The Science of Snow
Written by Santa's Quarters™
Snow occurs when water vapor freezes. This precipitation is not a form of frozen rain; it is a crystallization of the ice that forms in clouds. It usually occurs when the temperature drops below 0 degrees Celsius or 32 degrees Fahrenheit and the atmosphere has a sufficient amount of humidity to condense water vapor into ice. After an ice crystal has formed in the clouds, it absorbs and freezes additional water vapor before falling to Earth and coating the ground as snowfall.
Moisture must be present in the atmosphere for snow to fall to the Earth. Snowstorms heavily rely on the temperature in the area, but that doesn't necessarily mean that the temperature you feel on the ground is the same as the temperature in the atmosphere. The atmosphere can have a temperature that is at or below freezing, but the ground temperature has to be at or below freezing as well for the snow to reach the ground and accumulate. As a general rule, if the ground temperature is at least 5 degrees Celsius or 41 degrees Fahrenheit, the snow will not form on the ground. It can be too warm to snow, but it can't be too cold for snow. Snow can occur at even the most incredibly low temperatures if there is a source of moisture and a way to lift the cool air. Because snow requires moisture, it means that even if an area is extremely cold, it won't snow. This is why Antarctica's Dry Valleys rarely receive snow, making them the largest ice-free portion of the continent.
How Snow Forms
Snow forms through an accumulation of packed ice crystals. How the packed ice crystals accumulate is determined by several qualities like temperature, color, and the water equivalent. However, when weather conditions change, that means the snow changes as well because weather directly affects the snow's characteristics.
Snow on the Ground
The characteristics of snow are dependent on the form of crystals from the weather conditions present in the area where the snow fell. If snowfall is accompanied by strong winds, it could be broken into smaller, more densely-packed fragments. Once on the ground, the snow will either melt quickly or persist for long periods of time if the temperature remains below freezing. They could also melt and refreeze over time, which will compress the snowflakes after multiple snowfalls. Snowpack is common in the winter season and typically develops a complex structure made up of a variety of different types of snow that is dependent on the weather and climate at the time it fell.
The Colors of Snow
Snow and ice typically have a white appearance because visible light is white. The visible light hits the snow's surface and gets reflected back without any color preference. Natural materials that absorb sunlight give it color, but when clean snow reflects the sunlight, it creates a white appearance. However, snow and ice are not always white, they can also appear as blue when light waves travel into the snow or ice and the grains scatter the light. This happens because when light travels over a distance, it has to survive scattering events and not be absorbed. The observer will see the light after it has been scattered and bounced around for some time before reflecting back. Usually only blue light makes it through after this. If there are other particles or organisms in the snowpack, then that could also affect the color. For example, in some places, there is a pink coloration called Watermelon snow. This occurs because of the presence of a form of cold-loving fresh-water algae called cryophilic that has a red pigment. A similar color also happens in Antarctica's Taylor Glacier, which has a deep red color caused by iron-oxidized saltwater leaking under the glacier from an old reservoir.
Snow and Sound
The snow's characteristics and age directly affect how sound waves travel. In some cases, the snow can dampen the sound while in other cases, it can enhance them. A thick layer of fresh, fluffy snow means that sound waves are absorbed into the surface which dampens the sound. If the snow melts and refreezes then that is a surface that can help reflect sound waves which will make them travel further and sound clearer. It's also common to hear snow crunching and creaking, especially when you step on it because it means you're compressing the grains and they rub against each other creating friction. At higher temperatures, when snow is about to melt, this friction is lessened so there won't be much of a sound if you step on the snow.
Snow Depth and Temperature
The surface temperature of snow is controlled by the air temperature, so the colder the air, the colder the snow layers on the top 12 to 18 inches of the snowpack. The further into the snowpack you go, the warmer the snow will be because it's closer to the ground which stores heat from summer.
How Much Water is in Snow?
There's no specific instance of exactly how much water is in snow. It varies more than most people realize. 10 inches of fresh snow could contain as little as .10 inches of water, or as much as 4 inches of water depending on temperature, wind speed, crystal structure, and other factors. In the United States, the majority of new snow contains between 4% and 10% depending on the weather conditions during the snowfall.
How Big Can Snowflakes Get?
Most snowflakes are as small as .5 inches across since they are technically an accumulation of multiple snow crystals. However, under certain conditions, especially near-freezing temperatures with light winds and an unstable atmosphere, then there are snowflakes up to 2 inches across that can form. Overall, there aren't any routine snowflake measurements taken so these numbers are based off of much smaller batches of data.
Types of Snow
The conditions of the atmosphere in the area affect how snow crystals form and what happens to them when they hit the ground. It could be large clumps of flakes or symmetrical, six-sided snowflakes depending on the weather. Once it reaches the ground, the snow will also take on different qualities dependent on temperature changes, wind speed, and how long the snow has been on the ground. Fresh snow is often loose and powdery, but the longer snow has been on the ground throughout winter, it may have dense layers from months of melting and refreezing.
Types of Snow Crystals
A snowflake is a single ice crystal or a cluster of ice crystals that fall to the ground from a cloud.
Polycrystals are a type of snowflake that occurs when many individual ice crystals merge together.
Hoarfrost happens when ice crystals are deposited on a surface where the temperature is lower than the surrounding air's frost point.
Rime frost occurs when cold droplets freeze on an exposed surface.
Graupel is a combination of rounded, opaque pellets of snow that form ice crystals in a large lumpy mass. It is often mistaken for hail, but they have a softer and more crumbly texture.
Types of Snowfall
A violent winter storm that lasts at least three hours is a blizzard. They tend to combine strong winds with freezing temperatures to create low visibility.
A snowstorm occurs when an area receives a large amount of snowfall over a short period of time.
A snow squall is a brief snowstorm with little accumulation but has the potential to be very intense.
A snow burst is a short, but intense snow shower that produces a lot of accumulation and very little visibility.
A snow flurry is a lighter amount of snow with a varying intensity that typically lasts for a short period of time.
Drifting snow is snow that the wind has blown from the ground to heights up to 8 feet above the surface. After that height, it becomes blowing snow.
Blowing snow is airborne snow particles where the wind can raise it to heights over 8 feet above the ground. It generally creates low horizontal visibility.
Types of Snow Cover
Snow cover or snowpack is the total accumulation of all ice and snow on the ground that hasn't melted.
New snow is the most recent layer where people can potentially recognize the original ice crystals form.
Névé is a more granular snow that has experienced being partially melted, then frozen over again, and finally compacted. It's particularly associated with glacier formations.
Névé that survives a full year is called Firn. It is well-bonded snow that is rounded and older than one year. It has a 55% density per meter.
Powder snow is new snow that is often dry and composed of loose ice crystals.
Perennial snow is snow that has persisted on the ground for years.
Seasonal snow is snow that lasts one season and then melts.
Old snow is deposited snow that has transformed so much that the original form of the snow crystals is no longer recognizable.
Types of Snow Formations
Snow is subject to weather conditions like changing temperatures, long periods of shade or sunshine, and blowing wind once it's on the ground. These elements change the shape of the snow surface.
A cornice is an accumulation of ice and wind-blown snow that hangs over the edge of a ridge or cliff.
Crust occurs when a hard snow surface lies on top of a soft snow layer. It's often formed by rain, sun, or wind.
Mega-dunes are large dunes that are more common in Antarctica and made up of large snow crystals.
Penitents are tall and thin pieces of hardened snow that develop in arid areas like the Dry Andes or the mountains around Death Valley.
Ripple marks look like the surface of a snowpack is corrugated, and they are caused primarily by strong winds. This is also common in sand.
Sastrugi occur when wind erodes or deposits snow in irregular grooves and ridges. They sometimes result in delicate and fragile snow formations.
A snow barchan is a snowdrift often found in the shape of a horseshoe with the ends of it pointing downwind.
A snow bridge is snow that has drifted across a crevasse and initially formed into a cornice before an arch is created as well. Sometimes a snowbridge can completely obscure a crevasse.
Snow rollers are rare occurrences that happen during specific weather conditions when strong winds blow chunks of snow across the ground to create snowballs that accumulate material as it rolls.
Sun cups are shallow, bowl-shaped hollows formed when the snow is present during periods of intense sunshine.
Snow and Weather
Thanks to increased technology, snow forecasts are better than ever but it's still a challenge for meteorologists due to the fact that even during heavy and intense snowfalls, it may only occur in small narrow bands. This is likely because small differences between temperatures in different areas can make a large difference in how much snow falls and if an area gets snow or rain. Due to this, each storm produces a different type of snowfall and snow won't fall evenly everywhere. Storms follow a storm track where parts of an area in the middle of the track will receive the most snowfall while the areas near the edges of the storm track will receive much less snow. Lighter snows with less accumulation are called flurries. Freezing rain occurs when the water in the air is super-cooled and it forms an ice storm when that freeing rain coats trees, roadways, and power lines with ice. A blizzard is a severe winter storm with heavy snow and wind.
Blizzards are much more than a simple snowfall, they can create dangerous conditions very quickly, especially if you're traveling by car as they can bring drifting snows and whiteout conditions. Blizzards often bring strong winds alongside their low temperatures, making an already cold day feel even colder. Wind chill charts can be used to estimate how much lower the temperature will feel thanks to the wind. These are all important details to remember because exposure to low wind chill can result in frostbite or hypothermia which can cause permanent damage to people. Aside from personal damage, these strong winds and heavy snow can cause utility lines to fall creating mass power outages, causing tree limbs to fall, and can make travel difficult.
Bomb cyclones have been around for years, but they've been more prevalent recently, typically in the Northern Hemisphere. They move counterclockwise around the core and bring snow and rain rapidly thanks to low pressure in the air. In some conditions, a bomb cyclone could become a blizzard.
Snowstorms typically occur in the winter and thunderstorms are more common in the summer, but there are instances where the weather conditions can create a phenomenon called thundersnow. This typically occurs when the temperature is cold enough to generate snow, but the turbulence in the atmosphere is reminiscent of a summer storm and it causes thunder and lightning.
An avalanche occurs on slopes when a large mass of snow slides down it. They are common from December to April in the Northern Hemisphere and can happen on any slope in the right condition. In some cases, depending on the location, they may not be as dangerous, but there are other locations where avalanches are more common and could injure or kill a person. The likelihood of an avalanche depends on several factors such as weather, temperature, slope orientation, slope steepness, vegetation, terrain, wind direction, and general snowpack conditions. Different combinations of these factors can create avalanche conditions that vary between low and extreme. They are often triggered by big snowstorms and earthquakes, but about 90% of avalanches are triggered by humans putting stress on the snow and collapsing a weaker layer of snow underneath.