Our Common Atmosphere: FREEZING RAIN
An ice coating on trees and bushes gives them a beauty fit for a fairy tale but, in reality, it is one of the most deadly of winter events. With accumulations of several centimetres, tree limbs may break, or power lines may come down, with the extra weight. All surfaces that we humans use become treacherously slippery. Freezing rain can be extremely dangerous to aircraft since the ice moulds itself to the shape of the airfoil. But how does this all happen?
Freezing rain occurs when precipitation in liquid form freezes on hitting any surface. For this to happen the rain water has to be super-cooled. Rain or snow aloft falls through a warm layer turning the snow into rain. As the rain falls, it has to pass through a very cold but shallow layer near the ground. The rain becomes super-cooled (cooled below its freezing point without freezing) at which point any stimulus (hitting a surface) when the surface is below zero degrees C causes instantaneous freezing.
Conditions have to be just right. If the cold layer near the ground is too deep, the rain will freeze and fall as ice pellets or sleet. If the cold layer is too shallow, it will just rain.
If the ice accumulates to a thickness of several centimetres, it is called glaze ice.
When windy conditions are present, the damage can be worse because tree branches not only have to support the extra weight of the ice but also resist the wind forces. The ice that forms on roadways makes vehicle travel dodgy because ice, unlike snow, provides no traction. In this situation, vehicles will slide even on gentle slopes. This ice is very difficult to remove because it conforms to the shape of the surface and is a continuous layer of ice.
Freezing rain on a large scale is known as an ice storm like the one that hit eastern Canada and parts of New York in January 1998 causing billions of dollars in damage or the one just prior to Christmas in southern Ontario in 2013.
Dr. Young has over 40 years of experience in weather and air quality studies and continues to provide services in these areas at RWDI. He welcomes feedback on his blogs as well as requests and suggestions for future topics. He can be reached at [email protected]. Please do not hesitate to contact him regarding forecasting of freezing rain.