With a wintry mix expected to impact the Portland area this week, there is a good chance that we'll see some ice. Freezing rain, sleet and snow are all bad for traffic conditions on roads and sidewalks, but they also weigh down tree branches and power lines, making them vulnerable to breakage. A significant accumulation of ice lasting several hours is called an ice storm.
To help us understand what is the difference between sleet and freezing rain, we turned to the National Weather Service, which provided this graphic with a simple explanation and a nifty diagram. The bottom line is that snow and sleet form differently than rain and freezing rain, but the main reason has to do with the temperature of the column of air the precipitation falls through.
Freezing rain forms when snowflakes fall into a layer of air above freezing and melt, then fall into another thin layer of freezing air closer to the ground where they freeze again on contact. This creates a glaze of ice on the ground and anything else in its path. It's the kind of ice that can easily cover the road surface, making it dangerous for drivers to travel on. It can also build up on trees and power lines causing them to break and knock out the electricity.
Sleet, on the other hand, starts out as snow well above the ground in the clouds. Then, as the snow melts and falls through a layer of water above freezing into a layer of water below freezing closer to the ground it refreezes and drops to the ground as little ice pellets. It can be a nuisance, but it is not nearly as treacherous as freezing rain.