lämminrintaman

Lämminrintama, or warm front, is the boundary where a warmer air mass advances over a cooler air mass. In synoptic meteorology it forms when warm air glides over the cooler air, creating a broad, gradual transition zone between the two masses. The front has a relatively gentle slope, often around 1:200 to 1:400, which leads to slow ascent of the cooler air ahead of the front rather than abrupt lifting.

As the warm front approaches, high clouds such as cirrus and cirrostratus may appear, followed by mid-level altostratus and lower stratiform clouds. Precipitation associated with a warm front tends to be widespread, light to moderate, and longer lasting, often occurring ahead of the surface front. In mature systems, the rain can persist for hours to days as the front moves slowly.

Temperature and humidity typically rise as the front passes. The dew point increases, and the air behind the front remains warm and humid. Winds usually shift from easterly or southeasterly ahead of the front to southerly or southwesterly behind it. Surface pressure often falls as the warm front approaches and may rise slightly after its passage.

Warm fronts are a key feature of mid-latitude cyclones and the warm sector. Their movement brings extended periods of cloudiness and steady precipitation, followed by warmer, more humid air. Forecasting the timing and impact of a warm front relies on tracking temperature, dew point, cloud development, wind shifts, and pressure changes.