Chilly Mornings, Warm Afternoons

There have been some pretty impressive temperatures swings the past couple of days as morning lows fell into the 30s and 40s but afternoon highs managed to warm into the upper 60s on Sunday and the middle and upper 70s the past couple of days.  Now it's not uncommon to see a large diurnal range (morning lows to afternoon highs) in the spring and fall but these have been well over 30° each day.  So why is that?

You have to look at our dew points.  Dew point is a measure of the amount of moisture we have in the atmosphere.  Our dew points this week have been very low, also creating low humidity.  This means there is a dry atmosphere in place across the Midwest.  Dry air heats up a lot more efficiently than air that has a lot of moisture.  The molecules in dry air are able to move around more freely causing them to heat up and the temperature to rise.  In a moist atmosphere there are more water molecules and they aren't able to move around as much causing a slower warm up.  Think of it as if you were running versus walking.  When you run you tend to get hot a lot more quickly than if you were walking.  Same concept is applied to our water molecules.  Now this trend is likely to continue for the next couple of days, but as southerly winds move in our overnight lows will only drop into the 50s versus the 30s and 40s.