One of the best things about this time of year is that faint possibility that it might snow. The promise of snowmen and snowball fights, of time of work and school, of that cheer that only snow can bring, is almost enough to get us through the long, dark, bleak winter. They say no two snowflakes are alike, and although some pesky scientists have proved that incorrect, the intricate crystalline structure of snowflakes is truly beautiful. Snow is also hugely important for wildlife and people, and plays an important part in keeping our climate stable. To say Merry Christmas from Curious Meerkat this year, here’s a look at the science of snow – from its formation to its recreational uses and its role in the healthy functioning of Planet Earth.
Snow is simply a form of frozen rain, or precipitation, which occurs when cloud temperature is at or below freezing. Snow crystals tend to form in heavy, moisture-rich clouds, containing dust particles. Ice crystals form around these dust particles, called ice nuclei as the water vapour in the cloud slowly condenses. Snowflakes can be formed from multiple crystals that have melted slightly and fused together, or through new water vapour condensing onto existing crystals. As ice crystals grow inside the cloud, they get heavier and heavier until eventually their weight causes them to fall from the cloud. If the air temperature at the ground is low enough (below about 2°C), these crystals will remain frozen all the way and land on the ground as snow. Often, snow flakes that form in the clouds will have melted by the time they reach Earth, and all we see is the rain that results.
Rain that freezes as it falls (it may well have started life as a snowflake that melted) becomes sleet. Hail, on the other hand, is formed by quite different processes. Hail is formed when stormy weather carries water droplets from the warmer bottom of a cloud to it’s cooler top causing them to freeze. These small ice crystals then sink to the bottom of the cloud (because they now weigh more) picking up more water before being carried back to the top of the cloud again by the storm. This process continues, adding layer after layer of frozen water to the growing hail stone, which eventually falls to Earth when the storm subsides or it becomes too heavy.
It’s All in the Bonds
Hydrogen bonding between water molecules means that ice crystals are always six-sided, but snow crystals can come in types according to the type of ice crystals that compose them: prisms, plates, needles, columns and dendrites, to name a few. Which type of snow forms depends on the humidity and temperature in the cloud when they form, and in the air as they fall. Contrary to popular mythology, snowflakes are not unique – in 1988 scientists found two identical snowflakes under the microscope – although their enormous variety of shapes and forms means it’s hard to find two the same.
Snow flakes range from less than a mm in diameter up to several inches, and the largest reported snowflake was an astonishing 15 inches in diameter (although this report is unsubstantiated – snowflakes of several inches are certainly possible). In 2012, chemists at the Max Planck Institute produced the world’s smallest snowflake – the smallest snowflake possible, in fact, which contained just 275 water molecules! In general, smaller snowflakes form at lower temperatures, often in higher altitude clouds. Temperature also influences snowflake shape – plate-like crystals are more common at warmer temperatures, whereas needles commonly form in colder air. Snow crystals continue to grow and change shape as they fall, and their final size and shape will be determined by the conditions they experience both in the cloud and in the air as they fall to Earth. Snowflakes falling through humid (moist) air will tend to grow more as water is needed to fuse snowflakes together. Dry air produces dry, powdery snow composed of small flakes; humid air produces wetter, ‘sticky’ snow with larger flakes. Since each arm of the crystal experiences the same conditions, snowflakes remain symmetrical, although they can loose hexagonal symmetry by fusing with other snowflakes or and branching off.
It is the many reflective surfaces in a snowflake that cause it to appear white – reflecting all visible light at every possible angle. Imperfections in snow such as soot, pollen or other particles int he atmosphere, can give snow a red-, pink-, yellow- or brown-ish tinge.
We’ve all experienced the disheartening moment when you find that the snow you’d so excitedly tried to form into the shape of a snowball or a snowman just won’t stick. As I mentioned earlier, sticky snow tends to form when the air is moist – it is actually the liquid water in snow that determines whether it’s good for snow frivolity like snowball fights and snowmen. Liquid water between the ice crystals allows them to ‘stick together’ – when you compact snow with your hands you warm it slightly and cause the snow crystals to fuse together as the liquid water freezes. Without those lose water molecules, the snow won’t stick.
When the air temperature is below freezing, the spare, liquid water in the snow freezes as well forming complex crystals of powdery, dry snow. Again, no good for snowmen! You can often tell that snow is quite dry because it squeaks under foot. Your snowball fights will be the best when the temperature is just slightly above freezing – around 1 or 2°C, and relatively high humidity. Dry snow is great for many winter sports, however, such as skiing and snowboarding, so it can still provide some entertainment!
Snow and frozen water in general is of huge importance to our planet. It alters habitats, moderates sea level, influences regional and global weather patterns, and produces glaciers that can shape entire landscapes. The cryosphere refers to any region of the Earth’s surface that is covered by frozen water – snow, sea ice, icecaps, glaciers, permafrost and icebergs. Most of the cryosphere by volume is found in Antartica, although winter snow and ice in the Northern hemisphere takes up a larger area. To maintain the cryosphere, these regions must remain below freezing for a certain part of the year, at least. Some parts of the cryosphere melt annually, while others have been frozen for thousands, even millions of years. Approximately 47 million km2 of the Earth’s surface is covered in snow!
Snow has a number of important properties that make it highly influential in regulating the Earth’s climate. Snow, being white, is highly reflective, meaning that large areas of snow and ice reflect solar radiation and cool our planet. Another key characteristic is the low thermal diffusivity of snow – snow diffuses heat very poorly, making it an excellent insulator. This means that snow can (counter-intuitively) protect vegetation from freezing and provide winter shelter for many animals, not to mention humans! Snow insulates the land and sea ice insulates the oceans. Snow and ice also evens out our climate and smooths the seasons. Frozen water has a high latent heat of fusion, meaning that it requires a lot of energy to melt ice and snow – this slows the warming of the land and sea in the summer.
The cryosphere is involved in numerous climatic feedback loops at both a local and global scale, and can influence weather patterns by altering air temperature and humidity. Snow is also important in maintaining water balance, and snow packs are a source of ground and running water for many parts of the world, providing over a billion people with fresh water. Melting of the cryosphere due to climate change has the potential to disrupt climate cycles and water balance, as well as cause sea level rise. Sea ice and permafrost melting may have only a minimal affect on sea level, although the loss of this part of the cryosphere could have a serious impact on ecosystems. But it is the melting of the glaciers that is a big concern for sea level rise. In the millennia following the end of the last ice age, global sea level rose by 120 meters, to roughly the level it is today. However, as the glaciers melt due to climate change, and sea water undergoes thermal expansion, the sea level is rising further, threatening to decimate island and low-lying populations such as Japan and The Netherlands. Increased flooding, particularly in cities located in river basins, such as …, is also a consequence of melting glaciers and thermal expansion. Thermal expansion happens because, like all liquids, the volume of water increases with temperature rise, because each water molecule has more energy and moves around more.
We are currently experiencing sea level rises of about 3mm each year – roughly half of that is due to glacial melt (including icecaps and icesheets) and the other half is due to thermal expansion of water. Most of this melt, so far at least, has come from relatively small, low altitude ice caps and glaciers and from the Greenland ice caps. Most of the Antarctic ice remains intact, although were it to melt entirely, the sea would be expected to rise by 70m! Medium-level IPCC climate scenarios (not the most optimistic, but not the worst*) predict sea level rise due just simply to thermal expansion by 2100 to reach around 0.2 m and roughly 0.5 – 0.7m due to glacial melting – totally about 1m of sea level rise. Global temperature rises of more than 4°C are almost certain to cause the complete melting of the Greenland ice sheet, representing about 10% of the cryosphere, with catastrophic impacts on wildlife and people alike.
*I’m referring to the RCP4.5 and RCP6, which represent atmospheric carbon dioxide levels of 650ppm and 850ppm respectively.
Snow has to be the best kind of water – its delicate, crystalline structure, its capacity to be shaped into snowballs and snowmen, its use for winter sports. But snow is more than just a pretty distraction from the bite of winter. It is more than just snowball fights and skiing – it is a crucial part of the functioning of our planet and our climate, and an important source of water, food and shelter for millions of people and animals globally. Projected sea level rises of 0.5m by 2050 (that’s really quite soon, you know!) could leave many cities underwater – Stockholm, Copenhagen, Tokyo, New York, Kuala Lumpur, Singapore, Melbourne, Sydney, Brisbane, Bangkok, Boston, Baltimore, Hanoi, Lima, Amsterdam, Buenos Aires, to name a few! We must respect our cryosphere, lest we lose it forever!
Want to Know More?
- National Snow and Ice Data Center Snow Science
- A Tour of the Cryosphere
- Chemists find Smallest number of water molecules needed to form an ice crystal
- Caltech Guide to Snowflakes and Snow Crystals
- IPCC Fifth Report
Featured image was created by Claire Asher from images in the public domain.