Reunasade

Reunasade is a Finnish term that translates to "edge rain" or "margin rain." It describes a phenomenon where precipitation falls only on the outer edges of a cloud or storm system, leaving the center relatively dry. This can be a puzzling sight, as typically rain is associated with the densest parts of a cloud.

The formation of reunasade is often linked to specific atmospheric conditions. One common explanation involves downdrafts from precipitation that falls from the upper parts of a cloud. As these downdrafts reach the ground, they can spread outwards, creating a ring of moisture that evaporates before reaching the center. Another factor can be the interaction of wind patterns with the cloud. If the wind is strong enough to push precipitation towards the periphery of the cloud, reunasade can occur.

This type of rainfall can be particularly noticeable in situations where a larger storm system is breaking apart or weakening. The outer edges, still containing sufficient moisture, may continue to produce rain while the core has dissipated or moved on. It can also happen in smaller, isolated cumulus clouds.

While not a common meteorological term outside of Finland, the visual of reunasade can be quite striking, often appearing as a halo of rain around a drier, or even sunny, center. It serves as a reminder of the complex and sometimes counter-intuitive ways atmospheric processes can manifest.