The Part 107 exam sometimes gives you a temperature and dewpoint from a METAR and asks you to estimate where the cloud base will form. There is a simple formula for this and once you know it, these questions are free points. Here is everything you need.
Why Cloud Base Matters for Drone Pilots
Part 107 requires you to stay at least 500 feet below clouds at all times. If you know where the cloud base is, you know your usable ceiling. Fly above that ceiling and you are in violation. The cloud base calculation is how you figure out that ceiling from the numbers in a METAR when there is no ceiling already reported, or when you want to predict where clouds will form as conditions change.
The Formula
Cloud base forms at the altitude where the temperature and dewpoint converge. As you go up in altitude, temperature drops faster than dewpoint does. They meet at the cloud base. The standard approximation used in aviation is:
Temperature and dewpoint are both in degrees Fahrenheit here. The number 4.4 comes from the fact that temperature drops about 4.4°F per 1,000 feet of altitude while dewpoint drops only about 1°F per 1,000 feet, so they converge at a rate of roughly 4.4°F per 1,000 feet of climb.
Some versions of this formula use Celsius with a slightly different constant. The exam typically uses Fahrenheit but the concept is the same either way. The key is knowing the formula and being able to apply it quickly.
Two Worked Examples
Example 1 — Fine day, high cloud base:
A METAR reports temperature 77°F and dewpoint 50°F.
Cloud base at 6,100 feet AGL. You can fly at 400 feet and have over 5,600 feet of clearance below the clouds. No issue at all.
Example 2 — Muggy day, low cloud base:
A METAR reports temperature 68°F and dewpoint 60°F.
Cloud base at 1,800 feet AGL. You can still fly at 400 feet — that gives you 1,400 feet of clearance below the clouds, which exceeds the 500-foot minimum. But the sky is going to look heavy and overcast and conditions could worsen as the day goes on.
The Celsius Version
If the METAR gives temperature and dewpoint in Celsius (which is standard — METARs always report in Celsius), convert to Fahrenheit first or use the Celsius formula:
Example using Celsius:
METAR reports temperature 20°C and dewpoint 12°C.
Two Ways the Exam Uses This
The exam approaches cloud base in two ways. The first is giving you a temperature and dewpoint and asking you to calculate the estimated cloud base. Run the formula and pick the closest answer.
The second is giving you a cloud base already reported in a METAR — like BKN018 — and asking whether you can legally fly at a given altitude. That one does not require the formula at all. BKN018 means broken clouds at 1,800 feet AGL. Subtract 500 feet and your usable ceiling is 1,300 feet. Since 400 feet is well below that, you can fly. The formula only comes into play when clouds are not already reported and you need to predict where they will form.
Quick Reference
| Temp/Dewpoint Spread | Estimated Cloud Base | Flyable at 400 ft? |
|---|---|---|
| 2°F (very close) | ~450 ft AGL | No — less than 500 ft clearance |
| 4°F | ~900 ft AGL | Yes — but barely, fog likely |
| 9°F | ~2,000 ft AGL | Yes |
| 18°F | ~4,100 ft AGL | Yes, comfortably |
| 27°F (wide spread) | ~6,100 ft AGL | Yes, no concern |
Weather math is easier with practice
FAA 107 Prep has practice questions covering cloud base calculations, METARs, and every other weather topic on the Part 107 exam.