Lightning

Description

Lightning is an electrical discharge between positive and negative regions of a thunderstorm. A lightning flash is composed of a series of strokes with an average of about four. The length and duration of each lightning stroke vary, but typically average about 30 microseconds. Typically, thunderstorms include rain, hail, or other forms of precipitation. However, it is possible for a thunderstorm to produce lightning with no delivery of precipitation. These events are called ‘dry thunderstorms.’

Intra-cloud lightning is the most common type of discharge. This occurs between oppositely charged centers within the same cloud. Usually it takes place inside the cloud and looks from the outside of the cloud like a diffuse brightening that flickers. However, the flash may exit the boundary of the cloud, and a bright channel, similar to a cloud-to-ground flash, can be visible for many miles.

Cloud-to-ground lightning is the most damaging and dangerous form of lightning, though it is less common than intra-cloud occurrences. Most flashes originate near the lower-negative charge center and deliver negative charge to earth. However, some flashes carry positive charge to earth. These positive flashes often occur during the dissipating stage of a thunderstorm’s life. Positive flashes are also more common as a percentage of total ground strikes during the winter months. This type of lightning is particularly dangerous for several reasons. It frequently strikes away from the rain core, either ahead or behind the thunderstorm, and can strike as far as 5 or 10 miles from the storm and occur in areas where common observers may not recognize the danger. Positive lightning also has a longer duration, so fires are more easily ignited. Positive lightning strikes usually carry a high peak electrical current, which may potentially result in greater damage.

The ratio of cloud-to-ground and intra-cloud lightning varies significantly between storms. Depending upon cloud height above ground and changes in electric field strength between cloud and earth, the discharge either stays within the cloud or makes direct contact with the earth. If the field strength is highest in the lower regions of the cloud, a downward flash may occur from cloud to earth. Using a network of lightning detection systems, the United States monitors an average of 22 million strokes of lightning from the cloud-to-ground every year.

According to the Colorado Division of Homeland Security and Emergency Management, lightning is the number one life threatening weather hazard. Each year, lightning is responsible for deaths, injuries, and millions of dollars in property damage, including damage to buildings, communications systems, power lines, and electrical systems. Lightning also causes forest and brush fires, and deaths and injuries to livestock and other animals. According to the National Lightning Safety Institute, lightning causes more than 24,600 fires in the United States each year. The Institute estimates annual damages from lightning to be approximately $4-5 billion in the US. Lightning is so significant in Colorado that the Governor declares an annual Lightning and Wildfire Awareness Week each summer. According to NOAA, Colorado ranks 5th out of all states in total lightning caused fatalities from 1959 to 2016. Additionally, NOAA ranks Colorado 19th in the nation in the number of cloud-to-ground lightning flashes and 32nd in the nation in overall flash density with 4.8 flashes per square kilometer.

Previous Occurrences

There are approximately 2,000 thunderstorms occurring globally at any one time, with 75-100 cloud-to- ground lightning strikes per second. The NCEI storm events database lists 33 significant lightning strike events since 1995 in Jefferson County. Impacts of these strikes generally can be drawn into two categories:

Strikes that are notable because of human injury or fatality (7 strikes). These primarily occur when the victim is unsheltered during a lightning storm.

Strikes that are notable because of property damage (12 strikes). Most damages occurred to single properties.

The selections below demonstrate some events which caused notable injury, death, or property damage, and those events which triggered wildfires. (See Section 4.3.17 for more information on wildfire risk.) These records, drawn from the NCEI database, illustrate the wide variety of impacts that lightning poses to the planning area.

May 29, 1995 – Lightning struck a soccer goal post and injured six adults viewing a soccer game. Although no one received a direct hit, one woman was hospitalized.

September 4, 1995 – Two people were injured when lightning struck their home. The lightning entered in the attic where it sparked a small fire. It then travelled through the walls exploding a mirror that sprayed glass on the residents. Damages were estimated at $4,500.

July 3 - 5, 1996 – Lightning from a fast moving thunderstorm blasted a large hole in the side of a house in Lakewood, southwest of Denver. Lightning sparked a small fire near Buffalo Creek. Only one acre was burned before the fire was contained.

September 2, 1996 – Lightning sparked a brush fire in the south buffer zone of the Rocky Flats Environmental Test Facility. No structures were damaged but the fire burned approximately 100 acres of grassland before it was contained.

July 29, 1997 – A woman received minor injuries when lightning struck her when it passed through the office window. She suffered temporary blindness for approximately 15 minutes.

August 13, 2000 – Lightning sparked three separate grassfires near Golden. The fires were quickly contained, however.

May 30, 2001 – Lightning ignited a fire which destroyed a luxury home on Bear Mountain near Evergreen, resulting in a recorded $1 million in property damage.

May 27, 2002 – Lightning sparked a wildfire near Deckers. Extremely dry conditions and very strong winds the following day allowed the fire to consume 3,860 acres before it could be contained. Thirteen structures were destroyed, including 4 homes. This incident is discussed further in the wildfire hazard profile.

June 19, 2002 – Lightning damaged the Evergreen Fire Protection District (EFPD) repeater. One microwave transmitter, the main fire channel transmitter and two solar panel controllers were ruined. Damage costs were estimated at $5,000.

August 1, 2001 – Lightning coupled with strong thunderstorm winds knocked out power to approximately 10,000 Xcel Energy customers in Golden.

May 29, 2004 – A father and son practicing on the driving range at the Meadows Golf Club were struck by lightning. The father was killed and the teenage boy was seriously injured. Three other people standing nearby only received minor injuries.

July 23, 2004 – Lightning caused a power outage in Arvada, leaving approximately 9,800 customers without power for 90 minutes.

July 27, 2007 – A man was struck and killed by lightning while jogging at Matthews Winters Park in Morrison. The thunderstorm produced numerous lightning strikes and caused a power outage at Red Rocks Amphitheatre, which forced the cancellation of a concert later in the evening. Damages were reported at $5,000.

August 4, 2008 – Lightning sparked a grassfire that consumed 300 acres on the northern edge of Green Mountain. Gusty winds and very dry conditions allowed the fire to spread quickly and threaten several homes. Only minor damage was reported, caused by smoke and melted siding. Damages were estimated at $100,000.

August 16, 2010 – Lightning struck a tree in Morrison; separately, a lightning strike sparked a small grass fire near Quaker Street and Golden Road in Golden. It was quickly extinguished by emergency responders.

May 23, 2011 – Lightning struck a park ranger’s office in Evergreen and destroyed a nearby gasoline storage tank. Damages were estimated at $1,000.

June 6, 2012 – Lightning struck a home in Lakewood, causing extensive electrical damage. Damages were estimated at $20,000.

July 7, 2014 – A man in Arvada was injured by a nearby lightning strike while he recorded a video of a thunderstorm with his cell phone. He was standing in his garage, when a nearby lightning bolt knocked him out. He suffered overall body aches and had a ringing sensation in one of his ears.

August 8, 2014 – A man in Evergreen suffered minor injuries when he was struck by lightning, which entered through his finger, traveled down his body, and exited his foot.

July 19, 2016 – Two men at the Indian Tree Golf Course in Arvada were struck by lightning when they sought shelter beneath a tree during a rapidly developing thunderstorm. One man suffered minor injuries, while the other died from his injuries.

Geographic Extent

The geographic extent for lightning may be examined in two ways. In one regard, ‘lightning’ is a regional hazard measured by the possible places of occurrence. In the other, ‘lightning incidents’ refer to single- point occurrences and are measured according to density. Examining the density of the lightning flashes may yield more useful information, particularly when the impacts of the hazard are examined. According to the NOAA, Jefferson County averages 7,000 lightning strikes per year. This results in approximately

8.9 lightning strikes per square mile per year (7000/785 mi2). indicates that, for the most part, Colorado’s Front Range experiences an average density rating that is higher than the rest of the state and much of the country. Therefore, while 100% of the planning area is vulnerable to lightning strikes, the density of these single-point occurrences is fairly limited.

Based on this information, the geographic extent rating for lightning is limited. Figure 4-41 Cloud-to-Ground Lightning Density

Source: Vaisala 2020 Annual Lightning Report,

Probability of Future Occurrences

As identified earlier, lightning occurs thousands of times a year in Colorado alone. According to information retrieved from NOAA, the planning area receives an average of 8.9 lightning strikes per square mile. This means the planning area, which is 785 square miles in size, experiences an average of 6,987 cloud-to-ground strikes of lightning a year. Knowing that the probability of any lightning event occurring in the future is highly likely helps underscore the importance of increased public education about the hazard. In order to fairly compare the lightning hazard to other hazards in the planning area, the probability of future occurrences for a lightning event that causes damage should also be computed.

The NCEI database is the only available dataset for county-specific lightning incidents that includes property and fire damages. Although this dataset is probably incomplete, it will be used as the source for the probability of occurrence calculation below. If additional lightning data becomes available for Jefferson County, then this section may need to be revisited. However, as all other data sets available reflect information that is consistent with the NCEI effort, the information calculated below is expected to remain fairly consistent with the application of a more comprehensive dataset. There have been 33 NCEI- recorded lightning strikes in Jefferson County since 1995; of these, there were 10 damaging incidents reported in Jefferson County between 1995 and 2020. The methodology for calculating the probability of future occurrences is described in Section Based on this formula the probability of a damaging lighting strike occurring in any given year is 40%. This corresponds to a probability of future occurrences rating of highly likely.

Magnitude and Severity

Impacts for lightning are both direct and indirect. People or objects are directly impacted when struck, or indirectly damaged when the current of the bolt passes through or near the person or object. Other impacts include the ignition of wildfires. The Colorado Division of Homeland Security and Emergency Management estimates that more than half of all forest fires in Colorado are ignited by lightning, in addition to the rangeland and wheat-field fires that lightning causes. Lightning is most likely to cause wildfires during dry conditions or during dry thunderstorms. Records of previous incidents in the NCEI database indicates that most events damage only personal property, and do not significantly impact the availability of critical services or infrastructure, corresponding to negligible severity ratings in both categories. Isolated cases, usually those which trigger large wildfires, have a more significant impact on property damages, but the ratings are still classified as limited.

The National Weather Service Pueblo Lightning Page indicates that between 1980 and 2016, nine people have been killed and 38 people have been injured by lightning strikes in Jefferson County. This equates to 9.1% of all killed and 7.8% of all injured reports for the state. The majority of lightning strikes with casualties for Colorado occurred between the hours of noon and 5:00 pm, peaking between 2:00 and 4:00 pm. This correlates to the times when the population are most exposed, as well: during the temperate summer months, on days where people are most likely to be outside, during peak times of day where outdoor activities are expected to occur. The injury and fatality rates associated with lightning are the greatest indicators of magnitude and severity. It is particularly telling when the flash density of the State is considered. As discussed in the geographic extent section, Colorado experiences an average number of cloud-to-ground strikes when compared to the nation. However, Colorado’s injury and fatality ratings are consistently in the top five, or top three when adjusted for population. Therefore, the magnitude and severity of lighting on the population is critical.

Information from the event of record is used to calculate a magnitude and severity rating for comparison with other hazards, and to assist in assessing the overall impact of the hazard on the planning area. In some cases, the event of record represents an anticipated worst-case scenario, and in others, it is a reflection of common occurrence. For lightning, there is no outstanding event of record, so the overall magnitude and severity rating for the County is determined based on the comprehensive discussion of severity contained above. Lightning events typically damage less than 10% of the property in the County. The damages inflicted on critical facilities and services (critical infrastructure) typically result in a loss or disruption of serves for less than 24 hours. While direct impacts may be negligible, the indirect impacts listed above, particularly the link to wildfire ignition, raises the magnitude severity ratings for lightning strikes to limited.

Climate Change Considerations

According to the 2018 Colorado State Hazard Mitigation Plan, the future impacts of climate induced lightning in Colorado are still unclear. No clear projected trend in the frequency or intensity of warm- season convective storms has been identified for Colorado. Therefore, the intensity and extent of thunderstorm and lightning events is not projected to change. However, according to studies referenced by the National Lightning Safety Institute, it could be possible globally to see an increase of 10-20% in the incidence of lightning with each degree Celsius of global temperature increase. This could potentially lead to higher frequency of occurrence in Colorado.

Vulnerability Assessment

General Property

It is difficult to quantify where specific losses will occur due to the random nature of this hazard. Given the lightning statistics for Colorado and Jefferson County, the County remains at risk and is vulnerable to the effects of lightning. According to NCEI data, $1.44 million dollars in property damage and $12,000 in crop damage was reported in Jefferson County over a 20 year period.

People

Persons recreating or working outdoors during the months of April through September will be most at risk to lightning strikes. It is difficult to quantify future deaths and injuries due to lightning, other than to note that future occurrences are likely without increased public education.

Critical Facilities and Infrastructure

Critical facilities and infrastructure will have the greatest consequences if damaged by a lightning strike. The effect of wind, combined with lightning, rain and hail, on power delivery is a significant factor when assessing current development exposure. An analysis of this impact is described in the hail vulnerability section. According to the 2018 Colorado State Hazard Mitigation Plan, statewide between 2008 and 2017, the Office of Risk Management (ORM) reported that 48 severe thunderstorm and lightning events damaged state assets. In this timeframe, these events resulted in $1,041,989 in losses, some of which occurred to critical facilities such as within the state correctional system. Forty-five of the 48 events were due to lightning strikes, equating to $1,010,944 of the $1,041,989 in losses. These lightning strikes resulted in damages to building contents such as electric and power equipment connected to the electrical system more than causing structural damage.

Economy

Economic impact of a severe thunderstorm is typically short term. Lightning and high wind events can cause power outages and fires. Generally, long-term economic impacts center more around hazards that cascade from a severe thunderstorm, including wildfires ignited by lightning. Similarly with the previous section, lightning can cause structural damage or damage to electrical systems to private buildings as well as critical infrastructure.

Historical, Cultural, and Natural Resources

According to NCEI data, the average significant lightning strike in Jefferson County occurs every 1.5 years. The strike most likely occurs in the summer, between 12 PM and 5 PM. Thirty-eight percent of damaging lightning strikes cause damage to either property or crops. The greatest losses from lightning result from the secondary hazard of wildfire, which can have cascading impacts on natural resources.

Future Development

New critical facilities such as communications towers should be built with lightning protection measures. As the population continues to increase and the number of people exposed to the hazard increases, it is reasonable to assume that injuries and deaths will also increase proportionately. Construction of lightning shelters at outdoor venues and increased public awareness campaigns may help minimize increased effects of lightning on growing populations.

Overall Hazard Significance

Lightning strikes in Jefferson County have a range of impacts on the planning area. The most serious impacts are the potential for injuries and deaths, with the most serious indirect impact associated with wildfire caused by lightning. The geographic extent of the hazard is considered limited. The probability of future occurrences is considered highly likely and the magnitude/severity for the event of record is limited. The HMPC considers the hazard to have a low overall impact on the County. Together, this equates to an overall impact rating of medium. This rating recognizes that other hazards may be a higher priority for the County or may possess more actionable mitigation solutions, while still addressing the significant threat that lightning poses to personal life safety for the jurisdiction’s citizens. This is also consistent with the efforts of the Colorado Division of Homeland Security and Emergency Management to increase lightning safety and awareness.