Saturday, November 14, 2009

Why "normals" don't necessarily always seem normal

I make it a practice to almost never use the word "normal" when referring to temperature and precipitation trends. Even though it is a widely used term in Meteorology and Climatology, I prefer the more mathematically correct term "average." As far as I am concerned, there is nothing normal about a weather pattern that changes every day, no matter how minuscule the change may seem.

The latest weather patterns in the Mid-South are a good example. 2009 has been, overall, a very wet year for the region. In fact, last month ended up as the wettest October on record in Memphis. After recording 14 days of measurable precipitation for the month, the city went over its climatological "normal" precipitation for the year by the end of October, which means anything we get from November 1 until the end of 2009 just adds to the "wetter than normal" yearly total.

The problem is, since October 30, the city hasn't recorded a drop of rain. As of Sunday the 15th, the dry spell will reach 16 days, which will be the second longest dry spell of the year behind an 18 day streak from June 16-July 3. Our dry streak will likely end on Monday as a cold front moves in, but very wet spells, like September and October are often balanced by periods like we have just been through and the last part of June and August, which were exceptionally dry.

In sum, though 2009 will end up with above average precipitation, dry and wet spells can make a normal year seem unusual!

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