Understanding ice and snow

It’s another cold day, and as I look out my office window, I wonder if it will snow today. and, if so, what type of precipitation I might expect. So here are a few questions to ponder.

What is the difference between hail, sleet, snow, freezing rain, ice pellets and graupel?

Hail, consisting of small ice balls or ice fragments, is almost exclusively formed due to thunderstorm activity. The process is complex but involves water droplets repeatedly being bounced around in rising columns of air and changing temperature layers. The droplets freeze into small ice fragments. As more water vapour rises through the column, it too freezes against these fragments, increasing their size, until they get large and heavy enough to fall as hail. The largest hailstone recorded in world came from Nebraska USA at 48 cm in circumference and weighed almost a kilogram. The largest Canadian hailstone recorded had a 12.3 cm circumference and weighted 292.71 grams.

Freezing rain starts falling as rain but freezes once it reaches the ground. They are generally formed when super-cooled water droplets fall and burst apart as they hit something solid, flash freezing to its surface. However, they may also be formed when regular raindrops fall onto freezing surfaces.

Sleet or ice pellets are a mixture of rain and snow. These tiny pieces of ice are irregular in shape and are formed while rain or partially melted snow freezes, as it falls to Earth through a cold layer of air. Snow pellets are very similar but tend to be opaque or white in appearance rather than translucent or transparent.

Snow is actually comprised of small hexagonal crystals of ice. Their origins are in clouds containing both water droplets and ice crystals. At low temperatures (i.e. approx. -23°C (-10°F)), the water droplets are super-cooled onto nearby ice crystals, which become heavier and then fall to Earth. Can it ever get too cold to snow? No, but generally, severely cold periods indicate the presence of highs and clearing conditions, which obviously are not conducive to snowfall. This may lead to the incorrect impression that if it’s cold, it can’t snow.

Graupel is snowflakes encrusted in ice, which occurs when the snow passes through cold, wet clouds as it falls and water droplets freeze on them.

To summarize, according to the US National Oceanic and Atmospheric Administration, when a cold air mass is thick, raindrops will freeze into ice pellets called sleet. If the air mass is thinner, the rain will be super-cooled on its way down to earth and will freeze instantly when it hits the ground, creating a layer of ice. This is freezing rain and results in an ice storm.

What is ice fog? Known by many names, such as rime frost, frozen fog, pogonip, or frost flakes (don’t be confused by this one - it’s not a breakfast cereal!) Ice fog manifests itself as suspended particles of ice, and occurs at temperatures below 2°C (35°F), in clear calm weather.

What is the difference between a blizzard and a snowstorm? According to the US National Weather Service, a blizzard is a storm with sustained winds of over 35 mph (56 kmph), visibility of less than 1/4 mile (0.4 km), and it must last at least three hours. Snowstorms are less severe in all respects. The USA experiences about 11 blizzards annually, with North Dakota and Minnesota getting the brunt of those.

Why is ice slippery? Well, ice isn’t actually slippery by itself; it’s just frozen water, after all. What makes it slippery is, it's often covered in a thin layer of melted water. This is what makes it treacherous to walk on.

In closing, here are some strange weather facts:

Chionophobia is the persistent fear of snow.

Rain can fall at 30 kmph, while snow falls at 4.8 kmph.

The subway systems in Boston and New York were built, in part, due to the 1888 snowstorm, which dropped 100-127 cm of snow on the region.

So there you have it, more information on winter weather terms than anybody would ever care to know.

Geoff Carpentier is a published author, expedition guide and environmental consultant. Visit Geoff on-line on LinkedIn, Facebook and Instagram.