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”.
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.