It’s a good idea to avoid the clouds as much as possible. For IFR pilots, you’ll typically find a smoother ride in the clear. VFR pilots have no choice on the matter: per cloud-clearance rules, it’s the law! But what about that cloud out on the horizon? You don’t need a Sporty’s sight-level to tell if you’re going to hit that puffy cumulus up ahead. All you need is a finger!
Here’s the trick:
Step 1: Point
Take your pointer finger and literally point at the top of the cloud. Reach forward and physically touch the window at the top of the cloud.
Step 2: Wait
Keep your head and finger in the same position while maintaining unaccelerated flight: straight & level or steady & stabilized climb or descent.
Step 3: Observe
Observe the cloud’s motion relative to your finger.
If the cloud appears to move below your finger, you will pass above the cloud. But if the cloud moves above the finger, you can expect to go IMC in a few moments.
This whole trick is based on the old collision-avoidance rule that any airplane with no apparent motion must be on a collision course. It’s easy to “eyeball” another airplane and detect relative motion.. Because of the slower closure rates and enormous size of some clouds, it is a lot more difficult to judge motion.
If you don’t mind writing on the windows, you can also use a dry-erase marker to free up your hand.