Characteristics of Ice Storms in the United States

Abstract

Freezing rainstorms in the United States during 1949–2000 resulted in 87 catastrophic events, storms causing property losses of more than $1 million, with resulting losses totaling $16.3 billion. Catastrophes and their losses were greatest in the northeast, southeast, and central United States, and only 3% occurred in the western United States. A greater percentage of the freezing-rain occurrences in the deep South produced catastrophes than did freezing-rain occurrences in the northern United States, a result of differences in storm-producing conditions. A dense network of observers who measured ice storms during a 9-yr period provided definitive data on damaging storm areas and ice thicknesses on wires. The geographic distribution of the 368 damaging–ice storm areas matched well with that of catastrophes; both were greatest in a belt from the Southwest, across the Midwest, and into New England. Storm-area sizes ranged from 205 to 796 000 km2, with 50% being less than 21 840 km2. Most storm-area shapes were elongated with length–width ratios from 2:1 up to 15:1, although 18% were oblate shaped, particularly those in the northern High Plains and upper Midwest. Two-thirds of the ice-storm areas and catastrophe loss patterns assumed one of seven spatial types. The most prevalent type was a storm restricted to the Northeast, the second had losses in the Midwest and Northeast, and third was a storm in both the Northeast and Southeast regions. Five large-sized catastrophes extended from the Southwest, through the Midwest, into the Northeast, and these caused $3.5 billion in losses, 21% of the 52-yr total. The radial thickness of damaging ice on telegraph wires was sampled at 1689 sites, and the greatest average (>1.3 cm) and maximum (>5 cm) sizes occurred in the deep South and southern plains where storms have long durations and ample moisture aloft. Maximum and average ice thickness were relatively large in New England and the Northeast. Ice-thickness values were least in the upper Midwest and Pacific Northwest. The results collectively show that the greatest risk of ice-storm damages (based on event frequency, ice thickness, storm size and shape, and financial loss) is in the northeastern United States, followed by the risk in the lower Midwest and that in the southern United States.