Minnesota rode the northern edge of a spring-like air mass Wednesday. And boy did it feel good.
Odd localized temperature patterns
The overall air mass was fairly uniform across Minnesota. But local differences in snow cover, landscape, and topography caused significant localized temperature differences.
Check out the map below. I’ve plotted meteorological station models over the GOES 1 km resolution visible satellite. Temperature is the number in the upper left had corner of each symbol. Notice the 50 degree warmth in Luverne in the southwest corner of Minnesota. Bare ground in extreme southwest Minnesota allowed the sun’s rays to efficiently heat the air near ground level.
Now look slightly to the north. Deeper snow cover just tens of miles away kept temperatures in the upper 30s. (blue boxes) So even with sunny skies, it was about 11 degrees cooler over the snow cover nearby.
All about albedo
So why an 11 degrees temperature difference in southern Minnesota Wednesday just a few miles apart? Blame it on albedo. That’s the metric on how much different colored surfaces on earth absorb or reflect incoming sunlight.
Snow cover reflects about 90% of inbound sunlight back into space. That means it can’t be used to heat the air mass as efficiently as a dark pine forest or bare ground, which absorbs and re-radiates about 90% of the sun’s rays.
That’s why temperatures in northern Minnesota’s coniferous forest reached the mid-40s. Thanks to the dark forests lower reflectivity, it was warmer in northern Minnesota than the snow covered prairies of southwest Minnesota.
On the map above you can also see how Two Harbors along lake Superior hit 52 degrees Wednesday afternoon. That’s likely the combined effects of dark forests and downslope winds. The ridge above the North Shore is about 1,000 feet above the surface of Lake Superior in many places. As the air drops in elevation at warms “adiabatically” about 5 degrees per thousand feet. That’s why Two Harbors got a little thermal boost.
It’s pretty unusual to observe the two warmest temperatures in Minnesota in the southwest and northeast parts of the state on the same day.
— MPR Weather (@MPRweather) March 14, 2018
You can thank snow cover and topography for today’s rare occurrence.