December 1st marked the first day of Meteorological Winter. The months of December, January, and February comprise this time frame. Meteorological seasons are easier to track and compare for record-keeping, unlike the astronomical seasons, which vary by a day or two and don't fall neatly into calendar months. So, with the start of this season, let's explore the types of weather we might encounter, and learn some fun facts along the way.
Snow blankets the Hintz Alumni Center on the campus of Penn State University. (my own photo)
Snow: Snow is synonymous with winter, but we don't always see a lot in a Tri-State winter. That's because it has to be cold enough for snow from the point of the snowflake's origin, the cloud, all the way down to the surface. While it can snow when the surface temperature is above freezing, it has to be below freezing right above ground level, 100 feet or so, in order for the snowflakes to remain frozen on impact. This combo is not easy to accomplish in a coastal area like ours; more on that later.
Did you know that snow's texture makes it hard to predict how much will fall in a storm? Typically, if an inch of rain were to fall as snow, it would pile up to around a foot, but the ratio varies widely. A forecast of 3"-6" of snow, for example, is equivalent to forecasting rainfall to within a couple tenths of an inch. Snow totals are also impacted by variations of temperature and humidity within the cloud, which produce different types of snowflakes (see figure below). These nuances are not fully resolved by the computer forecast models that meteorologists rely on.
Types of snowflakes/crystals; several of these can be present in the same storm.Credit: SnowCrystals.com The next two types of wintry precip we'll discuss, freezing rain and sleet, occur in a process called overrunning. Warm air is less dense than cold air, allowing a melting layer to sneak into many winter storms (see diagram below). This is most commonly seen in the warm front section of midlatitude cyclones. It's also a feature of a stable atmosphere, one that is not prone to much vertical motion.
This NOAA graphic shows varying types of precipitation that can fall when precipitation starts as snow at cloud level.
Sleet pellets are generally only a few millimeters in diameter. Photo from Britannica.com
Sleet: Sometimes mistaken for small hail, but formed in stable atmosphere, whereas hail only forms when an unstable, convective atmosphere is present. Sleet results when raindrops freeze into individual ice pellets before reaching the ground. This means there has to be a sufficiently thick layer of warm (above freezing) air in between the cloud and the ground, and there has to be a sufficiently thick layer of cold (below freezing) air between that melting layer and the surface.
Diagram of overrunning, and the types of wintry weather it can produce, from NOAA.
It sounds like this combination should be rare, but it's actually quite common around here. In the winter, ocean temperatures are frequently milder than air temperatures in the Tri-State. An onshore flow can inject mild air above the ground while it remains below freezing close to the ground (remember that cold air is more dense than warm air). And it doesn't take a lot of mild air to melt the tiny ice crystals in snowflakes. Sleet frequently mixes in during snowstorms, which has the impact of lowering snow totals since sleet pellets are more compact than snowflakes.
Image of the Great Ice Storm of 1998 from NOAA Vlab. Up to 3 inches of ice accretion was recorded in some parts of New England.
Freezing rain: The layer of cold air needs to be even more shallow to produce freezing rain. Temperatures aloft have to be too warm for the raindrops to freeze before reaching the ground. Freezing rain is likely to happen when it's in the 20s at the surface, with temperatures spiking into the 40s a couple hundred feet or so above the surface.
The "skew T diagram above is used by meteorologists to understand the vertical temperature profile of the atmosphere at one location. The image above (courtesy of NOAA) is typical for freezing rain. Freezing rain events are frequently referred to simply as ice storms, and they can cause a lot of damage. Ice gets heavy quickly; while a cubic foot of dry snow can weigh as little as a few pounds, a cubic foot of ice weighs about 57 pounds. And when ice coats every individual twig or leaf of a tree, the collective weight can easily take down large tree limbs and power lines, and cause roofs to cave in.
A layer of graupel on top of snow. (Image from NOAA)
Graupel (or rimed ice): 10 years ago, many of us had never even heard of this winter phenomenon, but as more people get interested in the weather, more weather terms are getting recognized (remember when everyone suddenly knew what the Polar Vortex is?). Graupel forms in a process similar to hail, with unstable air and strong vertical motion. This is very different than the stable setup described earlier, which creates sleet and freezing rain. Graupel particles are seeded by a snowflake or tiny ice crystal. An updraft then carries the ice crystal into supercooled water vapor, which condenses onto the snowflake or ice crystal. Graupel often looks like small balls of styrofoam because the dynamic process that forms each pellet smooths out the rough surfaces.
The image below looks like graupel, but these are snow pellets, which are snowflakes that partially melted and then re-froze while falling. It's not possible to tell the difference from a still image, but if you know whether the atmosphere creating the precipitation is stable or unstable, you can probably make a correct guess.
Snow pellets, which look similar to graupel. (my own photo)
For more on winter weather safety, check out the resources from
NOAA's Weather-Ready Nation page. And for a quick dopamine hit, I've included an unnecessary (but adorable) picture of my dog in the snow. Enjoy!
Gratuitous picture alert: my dog, Trace, experiencing his first big snowstorm in 2020. (my own photo)
