Freezing rain is a dangerous winter precipitation type given that only small amounts of it can produce enormous problems and lots of it can produce catastrophic problems. Freezing rain is rain that freezes on the ground surface very shortly after reaching the surface. Typically freezing rain will be reported even if only on some surfaces the rain is freezing. For example, the rain could be freezing on the grass and trees and not freezing on concrete yet freezing rain will still be the report. Freezing rain does not have to freeze instantaneously on the surface it strikes. The rain can flow on the surface before freezing such as when icicles form. The more below freezing the ground surface is, the faster and on more surfaces the freezing rain will accumulate. For example, a freezing rain at 20 F can result in a freezing on all surfaces and a quick freezing while a freezing rain at 32 F can result in only vegetation freezing and a slower freezing of the water. Some surfaces may not have rain freeze on them even when the outside temperature is well below freezing. This can happen for example in the day where some radiation makes it ways through the clouds and warms a dark colored roof. Places in the shade will freeze much easier since much less radiation is striking them. Bridges and overpasses are generally the first places to freeze in a freezing rain event especially if the temperature is not much below freezing outside. This is because bridges and overpasses are cooled from above and below. The soil can hold in heat such as cases when it was warm before the weather changed to produce freezing rain. The air temperature may have to drop well below freezing to make up for the soil and concrete being relatively warm. A heavily trafficked road may have the ice melted from the car tire friction and exhaust from the cars. The more lightly trafficked roads that include bridges and overpasses can be most problematic during a freezing rain event. A light accumulation of freezing rain can cause numerous accidents. Just a few hundredths of an inch of freezing rain/drizzle can paralyze a city, especially if that city has infrequent winter weather. Ice is extremely heavy in large amounts. A steady freezing rain over a vegetated region can paralyze the vegetation. Branches and leaves will fall. There is a very distinct cracking sound when branches break during a freezing rain event. It is a distinct and eerie sound when this type of damage is occurring. One of the most frustrating impacts of freezing rain events is the loss of electricity. The cold temperatures along with the loss of power makes living conditions extremely uncomfortable and dangerous if it is an extended power outage.  The diagram below shows the typical temperature profile that produces freezing rain. The vertical dashed line through the center is the freezing point of 0 C. To the right of this line the temperature profile is above freezing and to the left of this line the temperature profile is below freezing. The precipitation aloft starts as snow. This snow then falls through an above freezing layer and completely melts into rain. The above freezing layer is thick enough and warm enough to completely melt the snow. No ice nuclei are left in the falling rain. Thus, when this precipitation falls through a below freezing level near the surface the precipitation will stay liquid even if the precipitation drops below freezing. The precipitation stays a liquid while falling even when the temperature drops below freezing since there are no nuclei for the liquid water to freeze upon to produce ice. This below freezing liquid precipitation is called supercooled water. This supercooled water then reaches the ground surface. Once it reaches the ground surface then each raindrop has the ground surface to freeze upon. The precipitation can also be above freezing as it reaches the ground yet the ground is still below freezing. This can produce a freezing rain that takes longer to freeze once it reaches the surface. Also, it could be the case that the ground surface is above freezing due to warm weather before the freezing rain event moved it. In these cases there can be freezing rain on elevated objects such as in trees while the rain does not freeze on the above freezing ground surface.  |