Two inches of freezing rain causes power line failures and ice covered roads in central Nebraska.

WHAT is ice?

Ice can form on a surface when snow partially melts and refreezes. However, ice is most dangerous when it comes in the form of an ice storm. These storms are actually freezing rain (or freezing "drizzle") events that last for an extended period of time, usually from hours to days. Freezing rain is not a type of precipitation, but is instead just rain that freezes to a cold surface. This occurs in the winter when a layer of slightly warm air develops between a layer of cold air above it and a thin layer of cold air near the ground. As snow falls through the warm layer, it melts into rain drops. Because the cold layer at the ground is thin, it does not freeze before it hits the ground. Instead, it freezes onto the ground itself, as well as any human structures built on the ground. When this happens, A continuous sheet of ice on the ground is sometimes referred to as a "glaze" of ice.

Whenever water freezes onto roads, buildings, and other infrastructure, many hazards quickly present themselves. Ice on roadways greatly reduces traction, resulting in loss of vehicle control and collisions. Transportation delays caused by such conditions can result in millions of dollars of loss to urban economies in just a few days. Pedestrians lose their footing on slippery sidewalks, potentially falling and injuring themselves. Crops and other vegetation can be destroyed, resulting in significant economic losses. However, the greatest threat to both human life and economy during an ice storm occurs as the result of power and communication outages. Accumulations of ice on exposed utility lines can easily weaken and break these structures. The results are often devastating: entire states can be almost entirely without electricity and communication for several days, and sometimes even weeks.

Ice is also associated with low temperatures, which is dangerous to exposed skin, especially when wet from freezing rain or snow.


WHERE does ice occur?

In North America, ice storms are most common in the northern United States and southern Canada. In the northern United States, the states of New York and Pennsylvania have recorded the greatest number of ice storms, followed by Ohio and North Carolina. Although ice storms can also occur in the southern United States, they are much less common due to the greater number of days with above freezing surface temperatures. In general, mid-latitude regions between 40 degrees and 50 degrees latitude are most susceptible due to the frequent mixing of warm and cold air masses in these regions.

In January of 1998, the "Storm of the Century" in Canada was a freezing rain event that lasted for days, leaving four million people without electricity and costing over $5 billion (Canadian) in structural damage and economic loss combined. Over two dozen people died during this event, and approximately 1000 were injured.

WHEN does ice occur?

Ice is a threat to North America between the months of November and April. During this time, warm and cold air masses can mix over the central portion of the continent and provide the necessary conditions for freezing rain events. Besides these events, ice can also cover surfaces by the melting and refreezing of snow. This most frequently occurs during early and late winter when above freezing temperatures are common during the day, but below freezing temperatures still exist at night. Because this ice usually does not persist during the day when people are most active, it is considered slightly less hazardous than ice formed during ice storms, which is usually thicker and more difficult to melt.

HOW do we cope with ice?

The most important aspect of mitigating ice storms is forecasting and issuing warnings, for the purpose of community preparation. Communities can prepare for ice storms by stocking sand and salt to improve roadway and walkway conditions, advising people to use caution when leaving their homes, and advising people to stock food and batteries before a storm in case they lose power and are not able to leave their homes. In the United States, the National Weather Service issues winter storm warnings and freezing rain advisories when conditions are imminent or favorable for the development of an ice storm.

WHO has more information on ice?

• US National Severe Storms Laboratory Winter Weather Safety
  www.nssl.noaa.gov/edu/safety/winterweather.html
  
  
• NWS Quad Cities: Freezing Rain Climatology
  www.crh.noaa.gov/dvn/climate/ice/ice.php