First satellite image of a polar low

For some historical background, here’s what is probably the first satellite image of a polar low.  It was taken from the NASA Nimbus 3 satellite on 5 January 1970. The clouds associated with the polar low can be clearly seen to the north-west of Scotland.


Up to this point observational studies had been based on other sources of data such as weather balloons. This polar low was the subject of a case study in Lyall (1972), who used this image as part of the study. At this time there was uncertainty over whether polar lows were driven directly by heat transfer from the ocean (i.e. a mechanism similar to hurricanes) or mainly from rebalancing of atmospheric horizontal temperature gradients (i.e. a smaller version of the majority of mid-latitude cyclones). This picture suggested the latter. However, as more polar lows were studied and observed from satellite imagery it was found that some appear more similar in structure to hurricanes.

Lyall, IT (1972). The polar low over Britain. Weather, 27, 378-390.


Polar low and von Karman vortices

Check out the incredible detail in this image, downloaded from NASA’s MODIS Rapid Response System. The island at the top is Spitsbergen, the biggest island in the Svalbard achipelago. South of it there’s a polar low. It’s also cool to see the wake to the west of Svalbard. There’s probably very weak winds in that area, but there’s a strong tip jet around Spitsbergen’s south cape. I sailed through this jet when I was onboard a coast guard icebreaker in 2008. Standing on the bridge, I could see that the wind speed increased rapidly from 22 knots to 44 knots inside the narrow jet, and then it dropped down to 22 again as we sailed out of it.

Near the bottom of the image, the small island of Jan Mayen is seen, and downstream of it, some amazing von Kármán vortices are very evident. I zoomed in on these vortices here:

Cloud streets

Although there are no polar lows in this picture, I thought it was too nice not to share. It was captured by NASA’s Terra satellite and shows cloud streets off the coast of the eastern US [click here for their article]. These are cloud features that form during cold air outbreaks, when you get really cold air streaming off a cold continent (or sea ice) onto a warm ocean surface. Much in the same way that you get steam when you blow on hot soup, this leads to evaporation from the sea, and cloud formation. The most common cloud type during cold air outbreaks is Stratocumulus or Cumulus, but sometimes you get Cumulonimbus thunderstorms as well.

The clouds in the cloud streets are often called roll clouds. Here’s what Liu et al. (2004, Geophysical Research Letters) [read their paper here] had to say about them:

Convective roll clouds are one of the most common forms of shallow boundary layer convection. They typically develop during cold-air outbreaks that are characterized by large air-sea temperature and humidity differences and high surface wind speeds. These characteristics provide favorable conditions for the development of convection that is organized into long quasi two-dimensional rolls. This organization results in cloud streets or roll clouds oriented along the direction of the mean low-level wind. Typically, the roll clouds are observed to evolve into cellular convection as one moves downstream from the coastline or ice edge.

In NASA’s story you’ll find a high-resolution version of the same image. I’ve cropped the image to show some details:

Click the image for even more detail. What interests me here is that there is clearly a narrow band near the coast with no clouds. Liu et al. managed to capture that with their numerical model:

This means that “we” (not I) have a certain understanding of the physics behind roll clouds and cloud streets. And that’s important, because polar lows always form in cold air outbreaks.