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How does freezing rain form?

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Quick answer

Freezing rain forms when snow melts into rain in a warm layer of air, then falls through a shallow subfreezing layer near the ground. The raindrops become supercooled and remain liquid until they strike a surface below 0°C, where they freeze instantly into a glaze of ice.

Freezing rain forms when snow falls through a warm layer of air and melts into raindrops, then falls through a shallow subfreezing layer near the ground without refreezing. The drops become supercooled and freeze on contact with surfaces below 0°C, creating a glaze of ice.

Step by step

  1. Snow forms in a cloud where the air temperature is below freezing.
  2. The snow falls through a layer of air that is above 0°C and melts completely into raindrops.
  3. The raindrops then enter a shallow layer of subfreezing air near the ground.
  4. The drops cool below 0°C but remain liquid because they lack freezing nuclei and fall too quickly to freeze.
  5. When the supercooled drops hit a surface at or below 0°C, they freeze on contact, forming a smooth ice glaze.

Key things to understand

  1. Snow forms high in a cloud where temperatures are well below freezing.
  2. As the snow falls, it passes through a layer of air warmer than 0°C and melts into raindrops.
  3. Below that warm layer, a shallow layer of subfreezing air near the ground cools the raindrops below 0°C.
  4. The drops remain liquid because they lack freezing nuclei and pass through the cold layer too quickly to freeze.
  5. When these supercooled drops hit a surface that is at or below 0°C, they freeze instantly, forming a smooth glaze of ice.

Labelled diagram

  1. Snow

    Forms in cold cloud tops where temperatures are below freezing.

  2. Warm layer

    Snow falls through air above 0°C and melts into raindrops.

  3. Cold layer

    Rain falls through shallow subfreezing air and becomes supercooled.

  4. Surface

    Supercooled drops freeze on contact with cold ground, trees, or wires.

How freezing rain forms

Real-world examples

  • The 1998 Ice Storm in eastern Canada and the northeastern United States coated trees and power lines with up to 100 mm of ice, causing widespread outages.
  • In January 2020, freezing rain in Newfoundland, Canada, created treacherous roads and knocked out power to thousands.
  • Freezing rain often occurs in the Pacific Northwest, where cold air from the Fraser Valley meets warm, moist air from the Pacific.
  • Aircraft can encounter freezing rain while flying through supercooled clouds, leading to ice buildup on wings and requiring de-icing before takeoff.

Common mistake

✗ Myth: Freezing rain is just rain that freezes as it falls.

✓ Fact: Freezing rain remains liquid even below 0°C and only freezes when it hits a surface. If it froze while falling, it would be sleet or hail.

People also ask

What causes freezing rain?
Freezing rain is caused by a temperature inversion: snow melts in a warm layer aloft, then falls through a shallow subfreezing layer near the ground. The raindrops become supercooled and freeze on contact with cold surfaces.
How does freezing rain differ from sleet?
Sleet forms when raindrops refreeze into ice pellets before reaching the ground, while freezing rain stays liquid until it hits a surface. Sleet bounces, freezing rain coats surfaces in ice.
Why is freezing rain more common in some regions?
Freezing rain is more common in valleys and regions where cold air gets trapped near the ground, such as the Pacific Northwest or the St. Lawrence River Valley. These areas often have the right mix of warm and cold layers.
What is the difference between freezing rain and sleet?
Sleet forms when snow melts into rain and then refreezes into ice pellets before reaching the ground. Freezing rain stays liquid until it hits a surface, where it freezes into a clear glaze.
Why is freezing rain dangerous?
Freezing rain coats roads, trees, and power lines with a heavy layer of ice. This can cause car accidents, broken branches, and widespread power outages.
Can freezing rain occur at temperatures above freezing?
Yes, if the surface temperature is slightly above 0°C, rain can still freeze on contact if the surface itself is below freezing, such as a cold road or bridge. However, typically the air temperature is at or below 0°C.
How does freezing rain affect aircraft?
Freezing rain can cause ice to build up on aircraft wings and surfaces, disrupting airflow and adding weight. This is why planes are de-iced before takeoff in icy conditions.

Part of the guide: Water & Weather Phenomena →