FAA Ground School Practice Test

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

1. 4,000 ft MSL

2. 5,000 ft MSL

3. 6,000 ft MSL

4. 7,000 ft MSL

The correct answer is: 6,000 ft MSL

To determine the approximate base of cumulus clouds, it is essential to understand the relationship between air temperature, dew point, and altitude. The base of cumulus clouds is typically found at the altitude where the temperature of the rising air cools to the dew point, leading to condensation. Using the given temperature of 70 degrees F at 1,000 feet Mean Sea Level (MSL), we can apply the general rule that for every 1,000 feet the air rises, the temperature decreases by approximately 3.5 degrees F until reaching the moisture level where saturation occurs (which is represented by the dew point). First, we need to find the temperature difference between the air temperature and the dew point. The difference is: 70 degrees F (air temperature) - 48 degrees F (dew point) = 22 degrees F. To determine the altitude of the cloud base, we can then calculate how high the air needs to rise to cool from 70 degrees F to 48 degrees F. Since the air temperature drops about 3.5 degrees F per 1,000 feet, we find: 22 degrees F / 3.5 degrees F per 1,000 feet = approximately 6.29, which suggests