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CFI Tips

Stability is the key

Teaching weather made easier

Weather isn't as sexy as aerodynamics, and it isn't nearly as much fun as chandelles or lazy eights, but let's face it--good, solid weather knowledge is essential to safe flying. So let me tell you how I explain it to my students by breaking it all down to a basic element of weather--stability.

I teach aviation weather at a local college in Southern California. Recently we were having a discussion about the relationship between atmospheric stability and the dry adiabatic (ad-ee-uh-bat-ik) lapse rate. I'm pretty sure that I overheard one of the students in the back of the class say to his buddy, "This guy is a dry adiabat!" While he may be right about me, there is a great deal to learn from this lapse rate.

Let's consider a parcel of air the size of a party balloon. If we inflate this balloon about halfway at sea level and then magically lift it in the atmosphere, we would observe some interesting phenomena.

Think about those air molecules in the balloon. The number of molecules in that balloon is fixed--no more can enter but none can escape, either. Picture a dance party of, say, 200 people in your garage. Everyone would be hot and sweaty, bumping into each other. Now, take those same 200 people and move the party into a big gymnasium. People wouldn't be bumping into each other, everyone would spread out, and the temperature would go down.

The same concept applies to the air molecules in our balloon. As we go up in the atmosphere, pressure decreases. The higher pressure inside the balloon will force it to expand as the pressure outside decreases. The molecules aren't bumping into each other as much so the temperature cools down.

The reverse is obviously true as we lower the balloon to the ground. As the pressure outside increases, the balloon shrinks and the molecules begin bumping into each other again. This, in turn, increases the temperature.

This cooling by expansion and warming by compression is what we refer to as the dry adiabatic lapse rate (DALR). We are assuming that the air is completely dry, hence the name dry adiabatic lapse rate. This gives us a point of reference to compare actual air samples in the atmosphere, which contain water vapor, to our theoretically dry air parcel.

The DALR gives pilots an indication of the stability of the atmosphere in which we will be flying. Stability of the atmosphere can tell us about several useful and important conditions that affect pilots, such as turbulence, cloud types, height of clouds, types of precipitation, and IFR or VFR conditions.

How can we tell if the air is stable or unstable based on this information? Easy. The DALR is a 3-degree-Celsius temperature change per 1,000 feet. Twice a day the National Weather Service takes readings in the atmosphere to determine the actual lapse rate. If the actual lapse rate is greater than the DALR, then the air is unstable. Unstable air means that it wants to go up in the atmosphere. Therefore, you can expect good visibility in between puffy, white clouds and a bumpy ride. Watch out for thunderstorms! If the actual lapse rate is less than the DALR, expect the air to be stable. Stable air resists vertical motions, with IFR conditions likely, steady precipitation, and a smooth ride.

Now, you don't have to rely on the National Weather Service for your readings. On your first instructional flight of the day, notice the rate of temperature change in 1,000-foot increments. Relating this real-time knowledge to subsequent students can be valuable in cementing their concept of stable and unstable air. It can also be helpful on long cross-country flights where conditions can change rapidly, for in-flight decision-making, and for routine preflight planning.

There you have it, a quick reference guide for determining the stability of the atmosphere. While weather admittedly may not be the most exciting subject matter, the more pilots understand of the basic concepts, the better the preflight and in-flight weather decisions they're likely to make.

An air traffic controller for 18 years, Paul Langston is an instrument-rated commercial pilot who is working on his CFI.

By Paul Langston

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