Page 260 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

224 part II The Water, Weather, and Climate Systems
0.0 0.1 0.2 0.4 0.6 0.8 1.0 2.0 4.0 6.0 8.0 10.0 15.0 20.0 30.0 40.0 50.0 70.0 Flashes km−2 · yr−1
(a) Map of total annual lightning strikes (flashes) from January 1998 to February 2012. The lightning imaging sensor aboard the TRMM satellite combines optical and electronic elements that can detect lightning within individual storms, day or night, between 35° N and 35° S latitudes.
◀Figure 8.13 Global lightning strikes, 1998 to 2012. [(a) Lightning imaging Sensor-Optical Transient Detec- tor (LiS-OTD) global lightning image obtained from the NASA eOSDiS global Hydrology resource Centre DAAC, Huntsville, AL. reprinted by permission of richard Blakeslee. (b) Keith Kent/ Science Source.]
areas. Hail typically occurs in Canada in heavy but local- ized showers associated with mature thunderstorms. Most hailstorms develop in the continental interior, namely central Alberta’s “hailstorm alley,” in the lee of the Rock- ies and the southernmost part of Saskatchewan, east of Cypress Hills. These areas experience four to six major hail events each year. The period of most frequent hail occurrence is May to July, with nearly three quarters of all hailstorms occurring be- tween noon and 6:00 p.m.
In Canada, hailstorms cause the greatest economic losses of any natural hazard in terms of property and crop
(b) Multiple lightning strikes in southern Arizona captured in a time-lapse photo.
to softballs (11.43 cm). For larger hail to form, the frozen pellets must stay aloft for longer periods. The largest authenticated hailstone in the world fell from a thun- derstorm supercell in Aurora, Nebraska, June 22, 2003, measuring 47.62 cm in circumference. However, the larg- est hailstone by diameter and weight fell in Vivian, South Dakota, in July 2010 (Figure 8.14).
Hail is common in Canada and the United States, although somewhat infrequent at any given place. Hail occurs perhaps every 1 or 2 years in high frequency
damage. This, along with risk of droughts and floods, is why collectively, farmers pay millions of dollars each year for crop insurance. The pattern of hail occurrence across Canada and the United States is similar to that of thunderstorms shown in Figure 8.12.
Damaging Winds
Straight-line winds associated with fast-moving, severe thunderstorms can cause significant damage to urban areas, as well as crop losses in agricultural regions. Several terms are used to describe damaging winds, in- cluding straight-line winds, downbursts, microbursts, plough winds, or derechos. The first three terms are used commonly in eastern Canada while the term plough wind is more frequently used in the west. Microburst is used to describe damage to an area less than 4 km2. Downbursts from convective storms can produce groups of down- burst clusters from a thunderstorm system that can cover very large areas. These winds pose distinct hazards to summer outdoor activities by overturning boats, hurling
>Figure 8.14 Largest-diametre hailstone ever recorded. This hailstone measuring 20.32 cm in diametre fell in South Dakota during a supercell thunderstorm with winds exceeding 129 km∙h−1. its circumference measured 47.307 cm, just under the world record. read more about this hail event at www.crh.noaa.gov/abr/ ?n=stormdamagetemplate. [NOAA.]