This afternoon's satellite imagery shows an incredible slug of dry air moving across the south and east behind a cold front, bringing beautiful weather to millions of people. In fact, the air is so dry at 10,000 feet above sea level that its humidity is near zero percent in some spots.
The leading edge of a cold front diving through the southeastern United States is very clear on water vapor imagery, with warm colors denoting dry air and cool colors denoting cold air. This cold front is responsible for intensely nice weather across much of the eastern two-thirds of the United States today, and the high pressure building in behind it will continue a stretch of gorgeous weather for most people through the rest of the week. This kind of weather pattern is great for the public but hell for weather enthusiasts, since there'll be nothing to talk about.
Just how dry is it? We can look at a forecast sounding (data from a simulated weather balloon launch in the model) to look at the slice of the atmosphere containing the driest air. This particular sounding is from the Rapid Refresh model valid for west-central Alabama (about halfway between Tuscaloosa and Mobile) at 2000 UTC, or 300 PM CDT.
The red line traces the air temperature while the green line traces the dew point as you ascend through the atmosphere. The temperatures are on the x-axis in Celsius, and the pressure levels are along the y-axis in millibars (along with heights in meters printed to their right). When the temperature and dew point lines close in on one another, the air is moist, and when they sharply diverge, the air is dry.
The forecast sounding shows incredibly dry air—especially for Alabama this time of the year—between 500 and 700 millibars, or between 10,000 and 18,000 feet above sea level. The RAP model forecasts a temperature -0.8°C and a dew point of -50.7°C, producing a relative humidity value of just 0.9%. That's dry. Really dry. That's nosebleed-and-cracked-skin dry.
It's even pretty dry at the surface—the surface observations in nearby Demopolis for the same time showed an air temperature of 29°C (84°F) and a dew point of just 7°C (45°F), producing a relative humidity of 25%. Sure the air temperature is warm, but the low humidity makes it more than bearable.
Let's compare that to the ever-gross airmass situated over central Florida for 95% of the year, where it's hot, humid, and storming:
The end result of the extremely dry air is a gorgeous day completely devoid of any cloud cover whatsoever. For folks in most of the Deep South, today is one of those perfect fall days with a deep blue sky and comfortable temperatures.
It's the perfect day to go sit outside and talk or just stare at the sky. Or stay inside and blog alone all day. That's cool, too.