Here’s a pretty one. A large mid-latitude cyclone has moved in from the south and is positioned to the east of Iceland. On the left (west) side of the large cyclone, cold air is streaming down from the north-west. You can see the “cloud streets”, as they’re called. This kind of structure always means that cold air is flowing out over the ocean surface. What happens is that tremendous amounts of heat are transferred from the ocean to the air, much as what happens when you blow on hot soup. The clouds that form in such cold air outbreaks are normally Stratocumuli and Cumuli, with some Cumulonimbi. These are the larger dots, where there are probably isolated thunderstorms and heavy precipitation (snow).
This PL has a very nice spiral-like structure. It would be interesting to know what influence Greenland had on this one. When you have northerly winds to the east of Greenland, you tend to get very cold air damming up along the sides of the steep topography. In this case, there was also cold air streaming out of the fjords; see the nice clouds mirroring the shape of the narrow fjords.
The region that this PL formed in is one of the PL hot spots in the North Atlantic. The water is quite warm (eehhm – relative to the air, at least) and you often get the cold, northerly flow that we see in the picture. You can see why people got interested in PLs – they’re beautiful!