It has a milky, pebbly appearance, and first shows up as a thin white line on wing leading edges or other airframe protuberances, such as outside air temperature probes and antennas. The icing process occurs when an airplane flies into clouds or precipitation composed of supercooled water droplets. Supercooled droplets are liquid, but at freezing temperatures. They remain liquid until an airplane flies into them. Then they quickly freeze on impact with the leading edges.
Rime ice is usually slower to build than clear ice.
The worst of the worst There are two additional types of icing that rank especially high on the danger scale. It's caused by rain, snow, or ice crystals falling through a warmer layer of air at lower altitudes. The big problem with freezing rain is that the very large droplets associated with this phenomenon run far back on airfoil surfaces and can quickly disrupt lift. The extra weight of the ice has a minimal effect on airplane performance. But as bad as freezing rain is, there's another phenomenon that's orders of magnitude worse.