Seinäpilvi

Seinäpilvi is a Finnish term that translates to "wall cloud" in English. It refers to a distinct, localized, and often persistent lowering of the cloud base in the rain-free area of a thunderstorm. This lowering is typically found on the inflow side of a cumulonimbus cloud, often near the updraft base.

Wall clouds are a significant feature for meteorologists and storm spotters because they can be indicative of rotating updrafts within the thunderstorm, a phenomenon known as mesocyclone. The presence of rotation within a wall cloud increases the likelihood of tornado development. However, not all wall clouds rotate, and not all rotating wall clouds produce tornadoes.

The formation of a wall cloud is believed to be related to the strong updraft of the thunderstorm drawing in moist air from below. As this moist air rises and condenses, it can create the visual lowering effect. Evaporation of precipitation falling from higher levels of the storm can also contribute to the cooling and further condensation, enhancing the wall cloud's appearance.

Identifying wall clouds is an important part of severe weather observation and forecasting. Their development, changes in shape, and rotation are closely monitored to assess the potential for severe weather, particularly tornadoes. The absence of a wall cloud does not guarantee safety, but its presence, especially with observed rotation, is a strong warning sign.