It's a cold front, but we're getting warmer. What gives?

CHARLOTTE, NC (WBTV) - When we think of a passing cold front, we expect showers, maybe thunderstorms, and cooler air.  So what gives with the higher temperatures behind the front?

It seems counterintuitive, but there are occasions where a passing cold front will actually lead to warmer temperatures.

When we see this setup, we're not talking about a blue norther.  We see this happen with very weak fronts where changes in temperature across the two air masses on either side of the front are only a few degrees.  What can often happen is that despite the slightly cooler temperatures behind the front, the real change is in the humidity.

Often, the first mass of air will be warm and very humid, almost tropical in nature.  Then, the front moves in and drops the dewpoints significantly without a drastic change in temperature.  The cold front has passed, but it's the humidity that's different.

With drier air in place, temperatures heat up much more quickly due to the lack of moisture.  Think of a desert.  It's very dry, and as a result we see big swings in temperatures.  Lows in the 40s and highs over 100 are normal in the desert Southwest.  Now, move East along the same latitude, into the Southeast.  There's lots more water vapor, but temperatures would more commonly range from the 60s to the 80s.

We can apply the same idea to a cold front.  Without that moisture, temperatures can widely fluctuate.  

That extra moisture moderates our temperatures because water vapor requires much more energy to heat than any other gases found in our atmosphere. The other gases are in roughly the same concentration all the time, but water vapor fluctuates widely.  When there's more of it, the humidity climbs, and temperatures well, don't.

Another idea to note is that with drier air usually comes more sun, which only aids in surface heating.