Brr! It's chilly outside! At least we're not getting the lake effect snow the eastern Great Lakes are seeing, but with a high temperature yesterday of 43º F - 20º below our average - we southerners can still complain a little. Amidst the gripes, I've heard people wonder aloud why it would be sunny on the cold days and rainy on the warm ones. I can't do anything about the weather, but I can definitely answer that question.
Imagine a parcel of air as a balloon. When the air inside the balloon is warmer than the atmosphere surrounding it, the balloon will rise because that's what warm air does. Cold air sinks, so when the balloon is colder than the air surrounding it, the balloon will drop closer to the surface.
The same is true for larger masses of air that are the size of geographic regions. The differences between masses of air are most pronounced along frontal zones, where they bump up against each other. If enough moisture is involved to create precipitation, the fronts are more noticeable to us.
The warm sector of a frontal zone is the area where the air is rising and usually where the most moisture exists, so that is the sector where the clouds and rain occur. The rising, moist air actually creates the clouds. Once a cold front passes, the air behind it sinks because it is so much colder than the air that came before it. Sinking air is also drier air, which brings sunny skies.
While these are the basic rules of the atmosphere, there are always exceptions, but in transitional seasons and even more so in winter, we are more acutely aware of the trend. The last few days of frigid temperatures brought the perfect example of what I call "deceptive sunshine." From inside, the day looks like it should be warm, but the reality is that the cold air is the reason the sun is so bright and the skies are so clear.