29 July 2008

No, it doesn't rain more on Saturday- It's Wednesday!

Posted by Dan Satterfield

Admit it.

Sometime in the past you have complained that it always seems to rain on the weekend. In the back of your mind though, you told yourself that it really doesn’t rain more on any one day of the week.


WRONG! (If your in the Southeast U.S. anyway.)

Seems that a group of scientists decided to test a theory that it should rain more during the latter half of the work week than on the weekend. This makes sense because rain drops form on aerosols. (Aerosols include dust, sea salt, and soot etc. from air pollution.) More air pollution-more rain.

There was a famous paper (well in the geeky science world that I live in) about this affect about 30 years ago. The paper looked at rainfall downwind from St. Louis, and found that rain amounts were noticeably higher in Illinois to the East. Something the farmers there have for some reason decided not to complain too much about!

Along comes the TRMM satellite. The Tropical Rainfall Measuring Mission satellite was the first time we put a good old fashioned weather radar on a rocket and blasted it into low Earth orbit. There has been a ton of science coming out of this radar. Including a study by NASA using our ARMOR Dual Polarimetric Radar.

Thomas Bell of Nasa Goddard, and several other scientists have written a paper about using the TRMM satellite to track rainfall variations by day of the week. They found that indeed there is a signal. Wednesday and Thursday are wetter than Saturday and Sunday over the Southeast. Not only that. The opposite is true in the Atlantic Ocean, to the east of the S.E. USA.

This makes sense for two reasons. First, the air pollution will get pushed eastward by the prevailing westerlies. So the weekend will have more aerosols in the air over the Atlantic.More aerosols-more weekend rainfall.

Secondly, (A bit more complex) is the atmosphere must maintain hydrostatic balance. In other words if air rises up into thunderstorms over the southeast, it has to sink back down somewhere else to compensate. Therefore, the Western Atlantic should be drier if we are wetter. This phenomenon is very noticeable around hurricanes. You usually see very litte airmass thunderstorm activity outside the circulation of a tropical storm.

So yes, all that junk we spew into the atmosphere not only reduces the visibility, it makes it rain more. (Visibilities in the Great Smokey’s are less than half what they were 50 years ago!) Look on the bright side. The weekend is ever so slightly drier and sunnier!

I found the paper in pdf form on Doctor Bell’s website:


NASA has a summary here: http://www.nasa.gov/centers/goddard/news/topstory/2008/midweek_rainfall.html

If Math scares you, use the second link.