Extreme humidity, heavy rain

Take a look at the balloon data from NWS BMX this morning:

The red line is temperature, and the green line is dewpoint.  The dewpoint in BHM this morning was around 75.  On a lot of days, as it heats up and low-level air mixes, drier air will make it down to the surface.  But, this is not the case today, as the atmosphere is almost completely saturated up to 550 mb (about 15,000 feet).  A good measure of total atmospheric moisture is to figure out how much rain would fall if you squeezed all the moisture in the atmosphere above a point out (called precipitable water).  At BMX this morning, it was 2.2 inches!  So, when storms form, since they are scattered, some places will get more than that, as air moves in from other areas, replenishing the storms with more moisture, while other places may get no storm at all.  A few spots could get 3″ of rain this afternoon and evening, along with vivid lightning.  The precipitable water is high all over Alabama, as high as 2.5″ in MGM and Greenville (James’ old stomping ground, see blog on weather hype a few blogs down).

Given the rain that occurred yesterday, the dewpoint has actually gone up this afternoon, with some of the water evaporating out of the soil.  The dewpoint in BHM at 3 pm was 77 degrees.   At a dewpoint like that, the atmosphere is 2% pure water vapor, making water vapor the 3rd largest constituent in the atmosphere today (behind nitrogen and oxygen).   Even on a rainy winter day with a dewpoint of 50, the atmosphere is only 0.7% water, making water 4th behind Argon.  On a dry cold day in winter, a dewpoint of 20 means the air is only 0.2% water.  So, there is 10 times as much humidity in the air today as there is on a dry winter day.

This kind of humidity makes the air conditioner work harder (it has to condense water out of your house and cool it), and makes your sweat less efficient in cooling your body.  So, drink plenty of fluids this week, even if you work indoors.

Category: Met 101/Weather History