Why there are no satellite images in northern latitudes during winter-time?

The reason is polar nights. The satellite data of the Tarkka service are mainly based on optical instruments that measure the amount of light reflected from a terrain target. Adequate lighting conditions are required to obtain observations. At the darkest time of the year, November to January, the amount of sunlight is too low at northern latitudes and no observations can be made with optical instruments. The amount of light is affected by the angle of the sun, which is low in winter. During February, the sun is already high enough and from then on, observations will be received from all over Finland.

Area detected by Sentinel-2 satellite in the last days of the winter months (2019-2020).

Optical instruments include, for example, Sentinel-2 MSI, Sentinel-3 OLCI and Landsat-8/9 OLI, which are shown in Tarkka. The exception is the Sea Surface Temperature data produced by Landsat-8/9 TIRS and Sentinel-3 SLSTR instruments. They do not require light to function and therefore produce observations throughout the year.