Saturday, January 26, 2013

Not Our Most Intense Cold But It's Been A Nice Streak

Today's 29 degree high marks the fifth consecutive day of subfreezing high temperatures in Philadelphia and the fifth straight day of sub 30 degree weather.  In terms of freezing or subfreezing temperatures, it is the longest streak since January 2011 (seven days) but in terms of sub 30 degree weather, this is the longest streak since January 23rd-28th, 2004 (six days).  We haven't had as intense a cold as we did in 2011, when we dropped to 8 degrees on January 24th and the Lehigh Valley dropped below zero, but this is our lengthiest cold wave in a while.

In playing with stats, you saw our "heat wave index" scale that we developed last summer to measure apples to apples on heat waves that occur in Philadelphia during the summer, measuring the progressive impact that heat can have on the body.  Cold doesn't quite work the same way -- you can dress for cold and layer up if you are in proper shelter and not out on the streets although there are other side effects that persistent cold weather can bring on the general public through increased susceptibility for viruses and so forth.  Of course, cold can impact homeless populations dramatically (as can heat) but you have fewer deaths from cold snaps than you do from heat waves, in general.  So, measuring a progressive impact of cold is a bit different than measuring heat.   That said, having had some "fun with stats" moments on a Saturday evening, I've come up with a bit of quick and dirty scale to measure cold wave impact.

An average temperature of 25 between the high and the low is seven degrees below our coldest average temperature in Philadelphia and a pretty reasonable benchmark for what is considered rather cold.  It's akin to what 90 degree days are in the summer although the standard for 90 in July compared to average is rather light.   For the sake of cold arguments, any day where an average temperature is 25 or below is a "cold plus" day.  You can call it arctic, brutal, bitter, biting, nasty, a pain in the rear...it's colder than usual.

Last winter featured two such days where the average fell below 25.  This winter is already at five (our streak).  This is the longest streak of "cold plus" since 2007 when we went six days in a row with an average daily temperature of 25 or below.  The longest streak in the last 15 years was back in January 2004 -- ten straight days where the average daily temperature was below 25.

In terms of days each winter where our average temperature drops below 25, we historically average 14.7 days per year going back to 1874.  Since 1981, the average is 12.3 days through 2010.  We still have a ways to go to get to "average" and given a general warming trend in our climate...and the concrete "heat" helping prevent a complete bottoming out in temperature, the trend may be for fewer such days in the years to come.  They will never go away but they may decrease in frequency...which makes streaks like this one worth noting.