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Interactives
WEATHER: What forces affect our weather?
Ice and Snow
Introduction
Atmosphere
Water Cycle
Powerful Storms
Ice and Snow
Forecasting
Changing Climate
Resources


In January 1996, parts of Virginia recorded up to four feet of snow during a record-breaking blizzard. When the Blizzard of 1996 was over, nine states had declared a state of emergency. One hundred people were dead.

Blizzards such as this one are the most severe forms of snowy weather, with winds of 35 miles per hour or greater and visibility of less than 1/4 mile. Winter storms and blizzards may not always be as dramatic as the one that blanketed the northeastern United States in 1996, but their potential to disrupt everyday activities is great. Schools close. Commerce is interrupted. Property damage can occur from flooding or structural collapse. These storms may not rival hurricanes or tornadoes in destructive power, but they are an effective annual reminder of our planet's dynamic climate.

How does snow form?

Snow begins in the atmosphere as water condenses into a tiny droplet. As more and more water vapor condenses onto its surface, the droplet grows. Cold air then freezes this water into an ice crystal.

Each ice crystal has a unique shape that depends on the surrounding air's temperature and water vapor content. If it is below freezing and there is a lot of water vapor in the air, the crystal grows six evenly spaced branches. More and more water vapor collects on these branches and freezes, making the ice crystal increasingly heavy. Eventually, the ice crystal falls from the sky, leaving the cloud of precipitation that it helped to form. As it falls, the crystal continues to grow by picking up more water vapor.

As it descends, the ice crystal can come into contact with warmer air that makes it melt somewhat. This melting acts like a glue, causing crystals to bond together into larger flakes, forming what many people think of as the "classic" fluffy snowflake. If the crystals melt too much and then refreeze as they get closer to Earth's surface, the precipitation falls as sleet instead of snow.

Once on the ground, snow will remain if temperatures are cold enough to keep it from melting. Glaciers that form on mountains, for example, are made up of snow that accumulates on the ground and eventually turns to ice.

Are ice ages cyclical? Should we expect another one anytime soon? Find out more in "Glaciers on the Move: Ice Ages in History."

 "Weather" is inspired by programs from Planet Earth.

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