Freezing Rain Vs. Freezing Drizzle: What’s the difference?

In the winter months, one of the most feared weather conditions is freezing rain. It causes ice to accumulate on roads and sidewalks, making travel treacherous. While we all know freezing rain is dangerous, what makes it different from freezing drizzle, or even black ice? In this blog post, we investigate that question!

When you consider the difference between freezing rain and freezing drizzle, the easiest way to visualize the difference is to think of the difference between rain and drizzle. Rain consists of large drops, while drizzle is made up of much finer drops, almost like a mist in some cases. This probably gives the impression that freezing drizzle is just describing freezing rain at a lighter intensity. However, there is more to it than that.

Drizzle consists of water droplets that are less than 0.5 mm in diameter (image from: https://www.weather.gov/jetstream/preciptypes)

 

Rain consists of water droplets that are more than 0.5 mm in diameter (image from: https://www.weather.gov/jetstream/preciptypes)

To understand the difference between freezing rain and freezing drizzle, we actually need to briefly discuss a complex topic called precipitation microphysics. In short, this is referring to the very small-scale processes in clouds that cause precipitation to develop. One key concept regarding this topic is the fact that liquid water can actually exist in the atmosphere down to -40C, and the atmosphere is all liquid water at temperatures above -4C. Between -4 and -40C, there is a mix of ice and liquid, with more ice the closer the temperature gets to -40C. While a discussion about why this occurs is beyond the realm of today’s topic, keep this in mind as you continue reading.

To the average person, freezing drizzle is just low-intensity freezing rain. However, to a meteorologist the difference is much larger. Freezing rain occurs when snow is generated high up in the atmosphere, then falls through a warm layer above the ground (with temperatures above freezing), causing the snow to melt into liquid water. Before the now liquid drop of water reaches the ground, the temperature suddenly falls back below freezing, but not quickly enough for the water droplet to refreeze. As a result, the water droplet hits the ground as liquid, but immediately freezes because the ground is below-freezing. If the water droplet did have enough time to refreeze, it would be called an ice pellet (aka sleet), but that is a discussion for another day.

Freezing rain occurs when snow melts on its way to the ground and then freezes when it hits the surface (Image from: https://www.skybrary.aero/index.php/Freezing_Rain)

So if freezing rain is caused by snowflakes melting before they reach the ground, how is freezing drizzle different? The difference goes back to the microphysics that were discussed earlier. In the case of freezing drizzle, a low-level cloud deck is usually present in an atmosphere where the temperature is between 0 and -10C. Since the cloud in this case is not far below freezing, it is almost entirely (if not entirely) made of liquid water. Therefore, if the cloud contains enough moisture to produce drizzle, those small liquid particles will be supercooled – below freezing – as they head toward the ground. Once they reach the ground, the drizzle droplets are deposited as ice, sometimes called “glaze” because it is often a very thin layer of ice.

Freezing drizzle occurs when the entire atmosphere is below freezing (Image from: http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cld/prcp/zr/prcs/swrp.rxml).

So we’ve learned about freezing rain and freezing drizzle, but what about black ice? Black ice isn’t actually a weather phenomena, but rather a road phenomena. According to the American Meteorological Society:

A popular alternative for glaze. A thin sheet of ice, relatively dark in appearance, may form when light rain or drizzle falls on a road surface that is at a temperature below 0°C. It may also be formed when supercooled fog droplets are intercepted by buildings, fences, and vegetation.

Black ice can form in numerous different ways. It can be the result of freezing rain or freezing drizzle, but could also be from existing water on the road freezing, or even the deposition of ice onto the road from vehicle exhaust. Regardless of its source, black ice is a very dangerous road condition because it can easily be mistaken for a wet road, or even a dry road!

When producing winter weather forecasts, the professional meteorologists at Weatherlogics closely monitor the potential for freezing rain, freezing drizzle, and black ice. While to the average person there may not seem to be much difference between freezing rain and freezing drizzle, the difference is quite important to meteorologists. The impacts are also different, as freezing rain events can be quite significant (e.g. the 1998 Eastern Canada Ice Storm), while freezing drizzle events are usually much less severe. It is especially important to consult a professional meteorologist if you are affected by these types of precipitation, since they are among the most difficult aspects of weather to predict. Our Road Weatherlogics service also provides future predictions of all winter precipitation types, including snow, freezing rain, and blowing snow.

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