Freezing rain will hit some spots Wednesday, especially the Columbia River Gorge. Here's an explanation of freezing rain and how it differs from other types of cold weather precipitation:
Freezing Rain is simply liquid rain in the air that turns into a glaze of ice when the drops make contact with freezing objects on the ground. This is the precipitation type that can cause the most damage, coating trees and power lines with ice until the weight brings them crashing down (pictured). Freezing rain creates the classic ice storm; just over a trace of freezing rain can coat roadways and sidewalks with a sheet of ice, making driving and walking nearly impossible. Forecast models give meteorologists a temperature profile showing all warm air just above the surface. Under such conditions, when ground temps are 32 degrees or less, freezing rain will be in the forecast. The worst ice storms usually occur with ground temperatures in the 20s.
Sleet occurs when a warmer layer of air in the atmosphere melts snowflakes into raindrops, after which the rain falls into a freezing layer and turns the raindrops into ice balls or pellets. The frozen pellets then fall to the ground. Sleet, although formed differently, does look like hail stones to the casual observer. Sleet can fall onto a frozen or above-freezing ground surface. Heavy sleet falling onto frozen roadways can coat the surface with thick ice, also making driving difficult.
Hail appears at ground level much the same as sleet. The difference between the two is how they're formed. Hail is formed through convective weather, and for much of the country is most common in severe spring and summer thunderstorms. Liquid droplets in a cumulus cloud formation are carried high in the atmosphere, into freezing temperatures that cause the droplets to become icy balls known as hail stones. The hail stones fall from the top of the cloud to the bottom and back into above-freezing temperatures. They're then coated with liquid and carried by updraft winds back to the top of the cloud into freezing temperatures. The new liquid coating freezes into ice, making the hail stone larger. And the process repeats until the hail stone is too heavy to be supported by updraft winds and falls to the ground. Single hail stones can be as small as peas and as large as softballs. In the Northwest, hail is most common during the fall, winter and spring months when lower freezing levels cause convective activity.
KGW Meteorologist Rod Hill