Cold air outbreak in amazing detail

The current cold air outbreak, the first proper one this winter (see previous post), reveals incredible small-scale structure in the satellite image below. The land in the upper middle part of the picture is the Svalbard archipelago in the Northeast Atlantic.Image

The resolution of the original image (which you can see by clicking on the image above or go here) is 500 metres, which means that each pixel in the image is 500 metres wide. For maximum detail, check out this image at a resolution of 250 metres. There are so many things to see in the image, but here are some of the highlights.

In the upper left part of the picture, the white tendril-like features are sea ice. We also see some large floes. It’s all being blown southwards and at the same time broken up by the strong surface winds.

The north-south-oriented stripes that are made up of individual white dots are what we call cloud streets. They only occur when there’s a lot of convection (rising air) going on, and the dots themselves are cumulus clouds. Further south, away from the sea ice edge, the convection becomes less intense, and the cumulus clouds merge together into large patches of stratocumulus clouds. Cloud streets are a tell-tale sign of strong surface winds.

Downstream from Svalbard and in the lower part of the picture, the clouds get organized into a vortex. This is because there’s a small low-pressure system there, and the air starts to move in spiral-like, anti-clockwise patterns around the low (because of the earth’s rotation and the Coriolis force). The beautiful pattern we see in the image is, in other words, a polar low. Not a big one, but a polar low nonetheless.

Another interesting thing in the image are the wave clouds over and downstream from Svalbard. They form because the air moves over the mountains, much in the same way that waves form when water in a river hits rocks.

If you want to see more images from this region, take a look at NASA’s Hornsund subset.


First polar low this season

The satellite image on the left shows what I think must be the first polar low in the Northeast Atlantic this winter. At least it’s the first time snow is forecast for large parts of Norway, a sure sign that there’s a cold air outbreak in its way. The cold air outbreak itself is also clearly visible in the image, with its patchy or dotty structure. These are clouds that are organized into what we call “convective cells”.

A satellite image of the Northeast Atlantic, taken at 0940 on 23 October 2012. Downloaded from the Dundee Satellite Receiving Station in Scotland.

Convection is the meteorological term for rising air, and rising air is what creates all kinds of clouds. The convective cells in the picture are cumulus clouds which are formed because the cold air is heated from below by the (relatively) warm ocean surface, much the same as what happens when you blow on a bowl of hot soup. When air is heated, it gets lighter, and therefore it rises, letting colder air flow in from all sides to replace it.

So, the cold air outbreak in the picture sets up a lot of rising motion in the air masses. At the same time, there’s evaporation going on so that the air gets humid. And when humid air rises fast to create cumulus clouds, you get precipitation, in this case in the form of massive snow amounts forecast for Norway. Along the coast, people are going to witness tall cumulus and cumulonimbus clouds marching past them on their way south. Cold air outbreaks don’t always come as far south as Bergen, but I sometimes see these majestic clouds trotting along. It’s a very nice spectacle.