- Written by Dave Hayes, otherwise known by the moniker "Dave Hayes The Weather Nut”
- Published: 14 February 2020
Good day to you. My name is Dave Hayes and I wanted to write an educational piece for you about wintry and/or icy precipitation (a/k/a “hydrometeors,” or water-based meteors), given that we’re in the winter season. Many times, folks get confused about what kind of precipitation is falling out of the sky, what it’s called, and how it forms, so I will go through them one by one below, starting with snow. I hope you enjoy it, and thanks for reading!
For any type of precipitation or hydrometeor to form, you need what is called condensation nuclei. Condensation nuclei form the structural basis around which a rain droplet or snowflake can form, grow large enough, and then fall to the surface around us. But what are these condensation nuclei that you speak of, Dave Hayes? Well, I’m glad you asked! They can be pollen grains or dust particles that swirl around in the atmosphere. Or, they can be supercooled (and super tiny) water droplets that freeze. Although these supercooled droplets can remain liquid down to 0 degrees Fahrenheit, when these droplets freeze, they become another nucleus around which ice crystals can form. Through a process called deposition, water vapor will transform into a solid ice crystal directly on the nucleus, grow through further deposition, and fall to the ground as snow.
Interestingly, both the temperature at which snowflakes form, and the relative humidity at the time of their formation, plays into what types of crystals grow and the resultant snowflakes we see float to the ground. Common shapes are flat plates, long spindly needles, columns, fat fluffy dendrites, as well as many other snowflake types! Typically, snow that forms in warmer and drier air will produce simpler crystal structures like needles, columns and plates, whereas snow that forms in colder and moister air creates more ornate structures, like dendrites – the fat fluffy flakes that contribute to the “snow globe” effect.
It’s easy to think of these ping-y ice pellets as being called “freezing rain,” because they are, in fact, frozen rain droplets. However, freezing rain describes a different wintry precipitation type which I’ll describe below. Sleet occurs when snow falls into a milder, above-freezing air layer aloft, and subsequently melts into rain drops before reaching the ground. These rain drops then continue to fall, and eventually descend into a below-freezing parcel of air, generally a few thousand feet above our heads. The bottom of this parcel of air is also in contact with our planet’s surface. These rain drops then freeze into ice pellets known as sleet, and bounce off of everything they strike on the surface of our planet, including our craniums! Sleet accumulations cause dangerous driving conditions, and are akin to driving on ball bearings.
Condensation nuclei form the structural basis around which a rain droplet or snowflake can form, grow large enough, and then fall to the surface around us.
Freezing Rain/Freezing Drizzle/Freezing Fog
Take the sleet-formation example above, but imagine that the below-freezing surface layer adjacent to Earth’s surface is much thinner. In this case, milder air has deeply penetrated into the region just above our heads, while our surface is still quite cold. Within this setup, the melted snowflakes that turned into rain drops don’t have enough time to freeze in mid-air before they reach the ground. They then land on the ground as liquid rain, but freeze instantly on contact, creating a translucent, icy glaze on every outside surface – trees, roads, driveways, cars, lawns, etc. Similarly, if fog or drizzle forms in this type of environment, immensely slippery conditions can create the absolute worst driving conditions imaginable, even worse than sleet or snow.
Like sleet and freezing rain, sleet and hail can also be easily confused! Hail is a type of ice pellet that forms in thunderstorms (usually) during the warmer months, whereas sleet only forms in the winter months. While hailstones can be small like most sleet pellets, they can grow to much larger sizes, even as big as a softball! Hail forms during convective events like thunderstorms where powerful updrafts send rain drops high above the freezing layer miles up. These rain drops then freeze into hail, and either fall to the ground as small hailstones or get blown back up into the freezing layer again, growing in size as they accrete more ice to their surface. Eventually, gravity wins out. Even the largest and heaviest hailstones fall to the ground, sometimes causing substantial damage to crops, foliage, houses, cars and unfortunately even people.
Graupel forms in a completely different process than hail. In this case, snowflakes fall from aloft and encounter what are called super-cooled water droplets. At times, liquid water droplets can be suspended in the atmosphere in below-freezing temperatures without freezing. If conditions are right, these snowflakes encounter layers of supercooled water droplets as they fall, which then freeze onto the outer edges of the snowflake, semi-encapsulating it in a type of soft ice which then reaches the ground as graupel. When you pick it up it tends to crumble if you touch or press on it. Graupel can occur during winter, or the warmer months if conditions are right.
I hope this educational article has helped to clarify the similarities, differences and variety of wintry and icy precipitation. Thanks for reading!
Dave Hayes is a deeply committed weather enthusiast, writer and reporter for the western Massachusetts region and beyond, and has lived here for over 30 years. Dave has won The Valley Advocate's Best in the Valley Reader's Poll for the last 5 years in a row for Best Local Meteorologist. His hyper-local daily reports and interactive in-storm coverage not only provides the info you need in a timely way, but answers the questions you have, which is extremely helpful in planning one's day. He often mixes in his personality by tossing in fun weather facts, humor, gratitude, and whimsy. In short, it shows that Dave cares about the community in which he serves. Dave currently resides in Deerfield, MA. You can find him on Facebook or at WesternMassWeather.com.