By Rod Hill on 2013-01-23
With talk of freezing rain in some spots today, especially the Columbia River Gorge, I want to clarify the differences between frozen precipitation types.
Freezing Rain: Simply liquid rain in the air that freezes 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. Freezing rain is the classic ice-storm. Just more than a trace of freezing rain can coat roadways and sidewalks with a sheet of ice making driving and walking impossible. Forecast models give meteorologist 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: A warmer layer of air in the atmosphere melts snowflakes into raindrops, after which the rain then falls into a freezing layer, turning the raindrops into ice balls or pellets. The frozen pellets then fall to the ground. Sleet, although formed differently does appear similar to hail stones to the casual observer. Sleet can fall onto a frozen or above freezing ground surface. Heavy sleet falling onto frozen roadways and lawns can coat the surface with thick ice, also making driving nearly impossible.
Hail: As mentioned above, hail appears at ground level much the same as sleet. The difference between sleet and hail is how the two are 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 where the droplets freeze into icy balls known as hail stones. The hail stones then fall from the top of the cloud to the bottom and back into above freezing temperatures where the icy stones are coated with liquid then 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. 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 soft balls! Here in the Northwest, hail is most common during the fall, winter and spring months when cold upper level lows, lower the freezing level and cause convective activity.
Oregon Snowpack Near Normal 