Appearance in Satellite Data

Polar Lows are small-scale cyclones in the arctic or polar air mass with some similarity to tropical hurricanes. They occur during the wintertime (October until April) far north of the polar front. Because of their northern position, their small scale and their appearance in the dark winter season, AVHHR-NOAA IR images are often the best way to detect Polar Lows. At latitudes lower than 70 degrees north, Meteosat IR and WV images are also useful for detecting such systems.
This Conceptual Model is based on existing literature and the investigations of 25 cases, most of them in the winter seasons of 1999-2000 and 2000-2001.
Three different phases in the life cycle of a Polar Low can be distinguished: the initial/development phase, the mature phase and the decaying phase.

  • A developing Polar Low has the strucutre of a cyclonic curl in a polar or arctic airmass far away from the main polar frontal zone. Upstream of the cloud band, Cold Air Cloudiness (CAC) with relatively low tops (dark in IR) is present. The inner part of the curl shows sharp cloud edges while the outer part is often capped by cirrus cloud.
  • The cloud structure of a mature Polar Low is often a pronounced vortex with a (partly) cloud free eye. This vortex consists of Cbs. The deepest Cbs with the brightest cloud tops in the IR images are located around the eye.
    A cirrus shield often partly covers small-scale Polar Lows.
    Clouds further away from the centre tend to be less bright which means lower cloud tops, caused by descending air motion.
  • A decaying Polar Low is indicated by the disappearance of the cirrus cloud and the eye feature, lower cloud tops (less bright in IR images) and the disruption of the vortex. Most Polar Lows will decay as a result of landfall.

Developing Phase
Mature Phase
Decaying Phase

22 February 2001/21.00 UTC - Meteosat IR image; Developing Polar Low near the Faroe Islands; 22/21.00 UTC - 23/12.00 UTC 3-hourly image loop; developing - mature phase
23 February 2001/09.00 UTC - Meteosat IR image; Mature Polar Low south east of Scotland
22 February 2001/21.00 UTC - Meteosat WV image; Developing Polar Low near the Faroe Islands; 22/21.00 UTC - 23/12.00 UTC 3-hourly image loop; developing - mature phase
23 February 2001/09.00 UTC - Meteosat WV image; Mature Polar Low south east of Scotland

If the scale of a Polar Low is not too small and the position is not too far to the north, it is even possible to trace Polar Lows with Meteosat images. In the images above, a Polar Low is developing just east of the Faroe Islands. The first IR images shows the development of a Polar Low in a band of cloudiness while a developed vortex is already present in the water vapour image. In the morning of 23 February the Polar Low reaches its mature phase over Scotland. The IR image shows a lot of Cbs encircling the center. In the water vapour image the vortex is still present.

Appearance in AVHRR imagery

As Polar Lows are phenomena which frequently occur in the dark winter season, the use of combined VIS-IR images is normally not possible, however, single channel NOAA IR images (e.g. Ch 4) are most appropriate for the detection of Polar Lows.

26 February 1987/04.28 UTC - NOAA CH4 image; Developing polar low north of Norway
27 February 1987/04.18 UTC - NOAA CH4 image; Mature polar low north of Norway
27 February 1987/12.32 UTC - NOAA CH4 image; Decaying polar low over Norway

The satellite images above show the three phases in the life cycles of a typical Polar Low. The development starts on an old occluded front extending from northwest Finland to the Southeast of Svalbard. A clear curl develops at the most northern part of the cloud band. The inner part of the curl shows sharp cloud edges while the outer part is blurred. West and north of the cloud band, Cold Air Cloudiness (CAC) is present.
A few hours later a mature Polar Low is positioned just north of Norway with a clear eye and numerous Cbs encircling the center. An area of clouds with low tops, as a result of descending air, surrounds these Cbs.
Finally, having passed across the north coast of Norway, the Polar Low starts to lose its well-organized structure. In the last satellite image, only some unstructured Enhanced Cumuli are left over north Norway and Sweden.