Twin Cities: Urban heat island today

There are some benefits to living in the city. Today, it's being a little less cold.

Calm air produced a clear urban heat island (UHI) effect in the metro this morning. Temperatures were a full 13 degrees warmer in St. Paul (2 degrees) than in Lakeville (-11 degrees) early today.

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!

PH