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«INTRODUCTION Hail ! the word itself sends feelings of frustration through Colorado farmers. Each year, millions of dollars of agricultural losses ...»

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Nolan J. Doesken, Assistant State Climatologist

(Colorado Climate publication, April 1994, Volume 17, Number 7, Special Feature



Hail ! the word itself sends feelings of frustration through Colorado farmers.

Each year, millions of dollars of agricultural losses occur when hailstorms s weep

across the Eastern Plains. Hundreds of Colorado wheat farmers can tell tales of disappointment about years when their crop had survived drought, windstorms, winter cold, and insects only to be wiped out by hail the day before harvest. If it wasn't last year or the year before, then it might be this year or the next.

Hail is a pain, but it's also an unavoidable part of life east of the Rockies. All the w ay from Alberta, Canada, south to eastern New Mexico, hundreds (maybe thousands) of hailstorms develop each year. There is no other place in North America with more numerous or more severe hailstorms, and Colorado is right in the middle of it.

There are areas in Wyoming, Montana, South Dakota, Nebraska and New Mexico that may challenge Colorado as the hail capital of the U.S., but more often than not, Colorado takes that honor.

Hail used to be viewed as primarily an agricultural problem. The past 20 years, ho wever, has brought one catastrophic hailstorm after another to Front Range population centers. The culmination came July 11, 1990 when Denver took a direct hit by a prolific hail-making thunderstorm. When it w as all over, damage totals close to $600 million were reported ! the greatest property losses from hail ever reported from one storm up to that time. Property damage in Colorado has exceeded $50 million in 5 of the last 7 years. Front Range car dealers tremble every time the summer skies turn dark. Insurance agents have nightmares about being buried alive beneath piles of claim forms.

Colorado hail can also be life threatening. A child was killed in Fort Collins in 1979 when struck in the head by a large hailstone. There have been many instances of lesser injuries. Livestock fatalities from hail are fairly common.


We receive literally hundreds of questions each year about hail in Colorado.

Questions like, "Where can I set up greenhouses where the risk of hail won't be too great?" or "How often will stones larger than one inch fall at such-and-such location?" are common. We also find ourselves on both sides of insurance claims. Individuals w ho are filing claims but can't remember when the hail fell will call us. Likewise, insurance investigators routinely call or write to verify if hail did indeed occur at a particular time and place.

As I have attempted to ans wer these many questions, I have always been frustrated by the lack of information about hail. Systematic observations of hail are taken at only a handful of stations in Colorado. The National Weather Service offices at 1 Denver, Colorado Springs, Pueblo, Grand Junction and Alamosa have gathered hail information for many years ! but only right at their offices. Some of the 200+ cooperative weather stations in Colorado also report their hailstorms. The Fort Collins w eather station, for example, has more than 100 years of local hail reports.

The data from these fe w locations are very useful. Unfortunely, if you ask for information from Boulder, Lamar, Breckenridge or most any other location in Colorado, w e probably won't have much data to refer to. Since hail occurs only briefly (typically, just a fe w minutes per year even at the most hail prone locations) and tends to be very localized (Colorado hailstorms are at most a fe w miles wide), many storms go undetected by the "official" weather stations. For example, Denver Stapleton Airport, the source of Denver's hail data since 1950, only had a fe w hail stones on 11 July 1990 ! the day that much of the city was pulverized.

The National Weather Service (NWS), as a part of their duty to warn citizens of the threat of severe weather, obtains reports of severe weather from pilots, la w enforcement officers, ne ws media, local civil defense organizations, volunteer storm "spotters," private citizens and any other credible source. These data are used in real time to help issue and verify severe thunderstorm and tornado warnings. At the end of each month, severe weather reports are assembled, checked and then transmitted to the National Climatic Data Center. Several months later, the publication "Storm Data" is published for the country providing historical documentation of significant storms.

Another source of information is the insurance industry. In densely or uniformly populated regions of the country, property and crop insurance claims give a detailed picture of the locations and frequency of hail that greatly compliments weather station data. Here in Colorado, population is far from uniformly distributed, and farmland is not uniformly distributed. Thus, insurance data are not much help in improving hail information. Also, many farmers choose not to insure their crops against hail since the cost of insurance is so high. Some farmers try to self insure their crops by spreading their fields several miles apart so the likelihood is small that all their crops will be hit at the same time.

Weather radar can be used to detect hail. It is difficult, time consuming and expensive to go through years of past radar data to try to reconstruct storm locations, frequencies and intensities, so fe w such radar climatologies have been completed.

New NWS radars no w being deployed may make this task easier.


Our hail problem here in Colorado shouldn't be a surprise to us. Long before the 20th Century, Native Americans living on the High Plains were familiar with hail. Native American folklore referred to "Ice balls from the sky when summer winds blow from the east." Early European settlers knew about hail, but surprisingly little was written about it.

Climate summaries written prior to the late 1930s contained almost nothing about hail. A very small number of scientists embarked on descriptive hail climatologies around 1940.

After World War II, Air Force and commercial airline studies of hail began. A special network of volunteer weather observers was established in the Denver area in 1949 and operated for at least 10 years. Several fascinating papers were written in the 1950s and 1960s by W. Boynton Beck with of United Airlines using this data set.

Interest in hail research in Colorado expanded rapidly in the 1960s, and a number of published references can be found. Much of the interest centered around the 2 possibility of reducing hail damage by seeding clouds with silver iodide. This interest culminated in a large scientific experiment, the National Hail Research Experiment (NHRE), to learn more about hailstorms. This early 1970s project attracted scientists from around the world to places like Grover and Keota, Colorado. Cloud seeding aspects of this project attracted much controversy. The project came to a premature end, and much climatological information gathered on eastern Colorado hail was never analyzed.

Since the 1970s, most research has turned toward modelling and predicting severe storms. Studies of Colorado severe w eather and tornadoes have brought noticeable improvements in forecasting severe storms, but little information to better define the risks of hail has been assembled. In the past fe w years, ne w meteorological radars in Colorado are paving the way for expanded studies of storm characteristics.


The Colorado Climate Center has been working to improve climatological information about hail for Colorado decision makers. Back in the March 1988 issue of Colorado Climate we compiled some information about hail in Colorado. All significant hail reports for Colorado for a 13 year period, 1973-1985 were revie wed. The following map and graph sho w some of the features of Colorado hail patterns derived from those data.

In recent months, with the help of part time assistance from Natalie Tourville (High School intern) and Jim Harrington, we have no w updated our Colorado hail statistics. Based on more than 1,200 hailstorm reports, 1986more detail can no w be offered to better describe the characteristics of hail in Colorado.

The data used for this study included point weather station data from a small number of sites in and near Colorado along with statewide data on severe hailstorms obtained from the national publication, "Storm Data." It is important to note that to be reported as a severe storm, maximum hailstone diameter must be at least 3/4 inch.

Therefore, the numerous storms that produce smaller stones were usually not included in the state wide data unless they caused significant crop damage or accumulated to significant depths. Some of the larger hailstorms reported during this 8-year period included several storm data reports. In some of the analyses that follow, multiple reports w ere combined to define a single storm.


The hail season in Colorado begins in March and ends in October.

Average monthly distributions of hail (all sizes included) for selected locations sho w that overall, June has the highest frequency of days with hail. However, some individual sites, such as Fort Collins and Grand Junction, have more frequent hail in May. MayBAugust accounts for the vast majority of Colorado hail events. It would be very interesting to have mountain stations to add to this comparison. July-August thunderstorms are common throughout the Colorado high country, and many of these storms are accompanied by small and usually soft hail or graupel. This type of hail rarely does damage and is sometimes even reported as sno w.

Statewide severe hail day statistics sho w a similar monthly distribution. Out of an average of 37 days per year with large hail, June is the peak month with slightly more than 10 days. July has almost as many hail days. However, if you look at the actual number of hailstorm reports, June is clearly the leader with an average of 46 storms. This means that the number of severe hailstorms per hail day is larger in June than any other month.

There have been an average of more than 130 reported severe hailstorms each year since 1986.


Hail is primarily an afternoon or evening phenomenon here in Colorado. 90% of all severe hailstorms reported 1986occurred bet ween 1:00 p.m.

and 9:00 p.m. MST. Previous studies of hail at Fort Collins and in the Denver area, including both large and small hail, sho wed about 80% of all hail fell during those same hours. The least likely hours for hail in Colorado are bet ween 2:00 a.m. and 10:00 a.m. Less than 2% of the reported severe hailstorms occurred bet ween 4 11:00 p.m. and 10:00 a.m. with most of those occurring before 2:00 a.m.

There are some variations in the preferred times for hail at different times of the year and in different parts of the State. Nearly all reports of morning (5:00 a.m.-10:00 a.m.) hailstorms have been in April and May with a fe w in September. Some of these storms, including one in the Denver area on April 25, 1994, have dropped large quantities of hail, but stones are typically small. There is a detectable shift in preferred times of day for hail as you move east ward across Colorado. Most hail (including small stones) in and near the mountains occurs bet ween 11:00 a.m. and 6:00 p.m. Out near the eastern border of Colorado, storms are most likely from 3:00 p.m. to midnight. The large majority of severe hailstorms reported after 9:00 p.m. in Colorado have occurred over the eastern quarter of the State.

–  –  –


At any given point, hail usually only falls for a fe w minutes. Hail that continues for more than 15 minutes is unusual. A study of 60 Fort Collins hail events sho wed the median duration to be 6 minutes. Just over 10% of the storms lasted for more than 20 minutes, but these included most of the severe storms that included large stones. An a wesome hailstorm that hit parts of the Denver area on 13 June 1984 dropped stones as large as baseballs for up to 40 minutes straight.

While hail at a given point is usually short-lived, the storm complexes that produce hail may last for for several hours. The 11 July 1990 storm that crossed the Denver area began near Estes Park and continued south ward to El Paso County. This system lasted for more than 3 hours and dropped hail for most of that time. Severe thunderstorm systems out on the Eastern Plains have produced severe weather for 6 hours or longer.


The distribution of hailstone size is of critical importance for evaluating hail damage potential. Crops can be damaged by almost any size of hail. Even pea-sized stones can damage tender crops, especially if propelled by strong winds. Windblown marble-sized hail has been known to effectively strip paint from buildings. To damage vehicles and roofs requires larger stones. The NWS hail criteria for severe thunderstorms equalling or exceeding 3/4" diameter is consistent with the size of stones that begin to be capable of more extensive property damage. Since this study was primarily limited to severe storm reports, most reports are at least 3/4".

–  –  –

The most common size range for damaging hail in Colorado is 1 to 1.5" in diameter.

This size range, which includes the classic "golfball" size, accounts for more than 1/3 of the severe hailstorm reports during this study. Slightly more than 1/3 of the storm reports included maximum stone diameters greater than 1.5 inches. These are truly large stones by any definition. Six percent of the reported severe hailstorms had maximum stone diameters of 2.5" of greater. Huge hailstones 3 inches in diameter or greater are not common, but they have been reported in 7 of the last 8 years and probably occur briefly and over limited areas every summer somewhere in eastern Colorado. These stones are commonly classified as "baseball-sized" or larger.

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