If the air temperature is 18°C and the dew point is 6°C, what is the approximate cloud base in feet AGL?

If the air temperature is 18°C and the dew point is 6°C, what is the approximate cloud base in feet AGL?

• A. 4,800 feet
• B. 5,200 feet
• C. 4,000 feet
• D. 4,500 feet

Answer: (A)

Think about how dew point spread controls cloud formation. The cloud base rises as the gap between air temperature and dew point grows, because you have to lift air higher to cool it enough to reach saturation. A handy rule of thumb is that cloud base height in feet AGL is about 400 feet for every 1°C of temperature–dew point spread. Here, the air temperature is 18°C and the dew point is 6°C, so the spread is 12°C. Multiply 12 by 400 to get about 4,800 feet AGL. So the cloud base is around 4,800 ft above the ground. The other options would come from spreads that don’t match the given 12°C difference.

Think about how dew point spread controls cloud formation. The cloud base rises as the gap between air temperature and dew point grows, because you have to lift air higher to cool it enough to reach saturation. A handy rule of thumb is that cloud base height in feet AGL is about 400 feet for every 1°C of temperature–dew point spread.

Here, the air temperature is 18°C and the dew point is 6°C, so the spread is 12°C. Multiply 12 by 400 to get about 4,800 feet AGL. So the cloud base is around 4,800 ft above the ground. The other options would come from spreads that don’t match the given 12°C difference.