Estimate cloud base in feet AGL when the ambient temperature is 22°C and the dew point is 12°C.

Estimate cloud base in feet AGL when the ambient temperature is 22°C and the dew point is 12°C.

• A. 3,000 feet
• B. 4,000 feet
• C. 5,000 feet
• D. 4,500 feet

Answer: (B)

The main idea is that cloud base is tied to how much the air must cool to reach saturation, which happens at the dew point. The difference between the air temperature and the dew point—the dew-point spread—tells you how much cooling is needed. A handy aviation rule of thumb uses about 400 feet of altitude per 1°C of this spread. Here, the temperature is 22°C and the dew point is 12°C, so the spread is 10°C. Multiply by 400 feet per degree: 10 × 400 = 4,000 feet. So the cloud base is roughly 4,000 feet above the ground. This is an estimate, and real conditions can shift it a bit, but it’s the most reliable quick calculation for practice questions.

The main idea is that cloud base is tied to how much the air must cool to reach saturation, which happens at the dew point. The difference between the air temperature and the dew point—the dew-point spread—tells you how much cooling is needed. A handy aviation rule of thumb uses about 400 feet of altitude per 1°C of this spread.

Here, the temperature is 22°C and the dew point is 12°C, so the spread is 10°C. Multiply by 400 feet per degree: 10 × 400 = 4,000 feet. So the cloud base is roughly 4,000 feet above the ground. This is an estimate, and real conditions can shift it a bit, but it’s the most reliable quick calculation for practice questions.