What’s a Skew-T, part 4

Posted: 5th March 2013 by Jason in Tutorial

dryDry Adiabats are next up. These are the first lines on the Skew-T that aren’t straight, and for what will hopefully be an obvious reason.  Dry Adiabats are drawn as a solid line starting in the south east corner of the Skew-T and moving to the north west and tend to curve northward in the middle. The adiabats are an interesting bit of science.

When you take a box full of air (you’ll usually hear it called a parcel, but box is more awesome) and you lift it high above the ground, a few things happen to the air inside of it.  Remembering back to the first axis of the Skew-T that we talked about (isobars), as you rise higher into the atmosphere the air pressure gets lower.  You probably learned in middle school, or will learn if you aren’t there yet,  that molecules pushed close together will generate more heat then molecules that are spread far apart.

So thinking back to our box, when we filled it full of air it had around 14.7 pounds per square inch of pressure on it from all sides.  By the time you’ve lifted that box to 900mb that box only has 13 pounds per square inch pressing on it. At 800mb it’s 11.7 psi and at 600mb it’s down to nearly half at 8 psi!  With less weight on our box of air, that air will start to expand and as it expands, it will cool.

These dry adiabat lines are showing us the rate of that cooling with height and plotting that decrease based on  our starting temperature.  Looking at our Skew-T then, if we started our bucket of air off at 1000mb on a day when it’s 30 C outside, and then lifted that box of air up to 600mb, that bucket of air will have cooled down to -10C or at roughly 1 C for every 100 meters. But usually we have humidity to deal with, which takes us to the moist adiabat.