Posted at 8:23 AM on February 11, 2010
by Paul Huttner
There are some benefits to living in the city. Today, it's being a little less cold.
The urban heat island effect is most pronounced on sunny days and clear calm nights. During the day, bright sun hits asphalt and concrete surfaces like roads and buildings. The sun's shortwave energy is converted into long wave energy which is more efficient at heating the lower atmosphere. That energy is stored during the day, then reradiated at night to keep surrounding urban environments warmer than outlying areas. Cities can be as much as 10 to 20 degrees warmer then surrounding areas at night, even with the same overall air mass in the region.
On May 11-12, 1997, NASA used a specially outfitted Lear Jet to collect thermal data on metropolitan Atlanta, Georgia. Nicknamed "Hot-Lanta" by some of its residents, the city saw daytime air temperatures of about 80 degrees Fahrenheit on those days, but some of its surface temperatures soared to 118 degrees Fahrenheit. In this image, blue shows cool temperatures and red shows warm temperatures. Pockets of especially hot temperatures appear in white. (Image courtesy NASA/Goddard Space Flight Center Scientific Visualization Studio.) Image obtained from NASA
To give you an idea of the UHI effect this morning, you would have to drive to Madison, Wisconsin to find temperatures as warm as the 2 above zero reading observed in St. Paul.
Look for more bright sun today. Some clouds will drift this way tonight, and we may see a light dusting of flurries in time for AM rush Friday. It doesn't look like we'll se much accumulation, but any snow these chilly temps can make roads a little greasy.
Enjoy the sun again today!
Posted at 4:47 PM on February 11, 2010
by Paul Huttner
It's amazing what a little snow, some sun and pine trees can do.
We've seen have seen huge temperatures swings today from early morning to afternoon. Temperatures in many locations rebounded anywhere from 25 to 40 degrees today. It turns out conditions are just perfect in the Upper Midwest today for what we call big "diurnal temperature variations."
This weather geeky term simply refers to the difference between the morning minimum and afternoon maximum temperatures. To get the diurnal variation, you simply subtract the daily low from the daily high. That gives you the diurnal range in temps for the day. Yes, these are the little weather geek games us meteorologists like to play. What's the deal Huttner...slow weather day?
Check out some of these numbers from today. Keep in mind that the average Twin Cities high and low today are 27/10. That's an average daily diurnal variation of 17 degrees.
So we saw anywhere from a 25 to 33 degree temperature recovery today in the Twin Cities to St. Cloud. In the case of St. Cloud, that's nearly double the average daily temperature range for this time of year.
The numbers get even bigger as you move north into pine and spruce covered nothern Minnesota towns.
That's a 45 degree swing in Cook in one day folks. Pretty remarkable.
So why the big daily temp swings this time of year?
It turns out several conditions are just perfect for generating the big temp changes.
-Fresh, Deep Snow Cover: Fresh snow radiates very well at night. That allows temperatures to plummet in the overnight hours on clear nights.
-Stronger February Sun: The noon sun angle is now 8 degrees higher than in late December. Overall solar output is more than twice as high as 6 weeks ago. The increased solar energy on sunny days helps boost afternoon temperatures more rapidly than a few weeks ago.
-Coniferous Forests: The pine tree effect up north help to greatly boost daytime temperatures in the northern forests this time of year. The trees are much more efficient "heaters' of the lower atmosphere as they turn incoming short wave solar radiation into longer wavelengths that are more efficient at heating the surrounding air mass.
Enjoy the big temp swings this time of year, and be happy that us weather geeks have something to do on quiet weather days like today. Can you say "pocket protector?"