What is the approximate base of cumulus clouds if the surface air temperature at 1,000 feet MSL is 70 degrees and the dewpoint is 48 degrees Fahrenheit?

• A. 6,000 feet MSL
• B. 4,000 feet MSL
• C. 8,000 feet MSL
• D. 10,000 feet MSL

Answer: (A)

Cloud base forms where rising air cools to its dew point, so the difference between the surface temperature and the dew point (dew point depression) tells you how far the air must rise to reach saturation. Here, the surface temperature is 70°F and the dew point is 48°F, giving a dew point depression of 22°F, which is about 12.2°C.

A common pilot rule of thumb is that roughly 400 feet of lift is needed for each degree Celsius of dew point spread. So the height to reach saturation above the surface is about 12.2 × 400 ≈ 4,900 feet.

Since the surface is at 1,000 feet MSL, add that to get an estimated cloud-base altitude of about 5,900 feet MSL, which rounds to 6,000 feet MSL.

So the approximate base of cumulus clouds is around 6,000 feet MSL. The other options would imply a smaller or larger dew point spread than we have, which doesn’t match the calculation.