Final answer:
The height of cumulonimbus cloud bases is most directly determined by the temperature and dewpoint spread. A smaller spread suggests lower cloud bases, while a larger spread indicates that clouds will have a higher base.
Step-by-step explanation:
The most important factor in determining the height of cumulonimbus cloud bases is b) The temperature and dewpoint spread. The spread between the temperature and the dewpoint indicates the level at which the air becomes saturated and clouds begin to form. When the surface air temperature is close to the dewpoint temperature, clouds are likely to have a lower base. Conversely, a larger spread suggests a higher base for cloud formation because the air needs to rise further and cool more before condensation and cloud formation occur.
Several factors influence the formation of cumulonimbus clouds, including unstable air, relative humidity, and the adiabatic lapse rate. However, the temperature and dewpoint spread provides a direct measure of when and where condensation will begin as air rises and cools at the adiabatic lapse rate, leading to cloud formation. This is crucial for predicting cloud base height and understanding potential weather developments such as storms and heavy precipitation often associated with cumulonimbus clouds.