Airframe icing occurs because water freezes to a sub-zero degree airframe, and the subsequent ice build-up decreases lift, increases drag, and increases the aircraft weight, all of which are bad effects. This can occur because of supercooled water droplets in a cloud (droplets of water below 0ºC but which are still liquid) come into contact with the aircraft, or like in this question, where there is rain falling (perhaps from an inversion layer above) and the outside air temperature is negative, and therefore the airframe temperature is likely to be negative also, leading to rain ice.

High level clouds such as cirrus do not cause airframe icing, as they are so cold, they are made up solely of ice crystals. The worst icing occurs inside cumulonimbus clouds, where very large super-cooled water droplets can exist.