raekuuroja

Raekuuroja are hail shower events described in Finnish meteorology as brief, intense falls of hail that accompany convective storms. The term raekuuro denotes a hail shower, and raekuuroja is the plural form used to refer to multiple such events or to a storm producing hail.

Formation and characteristics: Raekuuroja form when strong updrafts within a developing thunderstorm suspend small ice particles and allow them to grow through repeated freezing and layering of supercooled water. When the updrafts weaken, the hailstones fall to the ground. Hail sizes during raekuuroja commonly range from small pellets to golf-ball-sized stones, with larger stones being relatively rare. The events are often accompanied by gusty winds, heavy precipitation, and occasionally thunder.

Occurrence and climatology: In Finland, raekuuroja most often occur in late spring and summer, during periods of heightened atmospheric instability and moisture. They are more frequent in regions with frequent convective activity and can be linked to passing low-pressure systems or cold fronts that trigger thunderstorm development. Weather radar and storm spotter networks help detect and track these events.

Impacts and safety: Raekuuroja can cause damage to crops, vehicles, roofs, and windows, and may create hazardous road conditions due to reduced visibility and ice on surfaces. Insurance claims and municipal warnings are common responses to notable events. Preparedness includes securing outdoor objects, protecting vehicles, and staying indoors during active storm cells.

See also: hailstorm, convective storm, weather radar.