Here is a quick summary of the Earth’s snow cover between March 2000 and December 2018. These snow cover maps are made from observations collected by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite.
Some snow facts
Snow is precipitation that forms when water vapor freezes. Because snow is so reflective, it plays an important role in regulating climate: it reflects incoming sunlight back into space, cooling the planet.
Snow also supports life. Melting of seasonal snow (as well as glaciers) provides water for drinking and irrigating crops in many parts of the world. Snowmelt moisturizes soil and reduces the risk of wildfire. Too much snow, however, can lead to springtime floods when the snowpack melts.
Here are some interesting facts about snow:
1. Snow is not white
Snowflakes appear white as they fall through the sky or as they accumulate on the ground as snowfall. But, in fact, they are totally clear.
The ice though is not transparent like a sheet of glass is, but rather is translucent meaning light only passes through indirectly.
The many sides of the ice crystals cause diffuse reflection of the whole light spectrum which results in snowflakes appearing to be white in color.
And, in fact, it doesn’t always appear white. Deep snow can often appear blue. This is because layers of snow can create a filter for the light, causing more red light to be absorbed than blue light.
2. It doesn’t have to be freezing to snow
Generally, the air temperature does need to be at or below freezing for the snow to fall. However, if rain falls continuously through the air with a temperature as high as 6 °C (42.8 °F), it may cause the air temperature to fall low enough for the rain to turn to snow. This is because the rain that persists for some time will gradually cool the air that surrounds it.
3. The speed of snow
Most snow falls at a speed of between 1 – 4 mph (1.6 – 6.4 km/h) dependent upon the individual snowflakes mass and surface area, as well as the environmental conditions surrounding its descent.
Snowflakes which collect supercooled water as they fall can fall at up to 9 mph (14.5 km/h), but snowflake, as most people recognize them will tend to float down at around 1.5 mph (2.4 km/h) taking around an hour to reach the ground.
4. Snow cover more areas in the northern hemisphere
The main reason for this difference is the distribution of the Earth’s continents. Major snow-prone areas include the polar regions, the upper half of the Northern Hemisphere and mountainous regions worldwide with sufficient moisture and cold temperatures. In the Southern Hemisphere, snow is confined primarily to mountainous areas, apart from Antarctica.
5. Snowstorm intensity may be categorized by visibility
- Light: visibility greater than 1 kilometer (0.6 mi)
- Moderate: visibility restrictions between 0.5 and 1 kilometer (0.3 and 0.6 mi)
- Heavy: visibility is less than 0.5 kilometers (0.3 mi)