Private Pilot Private Exam

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

1. 4,000 feet

2. 6,000 feet

3. 8,000 feet

4. 10,000 feet

The correct answer is: 6,000 feet

To determine the approximate base of the cumulus clouds, one can use the temperature and dewpoint values provided. The base of cumulus clouds is typically found where the air temperature cools to the dewpoint as it rises, which signifies the point of saturation and the formation of clouds. In this scenario, the surface temperature is 70ºF, and the dewpoint is 48ºF. The difference between these two temperatures, known as the temperature-dewpoint spread, is 22ºF. A general rule of thumb is that for every 1,000 feet of ascent, the temperature typically decreases by about 2ºF. To calculate the altitude where the temperature reaches the dewpoint, you can use the temperature spread. Dividing the temperature spread (22ºF) by the rate of temperature decrease (approximately 2ºF per 1,000 feet), you will find that the air needs to rise about 11,000 feet for the temperature to equal the dewpoint. Since the starting altitude is 1,000 feet MSL, adding those 11,000 feet gives an approximate cloud base at around 12,000 feet MSL. However, since cumulus clouds typically begin forming lower in the