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Climate

Snow cover of Earth between Mar. 2000 and Dec. 2018

Snow cover of Earth
Blue Marble: Next Generation offers greater spatial detail of the surface and spans a longer data collection period than the original. The original Blue Marble was a composite of four months of MODIS observations with a spatial resolution (level of detail) of 1 square kilometer per pixel. Blue Marble: Next Generation offers a years worth of monthly composites at a spatial resolution of 500 meters. These monthly images reveal seasonal changes to the land surface: the green-up and dying-back of vegetation in temperate regions such as North America and Europe, dry and wet seasons in the tropics, and advancing and retreating Northern Hemisphere snow cover. From a computer processing standpoint, the major improvement is the development of a new technique for allowing the computer to automatically recognize and remove cloud-contaminated or otherwise bad data—a process that was previously done manually. Blue Marble: Next Generation improves the techniques for turning satellite data into digital images. Among the key improvements is greater detail in areas that usually appear very dark to the satellite (because a large amount of sunlight is being absorbed), for example in dense tropical forests. The ability to create a digital image that provides great detail in darker regions without washing out brighter regions, like glaciers, snow-covered areas, and deserts is one of the great challenges of visualizing satellite data. The new version also improves image clarity, and gives highly reflective land surfaces, such as salt flats, a more realistic appearance. Image: NASA Visible Earth

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.

These snow cover maps are made from observations collected by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite. Snow cover values range from medium blue (greater than 0 percent) to white (100 percent). Landmasses that do not sustain snow cover and places where MODIS did not collect data are gray. Because MODIS relies on visible light to assess snow cover, the sensor cannot collect data over the highest latitudes of the Northern Hemisphere during winter when no sunlight reaches those regions. Snow and ice cover most of the Earth’s polar regions throughout the year, but the coverage at lower latitudes depends on season and elevation. High-altitude landscapes such as the Tibetan Plateau and the Andes and Rocky Mountains maintain some amount of snow cover almost year-round. Land area is larger and snow cover is more variable in the Northern Hemisphere than in the Southern Hemisphere. Source: NASA Earth Observatory

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. The 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.

Snow cover in Belgrade Forest
Belgrade Forest (Istanbul) under snow. I took this photo in January 2012.

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.

Spain under a snow blanket (cropped)

Related: Spain under a snow blanket

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 that collect supercooled water as they fall can fall at up to 9 mph (14.5 km/h), but snowflakes, 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)

Sources

M. Özgür Nevres
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Özgür Nevres

By M. Özgür Nevres

I am a software developer and a science enthusiast. I was graduated from the Istanbul Technical University (ITU), Computer Engineering. In the past, I worked at the Istanbul Technical University Science Center as a science instructor. I write about the planet Earth and science on this website, ourplnt.com. I am also an animal lover! I take care of stray cats & dogs. This website's all income goes directly to our furry friends. Please consider supporting me on Patreon, so I can help more animals!

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