Yöpakkaset

Yöpakkaset, or night frosts, refer to periods when nocturnal air temperatures drop below freezing, causing frost to form on exposed surfaces. In Finland the term is commonly used to describe frosts that occur in autumn and spring, although frost can appear at other times during clear, calm nights.

The phenomenon is primarily caused by radiative cooling. On clear skies and with light or no wind, heat radiates away from the ground into space, allowing surface temperatures to fall below 0°C even if the air a few meters above remains slightly above freezing. Frost typically forms first on plant surfaces, vehicles, windows and other exposed objects. Humidity and surface conditions influence the amount of frost; wind tends to reduce frost formation by mixing the air and limiting rapid cooling.

Frost can occur as surface frost on plants and objects or as air frost measured higher in the atmosphere. Meteorological services monitor temperatures and issue frost warnings when conditions are favorable. Frost timing varies with latitude and season, and can be affected by local microclimates such as valleys, water bodies, and urban heat islands.

Impacts include potential damage to crops and ornamental plants, reduced growth of sensitive vegetation, and hazards for driving when roads, windshields or sidewalks frost over. Protective measures in horticulture and agriculture include coverings, moving vulnerable plants indoors, or using frost protection strategies. In everyday life, people may prepare by warming cars and ensuring outdoor exposures are minimized.

Yöpakkaset are a common feature of Nordic temperate climates, and their timing and frequency can be influenced by broader climate patterns and gradual climate change.