"How do I determine how much to deviate left or right of course to avoid a thunderstorm that’s 80 miles ahead of me?" - Walter S.
“The right answer is you turn far enough to stay away, but you can use a handy math trick to get a heading that’s roughly what you want. One degree of turn will displace you one nautical mile for every 60 nautical miles you travel. So one degree of turn to the left will put you 1.3 nautical miles left or right of your current course in 80 miles.
You want to pass the storm with 20 miles to spare, so let’s say you want to displace about 25 miles to the left by the time you get there. If one degree of deviation will get you 1.3 miles left of a storm 80 miles away, a 10-degree turn will get you 13 miles left of course, and a 20-degrees will get you 26 miles. Therefore, turning left or right as looks better by 20 degrees is a good start.
If the storm isn’t moving much, or it’s moving away from you, that’ll do the trick. If the storm is moving toward you, you can either add five degrees or just reassess as you go. It’s usually bad form to turn in the same direction a storm is moving and try to pass in front—similar to the bad form of crossing the tracks ahead of an oncoming freight train. If you must, add lots of extra room.
This trick can also be used when approaching an airport perpendicular to the landing runway and Tower tells you to enter a left base directly. Suppose you’re 10 miles from the airport, the runway is a mile long, and you want that base leg to be about half a mile from the runway. Half the runway length is half a mile, plus the half-mile for your base, means you want to target a point about a mile from the airport center.
If you turn just one degree right of that GPS-direct heading that was taking you to the airport center, by the time you travel 10 miles you’ll be displaced about one-sixth of a mile from the airport center. So if you turn six degrees right, after 10 miles you’ll be displaced about one mile—perfect to enter a left base as instructed.”
Which do you rely on most to navigate around thunderstorms?