Sumupilveä

Sumupilveä is a term used in Finnish meteorology to describe a haze-like atmospheric feature that blends characteristics of fog and low-altitude cloud. It refers to a state in which a dense near-surface layer of moisture coexists with a shallow cloud deck, producing a visible, misty veil that touches the ground while retaining cloud-like properties above.

Etymologically, sumupilveä combines sumu (fog) and pilvi (cloud), with the form sumupilveä representing the partitive case used in common descriptions and weather reports. The concept is used to distinguish this hybrid state from pure fog or standard stratiform cloud.

The phenomenon forms under specific conditions: moist air near the surface, light or calm winds, and radiative cooling that thickens the near-ground moisture, often in coastal or valley regions. A gentle inflow of humid air from above can feed a shallow cloud layer beneath or within the fog, leading to a layered structure. The result is a low, dense visibility-reducing layer overlain by a diffuse cloud deck that can remain connected to the ground at times.

Impacts include reduced visibility for transportation and outdoor work, along with potential changes in microclimate and local photography or tourism experiences. Measurement relies on standard meteorological tools such as visibility meters, ceilometers, and lidar to profile both the near-surface fog and the base of the overlying cloud.

Occurrences are most common in coastal and fjord landscapes with frequent moisture fluxes, and they are more likely to appear during transitional seasons when cooling nights meet humid daytime air.

See also: fog, cloud, radiation fog, stratiform cloud.