What is the most important factor in determining the height of cumulonimbus cloud bases?

The temperature and dew point spread is the most important factor in determining the height of cumulonimbus cloud bases because it directly influences the conditions necessary for cloud formation. Cumulonimbus clouds are often associated with thunderstorms and severe weather, and their formation requires moist, unstable air.

The base of these clouds forms when warm, moist air rises and cools, reaching its dew point temperature where condensation occurs. The greater the difference between the air temperature and the dew point (known as the temperature-dewpoint spread), the higher the base of the cumulonimbus will likely be. When this spread is large, it indicates that the air is relatively dry at lower levels, and significant lifting is needed for the air to cool to its dew point, thus forming cloud bases higher in the atmosphere.

In contrast, factors like unstable air, relative humidity, and the adiabatic lapse rate influence cloud development but are not specific indicators of cloud base height in the same direct manner as the temperature and dewpoint spread. Understanding the relationship between these elements helps in predicting weather patterns and the risks associated with cumulonimbus clouds, making the temperature and dew point spread key to forecasting.