Which metric is used to differentiate between "large-scale" and "small-scale" weather systems?

The Rossby number is the correct metric used to differentiate between "large-scale" and "small-scale" weather systems. This dimensionless number is a measure of the ratio of inertial forces to Coriolis forces within a fluid, in this case, the atmosphere. In meteorology, a low Rossby number indicates that the Coriolis force is a significant factor, which is typical for large-scale weather phenomena like cyclones and anticyclones. Conversely, a high Rossby number implies that inertial forces dominate, which is characteristic of small-scale weather systems such as thunderstorms or local wind patterns.

This concept is crucial in understanding the dynamics of atmospheric motions since large-scale systems tend to be influenced more by global circulation patterns, while small-scale systems are more affected by local conditions. Other metrics like barometric pressure, wind speed, or humidity index play important roles in weather analysis, but they do not provide the same framework for distinguishing between the scales of weather systems as the Rossby number does.