«INTRODUCTION Hail ! the word itself sends feelings of frustration through Colorado farmers. Each year, millions of dollars of agricultural losses ...»
The maximum stone size reported in this study was 4.5 inches. Such stones may fall at speeds of close to 90 miles per hour and can do incredible damage. Not only do these stones dent cars and break windshields, they can penetrate corrogated metal as w ell as asphalt shingle/ply wood roofs. Very fe w stones ever exceed this size, but the largest documented hailstone any where in the U.S. was found in Kansas. It was 5.5 inches in diameter and weighed nearly 2 pounds. (Note to all readers: If you ever become aware of a Colorado hailstone of a comparable size, please contact us immediately. Be ready to provide witnesses and photographic documentation.)
We have also performed a single-station analysis of hail size distribution using all reported hail of any size (see below). Based on hail data collected 1962-1993 at the Colorado State University campus weather station, we found that only 11% of the reported hail events included stones sizes of 3/4 inch or greater. Hail in excess of 1 inch diameter has occurred only t wice in the past 32 years. While large hail may be common somewhere within a large area, this suggests that at a point the risk of severely damaging hail may not be quite as great as we think. It may be possible for some of our roofs to gro w old naturally.
MONTHLY AND INTERANNUAL VARIABILITY
One of the big challenges of trying to deal with hail is its variability. An area can go decades without a severe hailstorm and then be hit three years in a ro w. The graph below gives an indication of year-to-year variations in hail frequencies at a point. More than 100 years of hail observations have been gathered by the National Weather Service in Denver. The annual number of hail days (including stones of any size) has ranged from 0 to 11.
7 The numbers seem to suggest an up ward trend in Colorado hailstorms. We predict this trend will continue, but not because hail is actually increasing. Rather, we believe that gro wing population, more cellular phones and greater a ware-ness will mean that more storms will be reported in the years ahead.
SPATIAL DISTRIBUTIONEach of the approximately 1,200 reported severe hailstorms was plotted as a single dot on the map below. This is not a totally appropriate method for displaying hail occurrences. Some storms were only severe in a very small area, but some storms produced long hail s waths. The point method is clearly inadequate for presenting spatial characteristics of hail, but we have no better data sources at this time.
8 Two features of Colorado hail are evident here. 1) Severe hail is not a problem state wide. Rather it is clearly limited to eastern Colorado beginning in the eastern foothills and extending across all the the Eastern Plains. Out of the more than 1,200 severe hail reports state wide in the past 8 years, only about 50 were in the mountains or on the Western Slope. Of these western Colorado hailstorms, fe w produced significant property damage and only a handful included stone diameters in excess of 1 inch. 2) Local details of storm concentrations east of the mountains are probably (and unfortunately) not realistic. Using the type of data available to us, hail patterns are strongly influenced by population density. The more people and personal property there are, the more severe hail reports we receive. Not only do towns and cities sho w up clearly on the map, so do highways. U.S. Highw ay 24 from Colorado Springs to Limon sho ws up clearly on the map even though fe w people live along that road.
To try to more accurately define the distribution of damaging hail in Colorado, the number of severe hailstorms per county were mapped. These values were then divided by the population (1990 Census) and expressed as hailstorms per 1,000 people. This paints quite a different picture of the spatial distribution of Colorado hail. While El Paso and Weld Counties were the leaders in reported storms, the greatest frequency of per 9 capita severe hail occurs in eastern Colorado near the Kansas and Nebraska borders.
But this, too, may be misleading.
Meteorological evidence (radar, satellite, historic w eather observations) points to the Palmer Ridge (high ground bet ween Denver and Colorado Springs that extends east ward beyond Limon) and the Cheyenne Ridge (high ground that extends east ward along the Colorado-Wyoming-Nebraska borders) as the most hail-prone regions of Colorado. Our study does not sho w these areas to be unusually stormy with respect to adjacent areas. However, except for U.S. Highway 24, these areas have little population, little transportation, and not much agriculture. Our experience with hail reporting also suggests that where people are most accustomed to hail, they are likely to only report extremely severe storms, so it remains very possible that these areas are indeed more hail prone.
Results of mapping hail, although somewhat dissappointing, still contain helpful information. For example, there appears to be a distinctly lower hail risk in Boulder and Longmont than in other Front Range cities. Also, despite relatively dense population and intense agricultural activities along the South Platte River from Denver north to Greeley, the number of hail reports there are relatively low. By comparison, the Lafayette area east of Boulder has had many hail reports. A relatively large number of severe hailstorms have also been reported north of Greeley along U.S. Highway 85. The Wiggins area along with Sedg wick-Julesburg have been especially active during the 1986-93 period.
There is considerable anecdotal evidence of preferred "hail paths" in eastern Colorado and along the Front Range. This might very well be true. At this point, we do not have the enough information to prove it one w ay or the other. Even when the results of the 1973-1985 study are combined, consistent patterns do not emerge.
MEMORABLE HAILSTORMSThe storms we remember most are the storms that get the most attention in the media. Many of Colorado's largest hailstorms plaster the Eastern Plains, flatten wheat fields, bruise cattle but pass unnoticed by most of us. I will list a fe w dates, locations
There are some very good reasons why Colorado and similar locations just east of the tall Rocky Mountain barrier are so prone to hail. Contrasting dry continental air masses and humid subtropical air from the Gulf of Mexico often clash just east of the Rockies in late spring and summer. This is a key ingredient for severe thunderstorm development. The nearby mountains serve as preferred initiation points for thunderstorm formation.
The high elevations of the western Great Plains also enhances hail potential in two ways. First, the high ground warms quickly under the intense western sunshine and provides an elevated heat source that intensifies convective updrafts. The greater the vertical speed of air within a cloud, the greater the hail potential. The cumulonimbus clouds (thunderheads) associated with Colorado's severe hailstorms frequently climb to heights of 45,000 feet or more above ground. Secondly, the high elevation means that hail does not have as far to travel to reach the ground. Thus, the chances of it melting are reduced. This is further supported by the dry air that typically lies just west of the Great Plains storms. Precipitation evaporating into the nearby dry air cools the air further, increases do wndrafts and increases the likelihood that the hail will hit the ground before it melts. Many spring and summer thunderstorms across the eastern and southern U.S. also contain hail, but that hail usually melts before it reaches the ground.
Many other graphs and data summaries were developed during the course of this research which cannot be shown in the compresses report. If you have more detailed questions or additional information about Colorado hail, please contact the Colorado Climate Center.