No Day Like a Snow Day - Wintry Precipitation

Recently we had the possibility of a snow day here in Norman, Oklahoma.?As I walked two very disappointed children to the bus stop in the rain, I was reminded: winter weather forecasting is hard.?As air temperatures approach the freezing point of water (0°C, or 32°F), fluctuations of one degree can make or break a forecast.?Schools and businesses may close, only to have cold rain dampen the mood.?On the other hand, rain could turn to snow, transforming a routine commute into driving on an ice rink.?So how does a small temperature change make such a big difference?

Wintry precipitation comes in several varieties: snow, sleet, freezing rain, and rain. The type of precipitation depends largely on the temperature profile of the atmosphere - in other words, how the temperature changes upward from the Earth’s surface.?Typically, air temperature decreases upward, away from the Earth’s surface.?However, there are several ways the opposite can happen.?For example, an area of low pressure can move air around in such a way that temperature warms with height for a time, which is called an inversion.?Generally, on the east side of low pressure, there is a warm front, where warm air slides above the denser cold air.?Conversely, on the southern/southwestern side of low pressure in the Northern Hemisphere (north/northwest side in the Southern Hemisphere), there is a cold front, where cold air pushes under warm air.?Both instances form an inversion. The relative warmth and coldness can have a profound effect on the type of precipitation that falls.???

During winter in the mid-latitudes (between 30° and 60° north or south), the atmosphere is below freezing a few thousand feet up where most precipitation forms, meaning all precipitation starts as snow.?What happens to the snow as it falls toward Earth is mostly controlled by the air temperature profile, or change with height.?Let’s take a closer look at how temperature changes with height for each precipitation type.

Snow

The image below shows an example of the temperature profile of the lower troposphere (usually within a few kilometers, or a few thousand feet, of the ground).? The bottom axis is air temperature, with values shown by the dashed green line.? The vertical axis is elevation.? The blue area indicates cold, sub-freezing air.? When an air column is entirely below freezing, snow remains frozen and reaches the ground as snow.? By the way, snow crystals come in many varieties and shapes (columns, plates, dendrites, etc.).? The flake’s shape depends on the air temperature and humidity where it forms.?

Sleet

Once a section of the troposphere gets above freezing, interesting things start to happen, all depending on how thick the “warm” layer is.? The image below shows a shallow, above-freezing, warm layer aloft, often referred to as a “warm nose”.? The layer is just thick enough to partially melt the snow.? Below the warm nose , there is another thick layer of cold air, which refreezes the precipitation.? The precipitation does not re-freeze into snow.? Instead, it forms ice pellets, also known as sleet.? Sleet looks like tiny frozen spheres that bounce off surfaces they hit.? Note sleet is not hail, which is typically larger than sleet and forms in much different conditions found within convective storms. Sleet is simply melted snow that has refrozen.???

Freezing Rain

As the warm nose air gets thicker and warmer, the snow completely melts and does not refreeze.? However, when a shallow layer of below-freezing air remains near the ground, the rain will freeze on contact with surfaces and structures, such as vegetation and power lines.??

Freezing rain can be very disruptive.?A thin glaze of ice can lead to increased risk of car accidents and falling injuries.?Significant ice build-up (generally more than 6mm, or 0.25 in) on tree limbs and power lines can lead to extensive tree damage and power outages.??

A major freezing rain event occurred in Oklahoma in December 2007.?An Arctic cold front slid southward, interacting with very warm, moist air farther south.?The warm, moist air moved over the cold air with enough energy to develop widespread thunderstorms. The thunderstorms produced heavy freezing rain, with much of Oklahoma receiving at least 1 in (25 mm) of ice accumulation.?Some areas reported ice accumulations over 3 inches (76 mm).?Power outages were common, including areas without power for several weeks after the event.

This situation is similar to sea spray icing which occurs at sea when well below freezing air temperatures freeze vessel surfaces combine with high winds (usually gale force or above), blowing sea spray on to vessel surfaces which then immediately forms ice.?This can result in severe stability issues which might be minimized by sailing close to land or changing course to steam with the wind.

Rain

When snow falls into a deep layer of above-freezing air that extends all the way to the ground, the snow melts and falls as rain.

The temperature and thickness of the warm nose as well as conditions at the surface influence what type of wintry precipitation an area receives.?Other concerns include whether there is enough lift in the atmosphere to form clouds and precipitation, the temperature of the ground surface, and the timing of when moisture and cold air arrive.?Forecasters must blend considerable amounts of information from models and observations, while keeping in mind any model tendencies that may influence how much the air cools or warms in the forecast.?Next time you hear a meteorologist call for snow, but you only get rain, please remember that just a degree or two can make all the difference, making for a difficult winter weather forecast.