Stratocumulus clouds can assume closed- or open-cell structures with strikingly different abilities to reflect solar radiation. While open cells have been linked to the presence of precipitation and low droplet concentrations, complete understanding of processes leading to their formation is lacking. Here we show that the structure of stratocumulus can be linked to two time scales: an updraft time scale (tup) and a rain initiation time scale (tr). When sufficient drizzle forms within updrafts (tr ≤ tup), cloud water in the outflow is depleted enough that an overcast cloud cannot be sustained. Using a simple parcel model, we relate these time scales to three observable parameters (droplet number concentration, cloud depth, and updraft speed) and derive a functional representation for the transition from closed to open cells. Eight well-documented observed and simulated cases of open- and closed-cell stratocumulus fit well into the classification based on our model.