Updated: Jul 10, 2020
Pilots must master many disciplines as they learn to fly. Weather is perhaps one of the most difficult disciplines to master. So it's quite important to teach it right the first time. But I often run into way too many poor examples of online weather training that is not only bad, but potentially dangerous. Here's yet another example from AOPA.
I received an email from AOPA that linked to a program called the Thunderstorm Challenge. It is developed by the Air Safety Institute. I typically like to go through these to see how others are teaching certain weather topics. Well, I got to this slide above and stopped. There are so many problems with this slide alone that it really shows how little they understand basic weather principles.
First, they mention "humidity" but do not tell you what kind of humidity. Yes, there are multiple kinds of humidity that include absolute humidity, specific humidity and relative humidity as you can read here. That's the least of the issues though. I assume they are referring to relative humidity. If so, the statement they make...
"...fly early in the morning when temperatures are cool, and humidity is relatively low. As the day progresses and heat and humidity increase, the chance of a pop-up storm also increases."
...is just the opposite of what actually happens. They appear to be conflating relative humidity with dewpoint temperature. Relative humidity is not a true measure of how much moisture is in the atmosphere. In Minnesota on a cold winter morning the relative humidity might be 100%, but in southern Florida on the same day the relative humidity could be 50%. So which situation contains more moisture? Turns out there's way more water vapor in the air in Florida than in Minnesota. That's important since water vapor is the "crude oil" for thunderstorms (actually it's the condensation of water vapor that releases the latent heat that fuels the storms).
As I mentioned, their statement is completely backwards. In the morning the relative humidity is generally at its highest. It's pretty common to have a relative humidity of 95% in the early morning hours during the summer when the temperature and dewpoint are nearly the same. Whereas in the afternoon, the lower part of the atmosphere called the boundary layer becomes well mixed and with the heating of the day, the relative humidity is typically at it's lowest. Mixing tends to bring down dryer air aloft and lowers the dewpoint temperature...in fact, the lowest dewpoint tends to be at the warmest part of the day.
Then, the most jaw-dropping part is when they try to connect the heat index with the potential for thunderstorms. How absurd!
The National Weather Service defines the heat index as -
"While dewpoint is a more definitive measure of moisture content, it is the relative humidity that commonly is used to determine how hot and humid it "feels" to us in the spring and summer based on the combined effect of air temperature and humidity. This combined effect is called the " Heat Index." The higher the air temperature and/or the higher the relative humidity, the higher is the heat index and the hotter it feels to our bodies outside."
Not unlike the wind chill, the heat index is really a comfort index and has no direct relationship to the potential for thunderstorms no more than the wind chill has a connection to airframe icing. There may be days where a high heat index may also be a day with a lot of thunderstorms, but that's a correlation, not a causation. Please do NOT use the heat index for any weather decision you may make as a pilot...other than whether or not you should use the air conditioner when you are taxiing to depart. It's better to use the dewpoint temperature as a measure of the available moisture in the atmosphere, not relative humidity or even high temperatures.
Here's an example that illustrates my point. Below is a forecast sounding for San Antonio, Texas that shows a Convective Available Potential Energy (CAPE) of 7,320 J/kg and lifted index of -9. This is an enormous amount of instability. The surface temperature in the late afternoon is expected to be 85°F and the surface dewpoint is expected to be a very juicy 76°F. That creates a relative humidity of 75%. All of these parameters are very high.
So it would seem that we have all the "perfect" conditions to create thunderstorms. Using the heat index chart below, the temperature outside feels very uncomfortable at 97°F. Yuk! But what about the potential for thunderstorms? Actually there's nearly a zero chance. So there's a high dewpoint temperature, high instability and pretty warm and humid air and this turns out to be a great day to fly...assuming you have an air conditioned cockpit. The lack of convection has to do with the capping temperature inversion between 5,000 and 7,500 feet, so it's important to understand the bigger picture.
Lastly, I really dislike the term "pop up" thunderstorm. I know it is also overused by broadcast meteorologists. However, it biases the pilot to believe that thunderstorms randomly pop up for no apparent reason just because there's no significant change of air mass. It's much better and more scientifically accurate to use the term "pulse" thunderstorm. That describes the process much better than an "air mass" or "pop up" thunderstorm.
Most pilots are weatherwise, but some are otherwise.™
Weather Systems Engineer
CFI & former NWS meteorologist