Page 259 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

180°
Bering Sea
170°
160°
150°
Nome
Barrow
ARCTIC OCEAN Beaufort
Sea
Inuvik
60°
>0
60°
1
1
>0
Arctic Circle
5
1
>0
1
50°
5
20
20 15
10
10
1
50°
1
10
5
20
5
1
40
5
20
5
20
10
20
1
20
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10 55
10
50
50 50
40
30°
60
30
140°
DAYS WITH THUNDERSTORMS
100 and over 80–99
60–79
40–59
20–39 Under
130°
PACIFIC OCEAN
Regina
Bismarck Fargo Duluth
Caribou 15 Québec
Montréal Portland
Halifax
15
Kodiak
Labrador Sea
15
50
50
30°
70
10
20
50
60
50
70
80
40
100
20
Gulf of Alaska
Whitehorse Juneau
Vancouver
Seattle Spokane
Fort Yukon Fairbanks
Anchorage
Portland Roseburg
Eureka
Sacramento San Francisco
Fresno
Los Angeles San Diego
120°
Missoula
Boise
Salt Lake
Great Falls Billings
Cheyenne Denver
Grand Junction
Winnipeg International Falls
Sault
Ste. Marie
Ottawa
Toronto Albany
Reno
City Milford
Columbus Charleston
Richmond ATLANTIC
Las Vegas
Yuma
St. Louis
Edmonton
Hudson Bay
Green Bay
Albuquerque Phoenix
Amarillo
Little Rock
Memphis Atlanta
Wilmington Columbia
20°
110°
80° 70°
Resolute
Baker Lake
Churchill
L. Winnipeg
Baffin Bay
30°
40°
50°
St. John’s
Minneapolis Rapid City
Hartford
North Platte
Des Chicago Lincoln Springfield
Pittsburgh Philadelphia
Abilene El Paso
Del Rio Antonio
Jackson
Fort Shreveport Worth
Houston
250
Jacksonville
Miami
▲Figure 8.12 Thunderstorm occurrence. Average annual number of days experiencing thunderstorms. Compare this map with the location of mT air masses in Figure 8.1. [Data courtesy of NWS; Map Series 3; Climatic Atlas of Canada, Atmospheric Environment Service, Canada.]
lightning and thunder An estimated 8 million light- ning strikes occur each day on Earth. Lightning is the term for flashes of light caused by enormous electrical discharges—tens of millions to hundreds of millions of volts—that briefly superheat the air to temperatures of 15 000°–30 000°C. A buildup of electrical-energy po- larity between areas within a cumulonimbus cloud or between the cloud and the ground creates lightning. The violent expansion of this abruptly heated air sends shock waves through the atmosphere as the sonic bang of thunder.
Lightning poses a hazard to aircraft, people, animals, trees, and structures, causing nearly 200 deaths and thou- sands of injuries each year in North America. When light- ning is imminent, Environment Canada and, in the United States, the NWS issues severe storm warnings and cau- tions people to remain indoors. People caught outdoors as
a lightning charge builds should not seek shelter beneath a tree, as trees are good conductors of electricity and often are hit by lightning. Data from NASA’s Lightning Imaging Sensor (LIS) show that about 90% of all strikes occur over land in response to increased convection over relatively warmer continental surfaces (Figure 8.13; see thunder.msfc .nasa.gov/data/data_nldn.html).
Hail Ice pellets larger than 0.5 cm that form within a cu- mulonimbus cloud are known as hail—or hailstones, after they fall to the ground. During hail formation, raindrops circulate repeatedly above and below the freezing level in the cloud, adding layers of ice until the circulation in the cloud can no longer support their weight. Hail may also grow from the addition of moisture on a snow pellet.
Pea-sized hail (0.63 cm in diametre) is common, al- though hail can range from the size of quarters (2.54 cm)
San
Detroit Moines Indianapolis
New York
Kansas City
Oklahoma City
Lexington Knoxville
Norfolk Raleigh
OCEAN
0
Birmingham
New Orleans
Tampa
500 KILOMETRES 90°
Chapter 8 Weather
223
Boston
60°
20°
Tropic
of
Cancer