Rain, Snow, Freezing Rain and Hail

From cloud droplet to raindrop

A cloud droplet is tiny, typically around a hundredth of a millimetre across. It is so small that the gentlest updraft keeps it aloft, and if it does start to fall, it evaporates long before it reaches the ground. A typical raindrop is a couple of millimetres across, and it takes something like a million cloud droplets to build one. That is why plenty of clouds drift by without dropping anything at all. Something has to make the droplets grow, and there are two ways it happens.

1. Bumping and merging

In clouds that are warmer than freezing, a few droplets end up slightly bigger than their neighbours. Being heavier, they fall a little faster, sweep up smaller droplets in their path and grow, like a snowball rolling downhill. This collision and coalescence process makes most of the rain in the tropics.

2. Ice crystals steal the water

Here is a surprise: cloud droplets don’t automatically freeze at 0°C. Without a speck of the right kind of dust to freeze onto, they can stay liquid well below zero. These are called supercooled droplets. When a few ice crystals form among them, something remarkable happens. Because ice holds onto its molecules more tightly than liquid water does, vapour leaves the droplets and deposits onto the ice. The crystals grow quickly while the droplets shrink, until the crystals are heavy enough to fall.

This ice-crystal process (named after Tor Bergeron, one of the scientists who described it) is how most precipitation in mid-latitudes like ours begins. Even a summer rain shower over Lethbridge usually started life as snow high in the cloud and melted on the way down.

Snow and the snow-to-water ratio

A snowflake starts as a single ice crystal. The shape it grows into, whether a flat plate, a needle, a column or the classic six-armed star, depends on the temperature and moisture where it forms. As crystals fall, they often tangle together into larger clumps. Near 0°C, flakes are wet and sticky and can join into big, soft clumps a few centimetres across. In bitter cold, the snow tends to be fine, dry and powdery.

Weather records measure snow two ways: its depth, and the amount of water you get when you melt it. The long-standing rule of thumb is 10 to 1: about 10 cm of fresh snow melts down to about 10 mm of water. Real snow varies a great deal. Heavy, wet spring snow can be closer to 5 to 1, while light, fluffy snow in cold air can be 20 to 1 or more. That difference matters to anyone shovelling a driveway, and to farmers wondering how much moisture a snowfall really brought.

The layers decide: snow, ice pellets, freezing rain or rain

In winter, precipitation almost always starts as snow. What reaches your driveway depends on the temperatures it falls through on the way down. Think of it as an obstacle course with warm and cold zones:

• Snow: the air stays below freezing all the way down, so the flakes arrive intact.
• Ice pellets: a warm layer aloft melts the snow, then a deep cold layer near the ground gives the drops time to freeze again into little balls of ice that bounce and rattle on your windows. (Americans call these “sleet.”)
• Freezing rain: the warm layer melts the snow, but the cold layer near the ground is too shallow to refreeze the drops. They arrive as supercooled liquid and freeze the instant they touch a cold road, car or power line, building a coat of clear glaze.
• Rain: the air is above freezing through a deep enough layer near the ground that the snow melts completely and stays liquid.

Freezing drizzle is a sneakier cousin. It often comes from shallow, low stratus clouds that are too warm and thin to make ice crystals at all, so the droplets grow by bumping and merging and fall as fine supercooled drizzle. It may barely show up on radar, yet it can glaze roads just as effectively.

Graupel and hail: ice made by the updraft

When a snowflake falls through a cloud full of supercooled droplets, the droplets freeze onto it on contact. This is called riming. A heavily rimed flake becomes graupel, also called snow pellets: soft, white, crumbly balls of a few millimetres that squash when you press them. Graupel is common in spring and autumn showers and is often mistaken for small hail.

Hail needs much more power. It grows inside a thunderstorm with a strong updraft, a column of rising air fast enough to hold up chunks of ice. A small ice particle or frozen drop, called the embryo, is carried up through a region thick with supercooled water. Droplets freeze onto it and it grows, layer by layer. When it becomes too heavy for the updraft to support, or drifts out of the strongest lift, it falls.

Slice a big hailstone in half and you often see rings like an onion. Clear layers form where plenty of supercooled water coats the stone and freezes slowly, letting air bubbles escape. Milky layers form where droplets freeze instantly on contact and trap tiny bubbles. The stronger and longer-lived the updraft, the larger the hail it can build.

Precipitation, southern Alberta style

Our location at the foot of the Rockies, on a prairie that rises gently toward the mountains, gives us a precipitation personality all our own.

Freezing rain is less common here than in Ontario or Quebec, because a warm layer aloft over a shallow cold layer is not our usual winter set-up. But it does happen, and freezing drizzle from low stratus during a cold, easterly upslope pattern is a familiar winter hazard on southern Alberta highways.