Snow

Snow is a kind of precipitation that falls when the temperature is below freezing -- during the winter months in the temperate zones, all year in the arctic, and never in the tropics save on the highest mountains. Made up of water that has frozen into soft, fluffy crystalline shapes, snow piles up over the winter in a white layer over the ground that may be only a few inches deep, or may heap up to several feet or even yards.

Snow falls in response to the same atmospheric conditions at rain, just at a lower temperature. Frontal boundaries are a major source of heavy snow, while lighter snow showers or flurries can occur whenever the air mass is fairly humid and unstable. Snow can also be caused by moisture swept up off large lakes and the ocean by strong winter winds and dropped on the nearest shoreline as lake effect snow.

Snowflakes are so beautiful, fragile, and marvelously symmetrical that they grip our imagination in a way that raindrops and other precipitation do not. These delicate sculptures of ice crystals are one of the most wondrous of nature's weather phenomena -- whether seen hugely magnified, revealing their shimmering perfection in all its detail, or as white fluffy motes drifting down gently in front of the dark green of pine trees on a quiet Christmas morning. They are also an important part of the annual weather cycle, building up moisture over the winter which allows temperate plants to grow quickly in the Spring, and can impact human activity both as a source of fun -- skiing, sledding, and snowball fights -- or as inconvenience when snow blocks roads and limits visibility to a few feet.

What a snowflake is

Although it exists in place of rain in winter storms, a snowflake is not a frozen raindrop. Frozen raindrops are small balls or chunks of ice and do not resemble snowflakes in the slightest. A snowflake forms from a frozen droplet of water on whose surface ice crystals grow by condensation. The process is complicated and takes some time, as you might expect from such an intricately detailed structure as a snowflake.

When the temperature is below freezing, the water droplets in the clouds become supercooled, meaning that they are colder than the freezing point but still liquid. At a certain point, as more cooling occurs, some of the droplets freeze while the others remain liquid. Since the cloud must be cooling in order for this to happen, the droplets that haven't frozen yet are giving off water vapor into the air as well. When the water vapor touches the surface of the frozen droplets, it also freezes, building up crystals that grow and branch outwards from the original core. Soon, a snowflake has formed, and when it gets heavy enough to offset the lift of the atmospheric instability that caused it in the first place, it will begin to fall.

Snowflakes have six arms because ice crystals are hexagonal, so the core is a hexagon whose corners are most likely to grow crystalline extensions. The shape of ice crystals causes them to grow in a hexagonal fashion, so snowflakes naturally end up as six-pointed stars. There are indeed no two snowflakes that are the same once the snowflake has grown to a certain level of complexity -- this is not a special quality of snowflakes, but is true of every complex structure, and even most structures that seem very simple to us.

Snow is not actually white but clear -- it is the reflection of light through the millions of tiny crystals that gives it a white appearance. There is much less water in a given volume of snow than there is in a given volume of rain, due to the spaces and the nature of the ice crystals themselves. Snow's density is about 8% that of water, meaning that 1 inch of rain is equivalent to 13 inches of snow.

The buildup and compaction of snow

When snow falls, it is initially 8% as dense as water, but the weight of the snow and the pressure of wind on its surface will soon compact it until it is 30% as dense as water. It can compact further while remaining snow, but this further compaction is caused by solar heating and refreezing.

Snow does not build up infinitely over the winter partly due to compaction, partly to occasional thaws, and partly to sublimation. Sublimation is evaporation directly from a solid to a gaseous state -- in other words, the ice crystals turn directly into water vapor without melting first. This happens when the air is dry enough to pull moisture out of the snowflakes directly -- a process which is helped by wind. Without these mechanisms, snow would heap up steadily until the Spring thaw and reach astounding depths in the northern regions.

The importance of snow

Snow is important for both natural plant life and human crops because it does tend to build up over the winter, then release a large amount of water in the spring, giving seedlings the head start they need to grow vigorously and survive. The fact that the ground is often frozen a short distance below the surface magnifies this effect, since it keeps the meltwater from draining off too quickly after the thaw. The meltwater from thawing snow is a crucial part of both natural vegetation's and human crops' life cycles.

Snow also provides people with opportunities for recreation of many kinds. Skiing, snowboarding, snowmobiling, snowshoeing, snowball fights, and snow sculpting all make use of the unique properties of snow. If a human's weight is distributed over a large enough area, snow can easily support it. This makes rapid travel over snow possible, especially since snow has very low friction and skis will slide over it under most normal circumstances. Snow therefore increases human enjoyment, provides some aesthetic and poetic pleasure, and has a positive economic effect in the areas that are popular for winter recreation.

Snow can also be a winter hazard. Roads are made slippery by snowflakes, or blocked by wind-raised drifts of snow, which can be quite hard and difficult to clear because of the crystalline arms of the snowflakes locking together under the pressure of the wind. Shoveling snow is hard work, and some elderly people are at risk of heart attacks when they are attempting to clear paths and driveways. Snow can even cause weak buildings to collapse if it is allowed to build up on their roofs. Like all of weather's phenomena, snow is a mixed blessing, but also remains crucial to the large-scale health of the planet's meteorological life cycle.