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How Freezing Rain, Freezing Drizzle, And Freezing Fog Form

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You might know the METAR abbreviations for freezing rain (FZRA), freezing drizzle (FZDZ), and even freezing fog (FZFG) but how are they different?

Freezing Drizzle & Freezing Rain: The Difference

At the surface, freezing drizzle might seem like just a smaller version of freezing rain. While this may be true in some ways, the process that forms each is different.

The simplest difference between freezing drizzle and rain is shown in the difference in their temperatures as you gain altitude.

Notice how freezing rain starts as snow aloft, then falls through an inversion layer which melts it, and then re-freezes as it passes through the cold layer of air near the surface.

But, looking closely at the temperature profiles you'll notice that the freezing layer on the drizzle stays below 0 degrees C the entire time.

Freezing drizzle relies on moist stratus clouds with weak updrafts to release small droplets as they become too heavy. Unlike freezing rain, freezing drizzle starts as SLD (supercooled large droplets) that coalesce and grow to a size large enough to fall from the clouds as drizzle.

Freezing Rain

According to the FAA, freezing rain is created when snow falls through a warm layer, then through a sub-zero layer of air - also known as a temperature inversion. These drops fall through the freezing layer so quickly that they don't have time to completely freeze before they get to the ground.

As the melted snowflakes fall through the below-freezing surface layer of air they become supercooled, creating a significant icing risk to aircraft. What is supercooling though?

Supercooling is a state where a liquid is below freezing but isn't a solid (in this case, ice), meaning that as the droplets fall through the atmosphere they can't crystalize. But when supercooled droplets impact a surface of your aircraft, they stick and freeze.

Why Are Supercooled Large Droplets So Dangerous?

Large drops, like you'd expect with freezing rain, can form a heavy glaze on your airframe (clear ice) that can be difficult to remove, especially if ice forms aft of de-icing equipment. Clear ice can also be hard to see because of its smooth, transparent appearance.

Inadvertent icing encounters with freezing rain can be deadly, the NTSB is still investigating a recent Cessna 210 crash in Lubbock, Texas where freezing rain is suspected to be a significant contributing factor.

Freezing Drizzle: Real-World Example

Recently in Boulder, northeast upslope winds kept a shallow moisture plume aloft. Pair this with the cold surface temperatures in Colorado, and freezing drizzle coated almost everything on the ground. Later in the day as the moist layer of air got thicker, ice crystals began forming aloft. Those ice crystals served as a condensation nuclei, and the freezing drizzle started sticking to the ice crystals aloft. As soon as that happened, the freezing drizzle was replaced by light snow.

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Freezing Fog

The process to create freezing fog is essentially the same as freezing drizzle. What's the difference? Once the visibility drops below 1/2 mile, METARs report freezing fog (FZFG) instead of freezing drizzle (FZDZ).

Just like radiation fog, freezing fog forms on clear, calm nights. Subzero temperatures supercool water droplets in the air. But without any cloud condensation nuclei to adhere to, these super-small droplets freeze to whatever they come into contact with. In most cases trees, the ground, roads, and your aircraft. Everything becomes covered in a glass-like glaze.

When freezing fog conditions exist, ice accumulation will increase as the wind increases.

Weather Products

The first indication that freezing precipitation might be a factor will be found on your airport's TAF or METAR. Here are the codes you'll see:

  • Freezing Rain: FZRA
  • Freezing Drizzle: FZDZ
  • Freezing Fog: FZFG

Without the proper de-icing tools the airlines have, it's best to stay on the ground when freezing rain, drizzle, or fog is headed your way.

Special Thanks: Dr. Andrea Orton, Purdue University and National Weather Service Forecasting Office, Boulder, CO.

Nicolas Shelton

Nicolas is a private pilot from Southern California. He is currently studying at Purdue University, where he is working on advanced pilot ratings. You can reach him at nicolas@boldmethod.com.

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