Figure 24. Freezing rain superstations in the Northeast.
present weather), Mount Washington Observatory (3-hourly weather data for most of the period
we defined the superstations shown in Fig. 24. While Barre was not included in a superstation, it
was ultimately grouped with other stations in a large amorphous region. The number of extreme
ice storms at these superstations is considerably less than the thousand that Peterka considers
ideal; however, sampling error is significantly reduced in the superstations compared to the indi-
vidual stations.
7.3 Extreme ice loads
Table 6. Probability of exceeding the load for a given
7.3.1 Ice loads for a 50-year return period
return period at least once.
A load that has a 2% chance of exceed-
pN = 1 (1 p1)N
ance in any year is called a 50-year return-
period load. A 50-year return-period load
Return period for load
does not recur every fifty years. It may hap-
Lifetime
pen two years in a row, twice in 30 years, or
(years)
50 years
100 years
300 years
only once in a particular 100-year period.
1
.02
.01
.003
There is a 64% probability that a 50-year
2
.04
.02
.01
load will be exceeded in any 50-year period
5
.10
.05
.02
and 39% and 15% probabilities that 100-year
10
.18
.10
.03
and 300-year return-period loads, respective-
20
.33
.18
.06
ly, will be exceeded in 50 years (Table 6).
40
.55
.33
.13
In the extreme value analysis, we found
50
.64
.39
.15
the 75th-percentile ice load to be a good
100
.87
.63
.28
threshold for extreme ice loads for this
300
1.00
.95
.63
region. It results in threshold uniform ice
41
Back to contents page
Previous Page
