September Haze

Crepuscular Rays. Sounds like a painful physical ailment. No worries. These "Twilight Rays" are rays of sunlight that appear to radiate from a single point in the sky. Often appearing at sunrise or sunset these rays become visible due to reflection, diffraction and scattering of light. Thanks to "Rayleigh Scattering" the shorter wavelengths of light (blues and greens) are scattered less than the longer wavelengths (reds and yellows). Around sunrise or sunset light travels through nearly 40 times more air than a midday sun, thus the red/yellow hues.


Talk about confusion: yesterday was cloudy, cool and windy when I left the Naval Academy in Annapolis, Maryland - I flew back into a hazy, smoggy, orange, almost LA-like soup - it felt like I was diving into a dirty aquarium as we landed. It was a complete role reversal: normally the DC area is warm, sticky and murky this time of year (I've seen people playing golf on New Year's Day - their climate is definitely milder than Minnesota's, please take my word for it).

This is what happens when it doesn't rain for 12 days in a row: pollutants: man-made and natural (pollen, dust, etc). builds up, no rain to bring this suspended crud to the ground and clean out the sky. Air quality was actually pretty bad on Labor Day, in spite of a lack of rush hour traffic - worst over Wisconsin. Some of America's biggest air pollution disasters have taken place in September and October, when winds can be quite light, allowing pollutants to collect, especially in river valleys and other nooks and crannies.

The Dog Days of September. Light winds at nearly all levels of the atmosphere allowed pollutants to build up over the last 2 weeks, no rain for 12 days in a row, nothing to pull dust, pollen and smog/pollution to the ground.

Minnesota has some of the cleanest air in in the nation: no natural barriers to trap pollutants, like Denver and Los Angeles, and precious little heavy industry to speak of. I start to get antsy when I travel elsewhere, especially out east, because I can't SEE the weather anymore. Between the hills, the haze, the smog, the wires and high-rise buildings, you're lucky if you can see a little patch of discolored sky overhead. Here in Minnesota? On a good day you can almost see tomorrow's weather taking shape on the western horizon. I love the fact that you can see 20, 30, even 40 miles in every direction. Forget Montana: Minnesota is really Big Sky Country.

Supercell. A photo taken from my dock in early August - that large thunderhead taking up the left 2/3rds of the image was a severe thunderstorm 85 miles to the southwest, producing golf ball size hail when this picture was taken. NWS Doppler Radar showed the storm rotating, but because of the curvature of the Earth, the Doppler beam intersected the T-storm cell some 12,000 feet above the ground, more than 2 miles above ground-level. This brings up a limitation of Doppler: unless the storm is close to the radar site (within 50 miles or so) any rotation observed may be so far above the ground it's impossible to know what's really happening at ground level. That's why SKYWARN weather observers are absolutely critical - to get "ground truth" that rotation is, in fact, spawning a wall cloud, a funnel, or an actual tornado.

I'll be checking with Pete Boulay at the MN State Climatology Office, but there's a good chance we just enjoyed (?) the driest Minnesota State Fair ever (previous record was .02" in 2003 during the 12 day run that summer). Yes, after a wet August the drought appears to be taking another turn for the worse, and here we are: Day #12 (in a row) with no rain!

We are sliding into a wetter, cooler pattern. Today will be the last warm, hazy, lazy, 80-degree(ish) day in sight this week. A few showers and storms arrive Wednesday (computer models print out a whopping .10" for MSP, but closer to .50" for St. Cloud). Cool front #1 arrives Wednesday; a secondary, reinforcing cool front sweeps across the state over the weekend, keeping temperatures in the 60s to near 70 (metro) with spotty showers Saturday - more numerous showers on Sunday. Some rain may hang on into the first few days of next week, weather systems in a temporary holding pattern. If I had to wager (preferably someone else's money) I'd bet on the weekend of September 19 being much nicer, sunnier, warmer and drier than the upcoming weekend - just a strong hunch. Stay tuned....

GFS Model Outlook for Sunday morning at 7 am. Computer predictions show a very slow-moving area of low pressure temporarily stalling out over Minnesota this weekend, meaning cooler temperatures and scattered showers. Right now Sunday appears to be the wetter day of the weekend.

Paul's Outlook

Today: Hazy sun, breezy, sticky and warm. Winds: S/SW 10-20. High: 83

Tonight: Getting cloudier, growing chance of a shower or stray T-storm. Low: 60

Wednesday: Mostly cloudy and cooler with a few showers (best chance morning hours). High: 73

Thursday: Plenty of sun, lukewarm and pleasant. High: 76

Friday: Fading sun, probably dry during the daylight hours. High: 79

Saturday: Breezy and cooler with a mix of clouds and sun - passing shower or two. High: 70 (holding in the 60s over most of central/northern MN).

Sunday: Mostly cloudy, cool and damp with more numerous showers. High: 67

Monday: Unsettled, more clouds than sun - another shower or sprinkle. High: near 70