Wednesday, July 29, 2015

It's Extremely Dry for Late July

If you've been suffering with a dry throat and drinking a lot of water since yesterday, there's a good reason.  It's extremely dry for late July.

We typically think of July as a dry month because we receive little precipitation (on average), but in terms of the total water vapor content of the atmosphere, late July is about as wet as it gets climatologically.

The precipitable water is the total depth of water that you would have if you were to condense all of the water vapor out of the atmosphere.  Below is the climatology for Salt Lake City, with red indicating the record high value for each day of the year, blue the record low, and black the average.  Note the peaks in late July or early August.

Source: SPC
Average values for this time of year are around 0.8 inches.  Record low values are around 0.25 inches.  In yesterday afternoon's sounding, we were sitting at only 0.24 inches, which is a record for the date and the 2nd lowest value on record during the last week of July.  The profile below shows why values are so low.  The dewpoint trace (green line) shows a surface dewpoint of only 28ºF (average for late afternoon this time of year is 47ºF), with even lower values aloft.  There's simply not much moisture out there.  

Source: SPC
As a result, if you are looking for a cloud, good luck with that.  We're talking "severe clear" conditions.

I don't look at time-height sections much in the summer, but check the one below from the NAM forecast through Saturday afternoon (time increases to the left).  That darkish yellow color indicates areas with a relative humidity below 10%.  Near the surface we're a little higher than this, and some higher humidity air creeps over us at upper levels beginning on Friday, but for the most part we're bone dry.

The bottom line is to keep sunblocking and drinking water despite the climatologically cool temperatures. 

No comments:

Post a Comment