Brrrrr.... and statistically cold, too!

Today's IAD sounding was very cold: except for a small blip just above the surface, the entire troposphere (and much of the stratosphere) was below freezing. The 500 mb temperature was -30C, which caught my eye: this early in the year (03 December), already -30C? How unusual is that? So I went looking in the sounding archive (IRGA), which fortunately for me I've been having one of my research students examine the data- so FORTRAN scripts were already written to quickly extract the data. The resulting histogram (see below) confirmed my suspicion: -30C *is* below average. Specifically, only 3.96% of all the November & December 500 mb temperature observations in the past 50 years at IAD had been colder than -30C. So we really are cold! Fortunately the ground (and nearby bodies of water) is still relatively warm- so our low this morning at the surface was still about 0C (and the high about 5C). If this same setup had occurred in late January, we would be COLD indeed, all the way to the surface. Now to hop on my bike and brave this statistically unusual chill.