How can a cold front bring warmer temperatures?

A weird thing happened in the weather yesterday.  

A cold front blew through Central Texas but the temperatures didn’t really cool down.  In fact, the high temperature on Wednesday – behind the cold front – was 101°! That’s only 1° shy of a record high temperature! 

So what’s going on? Well, it’s a combination of a very strong early September Texas sun, clear skies, and an atmospheric phenomenon called compressional warming. The compressional warming occurred due to the steady north winds that blew throughout the afternoon. 

Compressional warming is when air moves from a higher altitude with lower pressure into a lower altitude with higher pressure.  As it does so, the air’s temperature increases. 

This can all be described perfectly in an equation called the Ideal Gas Law. It states that the pressure and volume of gas (in our case air) are directly related to the temperature of that gas.   

It’s well known that air pressure decreases with height.  That’s why mountain climbers need oxygen tanks at the tops of the highest mountains. That pressure difference will help describe how the Ideal Gas Law works.  

If a constant volume of air – like an unsealed jar – is brought to a lower altitude, the pressure of the air in the volume will increase with the surrounding air pressure.  Since the volume of the air didn’t change but the pressure did, the temperature must increase as well.   

This can also be shown by placing an empty, open plastic bottle in the freezer.  

If after a few minutes you cap the bottle and remove it from the freezer the volume of air in the bottle will not change.  But your house is (hopefully) much warmer than your freezer so the air in the bottle will warm up.  As it warms the pressure inside the bottle will increase noticeably. 

That same thing happens as air travels downhill.  Cooler air at higher elevations can move to lower elevations thereby increasing its pressure, pushing temperatures up.   

But, you might say, Texas is very flat, so does that really have a significant impact?  It’s true that the elevation change from Dallas to Austin is only about 55 feet, but extend that to Oklahoma City and the elevation change becomes just over 800 feet!   

Yesterday’s north winds extended from northern Oklahoma all the way to the Gulf of Mexico.  That amount of elevation change won’t be enough to produce enormous differences in the temperature, but it will increase the temperature by maybe 1-4°.  In that space there was enough of an elevation change that allowed the temperatures to increase slightly, just missing out on a record high temperature. 

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