Freezing Rain Event “Coldilocks:” Three Brrrs Are Just Right

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The National WEather Service forecast for Friday morning calls for a high probability of freezing rain from New Mexico to Wisconsin. Forecast courtesy of NOAA.

The National WEather Service forecast for Friday morning calls for a high probability of freezing rain from New Mexico to Wisconsin. Forecast courtesy of NOAA.

Water in the liquid state is a nuisance, even a hazard if it accumulates in large enough quantities. But when liquid water turns to ice, it loses its coefficient of friction: things slide uncontrollably — even on a very thin layer of ice.

Later this week the atmospheric flows will conspire to create a long-lasting and widespread freezing rain event that Decoded Science is appropriately naming “Coldilocks,” because conditions have to be ‘just right’ to create freezing rain.

Earth Is In The Habitable Zone

It is a fortunate (for humanity) cosmic accident that on planet Earth the chemical dihydrogen monoxide exists in large quantities in its liquid state, commonly known as water.

It is also fortunate that Earth is enveloped in an atmosphere into which some of the world’s water evaporates as water vapor. Temperatures also exist on Earth at which dihydrogen monoxide undergoes a phase change to its solid state: Ice. Occasionally nature conspires to put all three phases of dihydrogen monoxide together to produce an ice storm — precipitation falls as rain and freezes on contact with a surface.

Why Is Ice So Dangerous?

Friction is the resistance to motion of bodies in contact. The amount of resistance is approximated by a ‘coefficient of friction,’ a number from zero (surfaces slide past each other with no resistance) to a little over one (surfaces lock together and cannot slide at all. The coefficient of friction between asphalt and rubber is close to one, so a car tire locks firmly to a road surface.

Ice has a coefficient of friction of about 0.03. A car whose tires have been firmly locked to the road when it is dry, suddenly slides freely over it when the road is covered with ice.

Though there is some dispute about the exact physics, the low coefficient of friction of ice has to do with the formation of a thin film of water created at the interface with an object. The coefficient of friction increases as the temperature falls because the water film is harder to form.

The important thing for commuters and walkers is that the everyday laws of physics, which involve high coefficients of friction between soles of shoes and sidewalk and between tires and roads, changes drastically when the surfaces are covered with ice.

The Conditions That Create Freezing Rain

The location of warm and cold air masses determines the type of precipitation. Graphic courtesy of National Weather Service.

The location of warm and cold air masses determines the type of precipitation. Graphic courtesy of National Weather Service.

Precipitation is very sensitive to how it is produced and the temperature at the surface on which it falls.

Snow and rain are the two most common types of precipitation, but there are intermediate forms, each of which arises from special conditions.

The Brrr Scale For Judging The Relationship Between Temperature And Precipitation Type

Decoded Science has developed the following scale of cold (values from one (Br) to five (Brrrrr)) and each value’s associated precipitation type.

Like Goldilocks, we’ll start with the extremes until the temperature is just right:

The Extreme Brs:

  • One Br: The temperature is well above freezing for a considerable height in the atmosphere, so that falling snow has melted during its descent and all precipitation reaches the surface as rain.
  • Five Brrrrrs: The temperature is well below freezing at all levels of the atmosphere and the ground is frozen. All precipitation falls as snow, which sticks to the ground and accumulates.

The Middle Brs:

  • Two Brrs: The temperature is above freezing at the surface but below freezing at higher levels. The precipitation falls as snow, but melts on contact with the warm surfaces.
  • Four Brrrrs. The temperature is below freezing at the surface and for a considerable distance above, but at higher levels there is a layer of warm air that melts the falling snow, which then re-freezes before reaching the ground as sleet.

The Three Brrrs: Conditions Just Right For Freezing Rain

Under certain circumstances, rain can reach a frozen surface and freeze on contact. The vertical temperature structure that allows this to happen is very specific and therefore relatively rare. A shallow layer of cold air at the surface freezes the ground. Above the cold layer, warm air (above freezing) is sufficiently thick to melt the falling snow. The cold air is shallow enough that the raindrops can reach the ground without re-freezing. The rain then freezes when it hits a surface — voila, freezing rain.

If the layer of cold air is too thick, the raindrops will freeze into freezing rain’s sister precipitation, sleet.

The Central US Is In A Favorable Location For Freezing Rain

The US from the Gulf Coast northward is a prime location for freezing rain. Warm, humid air moves northward from the Gulf of Mexico, while cold Canadian air surges south across the plains. Where the air masses meet, the warmer, lighter air rides over the colder, denser air — freezing rain is possible.

Where Will Coldilocks Find The Three Brrrs?

The forecast for Saturday morning shows a high probability of freezing rain in parts of Texas and Oklahoma. Forecast courtesy of NOAA.

The forecast for Saturday morning shows a high probability of freezing rain in parts of Texas and Oklahoma. Forecast courtesy of NOAA.

The stage for Coldilocks is being set by a large high pressure system off the north Atlantic coast which is driving warm air from the Gulf of Mexico into the center of the country. At the same time, a mass of cold air is sweeping southward across the plains.

The precise location of the three brrrs is never easy to forecast, but this will be a long-lasting event, and any location from New Mexico to New England could be affected between now and Sunday. According to the most recent forecasts, the highest likelihood for freezing rain is on Friday and Saturday in northern Texas and Oklahoma.

A Word About Names

The Weather Channel has absurdly called this weather situation Winter Storm Cara. There is no low pressure system with which to identify the so-called storm, most of the precipitation will fall as rain (not much wintry about that), and the major danger from this event is clearly from freezing rain.

Though The Weather Channel’s naming winter storms at first seemed like a helpful aid in communication, it now just looks like a publicity stunt.

Decoded Science will name significant weather events or patterns, and will try to use relevant, though occasionally whimsical, names.

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