Moisture depth is the region of the troposphere in the vertical in which significant moisture exists. This can be taken to mean air that is relatively warm and at or near saturated. Air with high dewpoints really adds to the moisture since high dewpoints indicates a large amount of moisture in the air since the amount of moisture that can be in the air increases at an increasing rate with temperature. Even cool or cold air that is saturated or nearly saturated can be considered to have significant moisture when it comes to precipitation forecasting since lifting this air will generate clouds and precipitation. This precipitation though tends to be lighter and without thunderstorm development (i.e. winter precipitation)

For a thunderstorm situation, what is typically needed for significant moisture is relatively high dewpoints in the lower troposphere. The lower troposphere tends to have the warmest temperatures (from between about the surface and 700 mb) and thus this warm air has the capacity to contain significant amounts of moisture. The forecaster sees if moisture is abundant by noticing the depth of the moisture profile. For example, if high dewpoints extend from the surface of 1000 mb to 925 mb, then this would indicate fairly shallow moisture. If on the other hand relatively high dewpoints exist from the surface of 1000 mb to 700 mb, then this would indicate a deep layer of moisture.

The significance of deep moisture is that it has the capacity to generate very significant rainfall when this air is dynamically and convectively lifted. Shallow moisture can also generate significant precipitation if the moisture is converged into isolated storms. Deep moisture though tends to generate a heavy rainfall of greater areal coverage and a more persistent rainfall when lifting remains in place for a significant amount of time.

Instability release, dynamic lifting and the vertical moisture profile all play a role in thunderstorm development. When it comes to moisture, the forecaster wants to answer questions such as: how high are the surface dewpoints? How deep in the moisture? How long does the moisture rich environment remain in place? Are there lifting mechanisms present while the moisture is in place? Is the troposphere unstable? Answering these questions will help with the precipitation amount, duration and intensity forecast.