since the temperature of low clouds are often warmer than the surface over high latitudes in winter, the clouds are radiating at a temperature considerably warmer than the ground, so clouds are a "double whammy". This works at night in the summer too, although clouds are not then warmer than the surface, so only the blanket effect works.

    Hope this helps,