Page 257 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 8 Weather 221
investigate severe weather forecasting, nowcasting (short- term forecast), disaster mitigation, ozone, risk assessment and prediction. Hazardous weather information can be found at the Environment Canada website—see www .ec.gc.ca/meteo-weather/default.asp?lang=En&n=15E59C08-1. In the United States, government research and monitoring of violent weather is centred at NOAA’s National Severe Storms Laboratory and Storm Prediction Centre—see www.nssl.noaa.gov/ and www.spc.noaa.gov/; consult these sites for each of the topics that follow.
Winter Storms and Blizzards
Winter storms and blizzards are types of violent weather that are generally confined to the mid- to high-latitude regions of the world. Winter storms are defined by En- vironment Canada (www.ec.gc.ca/meteo-weather/default .asp?lang=En&n=B8CD636F-1&def=allShow#wsDT6BFBCD1C) as major snowfall, or significant snowfall that is combined with freezing rain, strong winds, blowing snow, and/or extreme wind chill that pose a threat to public safety and property. Winter storm conditions may occur in the late autumn and early spring as well as through the winter season. In the United States, the National Weather Service further defines an ice storm as a particular winter storm in which at least 6.4 mm of ice accumulates on exposed surfaces. Ice storms occur when a layer of warm air is between two layers of cold air. When precipitation falls through the warm layer into a below-freezing layer of air nearer the ground layer, it may form a variety of freezing precipitation including freezing rain, ice glaze, and ice pellets (see The Human Denominator, Figure HD 8, at the end of the chapter). During a particular winter storm in January 1998, 700000 residents of a large region of Canada and the United States were without power for weeks as ice-coated power lines and tree limbs collapsed under the added weight. Freezing rain and drizzle continued for over 80 hours, more than double the typical ice-storm duration. In Montreal, over 100 mm of ice accumulated. Hypother- mia claimed 25 lives.
Blizzards are snowstorms with frequent gusts or sus- tained winds greater than 40 km·h−1 for a period of time longer than 4 hours and blowing snow that reduces vis- ibility to 400 m or less. These storms often result in large snowfall and can paralyze regional transportation both during the storm and for days afterward.
Thunderstorms
By definition, a thunderstorm is a type of turbulent weather accompanied by lightning and thunder. Such storms are characterised by a buildup of giant cumulonimbus clouds that can be associated with squall lines of heavy rain, in- cluding freezing precipitation, blustery winds, hail, and tornadoes. Thunderstorms may develop within an air mass, in a line along a front (particularly a cold front), or where mountain slopes cause orographic lifting.
Thousands of thunderstorms occur on Earth at any given moment. Equatorial regions and the ITCZ experience many of them, exemplified by the city of Kampala, Uganda, in East Africa (north of Lake Victoria), which sits virtually on the equator and averages a record 242 days a year with thunderstorms. In North America, most thunderstorms occur in areas dominated by mT air masses (Figure 8.12).
A thunderstorm is fuelled by the rapid upward move- ment of warm, moist air. As the air rises, cools, and con- denses to form clouds and precipitation, tremendous energy is liberated by the condensation of large quantities of water vapour. This process locally heats the air, causing violent updrafts and downdrafts as rising parcels of air pull surrounding air into the column and as the frictional drag of raindrops pulls air toward the ground (review the illustration of a cumulus cloud in Figure 7.18).
Turbulence and Wind Shear A distinguishing char- acteristic of thunderstorms is turbulence, which is cre- ated by the mixing of air of different densities or by air layers moving at different speeds and directions in the atmosphere. Thunderstorm activity also depends on wind shear, the variation of wind speed and direction with altitude—high wind shear (extreme and sudden varia- tion) is needed to produce hail and tornadoes, two by-products of thunderstorm activity.
Thunderstorms can produce severe turbulence in the form of downbursts, which are strong downdrafts that cause exceptionally strong winds near the ground. Downbursts are classified by size: A macroburst is at least 4.0 km wide and in excess of 210 km·h−1; a micro- burst is smaller in size and speed. Downbursts are char- acterised by the dreaded high-wind-shear conditions that can bring down aircraft. Such turbulence events are short-lived and hard to detect. In the United States, NOAA’s forecasting model, launched in 2012, gives hourly updates to improve predictions for severe weather
Georeport 8.3 Stormy Seas and Maritime Tragedy
February 2013 was a month of frequent winter storms across eastern North America. On February 17, the third storm
in two weeks travelled up the eastern seaboard of North America delivering wet snow, rain, and freezing rain to Nova Scotia
and heavy snow to New Brunswick. Winds of 160 to 180 km·h−1 downed power lines and closed schools and businesses. Along the coast, 10 m waves and high winds made conditions treacherous. Miss Ally, a fishing vessel from Woods Harbour, Nova Scotia, unable to return to shore amidst hurricane force winds and zero visibility, went down, taking her crew of five with her. The young fishers lost their lives as weather conditions severely hampered search and rescue efforts.