Sunday, November 23, 2008

The Subtleties of Clear Weather


It is clear over much of the reason, EXCEPT for the normally foggy areas, like the Willamette Valley...a bowl that tends to collect cool, saturated air (see pic). Some obvious fog in the valleys of NE Washington.
This morning there was frost outside but my thermometers read 37F. How is that possible? The reason is that on clear nights the earth radiates heat to space more effectively than the atmosphere above...and if the winds are light...as last night...an inversion forms where temps warm with height. So on such nights always be careful driving if the air temps drop into the mid 30s.
On such nights there are often large horizontal variations in temp...take a look at the reading around sunrise (see figure). The air temps range from the lower 40s over water to freezing inland. Not unusual at all.
Tomorrow looks like another dry, partly sunny day and a weak front with rain and clouds on Tuesday. Then we go dry. Not supposed to be this way in late November....but for those who suffer from SAD...seasonal effective disorder, a godsend. Winter ends here in mid-February...less than three months away.

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