📐 Rule of thumb: cloud base (ft AGL) ≈ (T − Td) °C × 400

• 🌡️ Worked example: temp 5 °C, dewpoint 2 °C → spread 3 °C → cloud base ≈ 1 200 ft AGL
• 🛸 Part 107 needs 500 ft clearance below clouds. If cloud base < 900 ft AGL at your flight spot, you cannot fly.

METAR gives you T/Td as 02/M01 (M = minus). Compute the spread, multiply by 400.