9.11.5
What is a blizzard?
Snowstorms are periods of intense snowfall with high winds. The most dangerous type is a blizzard. A snowstorm becomes a blizzard when wind speeds stay above 56 km/h or frequently gust over this speed for more than three hours. In a blizzard, visibility drops to less than 400 m, and in extreme cases, it can be less than a metre. Often, snow isn’t falling but is blown into snowdrifts that can bury people and objects. Blizzards form when warm and cold air meet, creating strong winds due to changes in air pressure.
SkillBuilder discussion Communicating 1. What happens when very cold air from a polar region collides with warmair? 2. How do snowflakes form in the clouds? 3. What happens when warm air rises over a cold ocean surface?
FIGURE10 How a blizzard forms
Ice crystals within a cloud collide and stick together, forming snowflakes.
Very cold air moves from a polar region and collides with very warm air.
As warm air rises, it cools, and clouds form.
If cooling continues, precipitation in the form of snow occurs.
Rising warm air
Winds blowing over cold ocean surfaces are colder than usual.
Snow can fall only when the air temperature is below 4 °C. At higher temperatures, the snow melts in the air and falls as rain or sleet.
Landmass
When cold air crosses a warmer landmass, the warmer air is forced to rise. Incoming cold air (from over the cold ocean) replaces the rising warm air.
Cold ocean surface area
9.11.6
Why are blizzards dangerous?
During snowstorms, snow accumulates. It can be difficult to know the depth of the snow, making movement difficult. There is an increased risk of falling through thin ice or into crevasses . Heavy snow on slopes can trigger avalanches (see FIGURE11 ), caused by earthquakes, noises or animals. Blizzards can cause whiteouts , reducing visibility to one metre. People and animals cannot tell the difference between the Earth and the sky, leading to disorientation. Exposure to the extreme cold increases the risk of hypothermia , frostbite and suffocation. In Antarctica, researchers face snow build-up; the Halley VI Research Station ( FIGURE12 ) is built on steel legs with skis to be raised and moved, avoiding snow dangers.
SkillBuilder discussion Concluding and decision-making 1. What type of building is shown in FIGURE12 ? 2. Why do you think the building is raised on legs? 3. How might this structure be useful in a snowy environment?
TOPIC9 Water in the world 321
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