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Freezing rain

earth science Maturity 5-7 Next article →

Freezing rain is a specific type of precipitation that occurs when liquid raindrops freeze immediately upon hitting a surface. Unlike sleet, which consists of solid ice pellets, freezing rain is made entirely of liquid droplets. These droplets remain in a liquid state even though they are at temperatures below the freezing point of 0 °C. This state is known as being supercooled. When these supercooled drops strike objects like trees, power lines, or roads, they undergo a process called nucleation. This process causes the liquid to turn into a smooth, transparent coating of ice known as glaze ice.

Ice Storm Chart
Ice Storm Chart

The formation of freezing rain requires a very specific arrangement of air temperatures in the atmosphere. It often begins with snow falling from higher altitudes. As the snow descends, it enters a layer of warm air located above the ground. This warm layer melts the snowflakes, turning them into liquid rain. As the rain continues to fall, it must pass through a shallow layer of subfreezing air near the Earth's surface. If this cold layer is deep, the rain might freeze into ice pellets before hitting the ground. However, if the layer is very shallow, the drops do not have enough time to freeze mid-air. They remain supercooled until they make contact with a surface below.

Ice Storm Chart
Ice Storm Chart

This weather pattern is frequently linked to the approach of a warm front. During these events, warm air is advected, or moved, into higher altitudes while cold air remains trapped near the surface. This can lead to a phenomenon called cold-air damming. In this setup, very cold and dry air is held in place by a high-pressure system. The warm air, often coming from the Gulf of Mexico, acts as the fuel for the melting process.

Freezing Rain skew-t
Freezing Rain skew-t

Meteorologists cannot see freezing rain directly on standard weather radar. Instead, they must use indirect methods to identify it. One method involves looking for a "brightband" on radar imagery. A brightband is a highly reflective area that shows where snow is melting into rain. By analyzing the height and the slope of this brightband, experts can estimate where melting is occurring. They can then use numerical prediction models to determine if that melting will result in freezing rain at the surface. Additionally, dual-polarized radar can help by sending both vertical and horizontal waves to deduce the shape of the precipitation.

Radar-bright-band
Radar-bright-band

When freezing rain accumulates in large quantities, it is classified as an ice storm. These storms can be incredibly destructive to infrastructure and nature. Glaze ice can reach a thickness of several centimeters. This weight is often too much for plants to support. Trees, especially pine trees with their ice-catching needles, may snap under the heavy load. This weight can also pull down power lines, causing widespread outages. For example, a massive ice storm in February 1994 caused over $1 billion in damage across the Southern United States. This storm impacted states like Mississippi, Tennessee, Alabama, and North Carolina.

Wingice
Wingice

Freezing rain poses an extreme hazard to aviation due to rapid structural icing. When ice builds up on an aircraft, it changes the shape of the airfoil. An airfoil is the curved surface of a wing that generates lift. As the shape changes, lift is reduced and drag is increased. This raises the stalling speed, making it difficult for the plane to maintain altitude. In 1994, American Eagle Flight 4184 crashed near Chicago after ice accumulated on its wings. To avoid this, pilots often try to climb into warmer air. However, because freezing rain is often accompanied by a temperature inversion, climbing can be a dangerous task.

Wingice
Wingice

Beyond its dangers, freezing rain can create unusual natural sights. In Michigan in 2019, observers found "ghost apples." These are icy shells left behind when freezing rain coats a rotting apple. Because the apple has a lower freezing point than water, the fruit inside may defrost and slip out while the ice shell remains. Similar shapes have been seen in other places, such as an ice shell left by a vehicle in North Carolina. These strange occurrences highlight how freezing rain can transform the physical world in unexpected ways.

Wingice
Wingice
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File:Ice Storm Chart.svg
Ice Storm Chart.svg
File:Freezing Rain skew-t.gif
Freezing Rain skew-t.gif
File:Radar-bright-band.png
Radar-bright-band.png
File:Wingice.JPG
Wingice.JPG
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