Recent Thunderstorm Rattles Island
by Nick Zachar, Meteorolgist,
RTS Staff
Mar 4 2011
An intense thunderstorm on January 30th produced a noteworthy display of lightning bolts and audible thunder during the afternoon hours. The sheer number of close lightning strikes caused schools to delay release times, postponed outdoor activities, and made quite a few residents nervous. In total, well over 100 strikes within 20 nm of Kwajalein Island were recorded during a 2-hour time frame.
While thunderstorms across Kwajalein Atoll seem to be rare phenomena, climatological averages suggest otherwise. In an average year, Kwajalein sees around ten thunderstorm days, meaning that thunder has been heard at the weather station in addition to observing lightning. Kwajalein experienced an above average year for thunderstorms in 2010 with a total of fourteen thunderstorm days.
Lightning strikes are most prevalent in the month of October, when an average of 1300 are recorded on the weather station’s Lightning Detection Sensors, which can detect strikes over 100 miles away. The month of February sees the lowest amount of lightning. January is a close second with 135 strikes each year when averaged over the last decade. The last time a thunderstorm was observed in January at Kwajalein was 11 years ago.
Normal dry season conditions inhibit the formation of deep convection and thunderstorms. But this has not been typical dry season weather. Strong surface wind convergence can generate shower activity any time of the year, but when favorable upper-level winds are also present, convection can develop and be sustained for extended periods of time. This long-lived convection is essential as it takes time for electrical charges to build between icy cloud tops and the surface of the earth. And with the temporary lack of the dry season temperature inversion around 8000 feet (which acts to stifle any deep convection), ideal conditions existed for long-lived thunderstorms.
What made this lightning event unusual was the high number and close proximity of cloud-to-ground strikes. On average, only 25% of lightning is cloud-to-ground, with much of the remaining lightning being in-cloud. However, lightning will strike the earth any time there is sufficient charge differential between the cloud and terrain.
In addition to the large number of lightning strikes, a thunder was also frequently heard by island residents. In the case of this thunderstorm, it was common for two or even three cloud-to-ground strikes to occur in short succession, sometimes thousands of yards apart. The sharp initial crack of thunder is caused by close strikes, while the rolling rumble of thunder is caused by the time delay of sound coming from different parts of a long stroke. The longer the lightning channels, the longer the rumble of thunder.
In normal atmospheric conditions, thunder is seldom heard further than 10 miles from an observer. Due to the flat terrain of the Marshall Islands, a person can often see lightning flashes along the horizon but remain far beyond the 10 mile limit for hearing thunder. Another reason that thunderstorms may not appear to have thunder is that most lightning strikes remain within the thundercloud. Thunder from in-cloud lightning tends to get diffused within the storm cloud, making it difficult to hear.
An observer can approximate the distance to a lightning strike by timing the interval between the visible lightning and the audible thunder it generates. In general, every second which passes between flash and sound equals about 1000 ft. For example, a lag time of five seconds indicates the lightning strike was about 1 mile away.
While no lightning strikes struck any land during this thunderstorm, a good portion did strike adjacent waters. This does not diminish any potential threat for boaters and beachgoers, as electricity from lightning has been known to kill people on and within surrounding water over thirty yards away from the initial strike. This occurs because salt water is a good conductor, so when lightning hits the water, it tends to spread along the surface of the ocean. In addition, an electrical charge can reach 10-20 feet in depth from the impact site, and can certainly kill or injure any living creature unlucky enough to be within that zone. This includes scuba divers.
Lightning is a very real danger for residents of Kwajalein Atoll. Individual bolts can travel ten miles through the air, so if a resident is close enough to hear thunder, they are close enough to be struck. Studies indicate that the most dangerous place to be during a thunderstorm is an open field, followed closely by the open ocean. Kwajalein Atoll consists primarily of these two locales, so every lightning warning should be taken seriously. During a thunderstorm, stay indoors and off the open ocean until the warning has been lifted. After all, when living on such a flat island, a person may often be the tallest object around.