# Can hail and rain nucleate cloud droplets?

## TL;DR

This study demonstrates that falling hail and rain can promote cloud droplet formation through homogeneous nucleation, with experimental insights from a sulfur hexafluoride and helium mixture modeling atmospheric conditions.

## Contribution

The paper introduces an experimental setup simulating atmospheric nucleation processes using sulfur hexafluoride and helium near critical conditions, revealing how falling drops influence droplet formation.

## Key findings

- Homogeneous nucleation occurs due to isobaric cooling of sulfur hexafluoride vapor.
- Falling hail and rain drops can enhance heterogeneous nucleation in clouds.
- Stable horizontal cloud layers can form under certain conditions separating different saturation regions.

## Abstract

We present results from moist convection in a mixture of pressurized sulfur hexa-flouride (liquid and vapor) and helium (gas) to model the wet and dry components of the earth's atmosphere. To allow for homogeneous nucleation, we operate the experiment close to critical conditions. We report on the nucleation of microdroplets in the wake of large cold liquid drops falling through the supersaturated atmosphere and show that the homogeneous nucleation is caused by isobaric cooling of the saturated sulfur hexaflouride vapor. Our results carry over to atmospheric clouds: falling hail and cold rain drops may enhance the heterogeneous nucleation of microdroplets in their wake under supersaturated atmospheric conditions. We also observed that under appropriate conditions settling microdroplets form a rather stable horizontal cloud layer, which separates regions of super and sub critical saturation.