Page 263 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

60
(b)
(103)
41
(53) (61)
29
PACIFIC OCEAN
120°
0.5–0.9 1.0–2.9 3.0–4.9 5.0–6.9 7.0–8.9
More than 9.0
(95)
60
135
PACIFIC CANADA OCEAN
CANADA
AVERAGE ANNUAL TORNADO INCIDENCE PER 10 000 KM2
Less than 0.5 0.5–0.99 1.0–2.49 2.5–4.99 5.0–7.49 7.5–10
Hudson Bay
(a)
MEXICO
Gulf of Mexico
90°
80°
(103)
54
CANADA
0
500 KM
200 180 160 140 120 100
80 60 40 20
(321)
180 (241)
171
UNITED STATES
ATLANTIC OCEAN
70°
AVERAGE ANNUAL TORNADO INCIDENCE
▲Figure 8.16 Tornado occurrence and frequency. Since tornado sightings are often concentrated around population centres, many tornadoes in rural areas may go unreported. [Data courtesy of
(a) David A. etkin, york University; (b) and (c) the Storm Prediction Centre, National Weather Service, and NOAA sources.]
because the air of the tropics is essentially homoge- neous, with no fronts or conflicting air masses of dif- fering temperatures. In addition, the warm air and warm seas ensure abundant water vapour and thus the necessary latent heat to fuel these storms. Tropical cyclones convert heat energy from the ocean into me- chanical energy in the wind—the warmer the ocean and atmosphere, the more intense the conversion and powerful the storm.
What triggers the start of a tropical cyclone? Cy- clonic motion begins with slow-moving easterly waves of low pressure in the trade-wind belt of the tropics (Figure 8.17). If the sea-surface temperatures exceed ap- proximately 26°C, a tropical cyclone may form along the eastern (leeward) side of one of these migrating troughs
(30)
17
PER 26 000 KM
2
(111)
96
0 (c)
20 22
JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
(27) (35)
30
0
250 500 KILOMETRES
(179)
109
of low pressure, a place of convergence and rainfall. Sur- face airflow then converges into the low-pressure area, ascends, and flows outward aloft. This important diver- gence aloft acts as a chimney, pulling more moisture- laden air into the developing system. To maintain and strengthen this vertical convective circulation, there must be little or no wind shear to interrupt or block the vertical airflow.
Physical Structure Tropical cyclones have steep pres- sure gradients that generate inward-spiraling winds toward the centre of low pressure—lower central pressure causes stronger pressure gradients, which in turn cause stronger winds. However, other factors come into play so that the storms with lowest central
Chapter 8 Weather
227
45
45
ATLANTIC OCEAN
60
45
40°
40°
30°
Average number of tornadoes per month (1950–2000)
(2003–2009)