Titan U glossary

Freezing Rain

Freezing rain is rain that falls as liquid, often as supercooled drops below 32°F (0°C), and freezes on contact with the ground, trees, power lines and other exposed objects, leaving a coating of ice. It usually forms when a layer of above-freezing air aloft fully melts falling snow, and the drops then fall through a shallow layer of below-freezing air at the surface.

Storm ForecastingStorms in the Field Winter Precipitation

Freezing rain is rain that falls as liquid, often as supercooled drops below 32°F (0°C), and freezes on contact with the ground, trees, power lines and other exposed objects, leaving a coating of ice. It usually forms when a layer of above-freezing air aloft fully melts falling snow, and the drops then fall through a shallow layer of below-freezing air at the surface.

How it forms

Snow forms in cold air aloft, falls into a warm layer and melts completely into raindrops. The drops then fall through a below-freezing layer near the ground that is too thin to refreeze them before they land.

Supercooled water can stay liquid below 32°F until it strikes something, then it freezes on that surface.

Freezing drizzle is drizzle that freezes on impact in the same way. Sleet is different: the drops refreeze into pellets before they reach the ground, so sleet bounces rather than coating surfaces.

How deep the warm layer has to be

As a rough guide from the National Weather Service (NWS) office in Louisville, a melting layer more than about 1,200 feet deep usually melts snow completely. Weather Prediction Center guidance adds that if the warm layer's wet-bulb temperature tops about 37 to 39°F (3 to 4°C), snow melts completely and falls as rain or freezing rain.

Large flakes or a barely-above-freezing warm layer may need a deeper layer to melt fully, so these are guides, not switches.

How the Weather Service handles it

Local NWS offices issue an Ice Storm Warning when ice is expected to meet locally set criteria, usually around 1/4 inch or more. As an example, the Baltimore/Washington office issues a Winter Weather Advisory for any amount of freezing rain.

Where the picture gets uncertain

Not every surface ices the same way. Water tends to run to the underside of a branch before it freezes, and if the ground is warmer than freezing while the air is colder, streets may stay wet while trees and lines ice.

Freezing rain also depends on a shallow cold layer, so small errors in surface temperature can separate an ice event from a cold rain.

Keep going

How Freezing Rain connects

Builds on 1

  • Warm Nose

    Freezing rain usually builds on a warm nose, an above-freezing layer aloft that fully melts snow before the drops fall into a thin cold layer near the ground.

Leads to 2

  • Ice Storm Warning

    An Ice Storm Warning is issued when freezing rain is expected to build up enough ice to meet the local office's warning criteria.

  • Winter Weather Advisory

    Freezing rain is one of the hazards a Winter Weather Advisory can cover, and some offices issue one for any amount of it.

Part of 1

  • Precipitation Type

    Freezing rain is one of the precipitation types forecasters sort winter precipitation into, along with rain, snow and sleet.

Contrasts with 4

  • Sleet

    Freezing rain reaches the ground as liquid and glazes surfaces, while sleet refreezes into pellets on the way down and bounces instead of coating.

  • Flash Freeze

    Freezing rain freezes as it lands, while a flash freeze ices water already on the ground after a fast temperature drop, even with no precipitation falling.

  • Freezing Drizzle

    Freezing drizzle can fall from a column that is below freezing at every level, while classic freezing rain needs an above-freezing layer aloft to melt snow.

  • Black Ice

    Black ice is a road surface condition, not a precipitation type, though freezing rain and freezing drizzle can coat untreated pavement and create it directly.

Measured by 1

  • Ice Accumulation

    The impact of freezing rain is measured by ice accumulation, the thickness of ice that builds on exposed surfaces like branches and power lines.

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