It's that time of year. One day it's summer, the next it is autumn. A stationary front extended from Lake Superior to Iowa and on to the Texas Panhandle at Noon today. Ahead of the front temperatures were in the 80s - definitely summer weather. Behind the front readings dropped into the 70s, 60s, 50s, and 40s.
The map above includes isotherms; dashed lines connecting areas with the same temperature. Notice the tight gradient along and behind the front. The lines are closer together because someone traveling from west to east (east to west) would experience a rapid change in temperature. Traveling parallel to the front someone would experience little or no change in temperature. The coldest temperatures at Noon were found in Wyoming where readings were in the upper 30s into the 40s.
This is a good example on how temperature extremes occur. Ahead of the front winds are from the south and southwest. Behind the front winds are northerly. It makes all the difference in the world which side of the front you are on. The colder air has plunged about as far south as the warmer air has moved north. We can expect more days like today during the next several weeks as we move from summer weather into autumn. Gradually the highs won't be as high and the lows will get colder during October and November when next up comes winter.