Description A polar low is a small, but fairly intense atmospheric low pressure system found in maritime regions, well north of the polar front. Its typical diameter is 100–500km and average life span is 18 hours. The polar low gives strong and rapidly changing winds and dense showers of snow or hail, and is generally more unpredictable than the larger and more common synoptic lows.

The average maximum wind speed is 46kt, which is a severe gale. 35-50% of the lows have storm force winds of 50kt or more, and the strongest recorded since 2000 had a wind speed of 70kt. The polar low is mostly found in the Norwegian and Barents Seas, with the majority being between 65N and 75N, from the 0 meridian to Novaya Zemlya. Other ‘hot spots’ are south of Iceland, southwest of Spitsbergen, in the Hudson Bay and in the northern Japan Sea. Some cases have also been seen in the North Sea. The season is from October to the end of May, with most polar lows occurring in the months of December to March. Typically 10 to 20 fully developed polar lows are seen in the Norwegian and Barents Seas during the season.

Formation
A polar low forms when unstable air in the lower atmosphere interacts with cold air above. A typical precursor to its development is cold Arctic air at low levels moving southwards over the Gulf Stream outside the Norwegian coast. Stability then decreases due to the heat from the sea and large convective shower clouds form. These have a strong vertical motion of air in the cloud and an influx of air under the base of the cloud. In areas of strong convection, the shower clouds organise in lines (troughs) with surface winds of 30–40kt associated with the influx. When the convection interacts with cold Arctic air at heights of 5–8km, the influx at the surface grows strong enough for a vortex to form, and a polar low is born.

History
Due to its violent and sudden nature, the polar low has probably been the cause of many losses at sea, although only one case with a fatal outcome is known since 2000. In the past, polar lows were extremely challenging to forecast. Their small size meant that they easily hid in the great expanses between observation points in the Arctic, and they did not have a sufficiently high visibility in the weather prediction models. Also, the physical processes were not well enough described in the models. This led to poor model performance and often erratic or absent indications in the numerical prognoses, as well as a general lack of confidence among forecasters. Subsequently, the lows were often omitted in forecasts to the public.

Later development
In recent years, the availability of satellite data as both images of cloud structures and wind data from the sea surface has greatly improved. Satellite data are now assimilated in the numerical models and, together with a finer resolution, this has led to a higher quality of short range forecasts.

Beyond 12 hours, there is still a lot left to be desired, and research is still strongly needed. Among Norwegian forecasters, a common methodology has been introduced, and work is continually taking place to improve understanding and awareness. Forecasts of polar lows are now routinely included in text forecasts of gale warnings, as well as in forecasts for aviation or maritime users.

All the data were collected by the Norwegian Meteorological Institute in Tromsø, except for the satellite images, which are courtesy of the NOAA.

Text: Gunnar Noer, Meteorologist,
Norwegian Meteorological Institute in Tromsø