More snowy science lessons

By Jack Williams ©2015

Does “blizzard” have an official definition?

The National Weather Service considers a storm to be a blizzard when it has winds of 35 mph or faster, low temperatures (no specific figure given), and sufficient snow in the air to reduce visibility to less than 0.25 miles.

When such conditions are forecast to last at lest three hours the NWS issues a blizzard warning. As the American Meteorological Society’s Glossary of Weather and Climate notes: “In common use in the United States and England, the term is often used for any heavy snowstorm accompanied by strong winds.

The Glossary also says that the wind, snow, and low-visibility hazards of a blizzard the threat to life “increases dramatically whe the temperature falls below 20 degrees Fahrenheit. The Glossary is available from the AMS online book store.

What’s a bomb storm?

When a strong storm threatens the East Coast you sometimes hear broadcast meteorologists and others talk about the storm being a “meteorological bomb” or say that the storm is expected to “bomb out.”

A meteorological bomb is an extratropical low (not a hurricane) “in which the central barometric pressure drops at least 24 millibars in 24 hours. Some storms have intensified as rapidly as 60 millibars in a 24 hour period. A few bomb cyclones even develop “eyes”, similar to the center of a hurricane,”  as I wrote in a aricle approximately 10 years ago.

Fred Sanders, one of the giants of 20th century meteorology, coined the term while working  as a transatlantic aviation forecaster for the U.S. Weather Bureau at La Guardia Airport in New York City. He died in 2006 and an MIT News obituary describes his career. He and John R. Gyakum described “bombs” in detail and discussed forecasting problems in the October 1980 issue of The Monthly Weather Review, in the article “Synoptic-Dynamic Climatology of the ‘Bomb'”.

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