~ National Lightning Safety Institute ~

Section 5.5.3

Lightning hazards can be mitigated by advanced planning. One part of this safety program may include an early detection and warning alarm package. Lightning detectors can give notice to shut down dangerous operations before the arrival of lightning. They also may signal "all clear" conditions after the lightning threat has passed. Some type of detection package may help you with Duty To Warn issues.

             No one lightning detector by itself constitutes optimum lightning protection. Detectors can be used as a part of an overall safety plan dealing with the lightning hazard. Depending on the site, additional measures should include air terminals, bonding, shileding, grounding, zone-defense surge protection, education and training, etc.  

            Lightning detectors vary in complexity and cost from large dedicated equipment packages costing in excess of $150,000 to inexpensive $20 Radio Shack portable weather radios. The Flash-to-Bang (F-B) Method requires no dedicated detector: only counting the time in seconds from seeing lightning’s flash, to seeing the associated thunder or bang. For each five seconds, lightning is one mile away. Thus, a F-B of 10 = 2 miles; 15 = 3 miles; 20 = 4 miles; etc.  

The distances from lightning Strike A to Strike B to Strike C easily can exceed more than 5 miles. How much time is needed to get to shelter? Suspension of activities is very site-specific. For most situations, we recommend activating your lightning defense at a F-B of 30 (lightning is six miles away). We also recommend waiting to resume activities 30 minutes after the last observed lightning or thunder. This protocol may seem excessively conservative in many situations…("we'll never get anything done under such strict guidelines…").  It is a case-by-case risk management decision. And yes, safety and productivity sometimes are mutually incompatible. Safety, however, always should be the prevailing directive. 

            Available technologies of the present day lightning detectors include: 

a. Radio Frequency (RF) Detectors. These measure energy discharges from lightning.  They can determine the approximate distance and direction of the threat. 

b. Inferometers. These are multi-station devices, much more costly than RF detectors. They measure lightning strike data more precisely. Usually they require a skilled operator.

c. NLDN. The National Lightning Detection Network covers all the USA/Canada and reports lightning  strikes to a central station. Local storm data is available by subscription.  Past strike information is archived and accessible upon request.

d. Atmospheric Field  Mill Monitors. These measure the potential gradient (voltage) changes of the earth's electric field and report changes as thresholds build to lightning breakdown values. 

e. e. Optical Monitors. These can provide earlier warning as they detect cloud-to-     cloud lightning that typically precedes cloud-to-ground lightning.

f.   Hybrid Designs. These monitors use a combination of the other single-technology designs.  Two or more sources of information may be better than just one.

g. Subscription Services. NLSI Recommendation - Rent a Meteorologist. Here hired professionals make the critical decisions and advise you. This method may blunt claims of Negligence if something goes wrong. Off-site detection-by-subscription is available from several capable vendors, including: Accuweather.com; Intellicast.com; Lightningstorm.com; and, Skyview-wx.com. 

            Beware of a false sense of confidence from detectors: none of them will detect all of the lightning all of the time. None of them will provide "first strike/Bolt Out of the Blue" information or forecast in advance the positions of lightning strikes on earth. Various detector detection receiver algorythms operate at different frequencies and wavelengths: Boltek Stormtracker in the Low Frequency Range 100-700 KHZ?; Vaisala GAI NLDN  at 100-400 KHZ; NMT Lightning Array at VHF 60-78 MHZ; LIS and OTD optical at 777.4 m; Vaisala SAFIR VHF 109-119 MHZ; Vaisala GAI LDAR II at 50-120 MHZ; GAI VLF at 20-50 KHZ; the UK Meteorological Office RDI at 9.8 KHZ; etc. An excellent summary of families of lightning detectors is at: http://thunder.msfc.nasa.gov/validation/instruments.html 

            Detectors can display early warning of lightning conditions to explosives operations. A signaling or alarm notification method is essential to alert field personnel of these dangerous circumstances. Two-way radios, remote sirens, strobe lights and other methods are available. 

            Essential companions to any type lightning detector include: 1) A  written Lightning Safety Policy; 2) Designation of Primary Safety Person; 3) Determination of when to suspend activities; 4) Determination of Safe/Not Safe Shelters; 5) Notification to Persons at Risk; 6) Education: at a minimum consider posting information about lightning and your organization’s safety program; 7) Determination of when to resume activities. 

            Understand the equipment’s limitations in the face of the capricious behavior of lightning. Even Total Lightning Information (TLI) as employed by strategic military meteorologists works on assumptions. [TLI = a combination of synoptic charts; modeling output statistics; local and upper air soundings; regional radar; NLDN et al; satellite interpretation; current synoptic situation; discussions within authorities; experience; training; etc.] 

Select the detector and/or signaling device that is site-specific to your requirements, easiest to use, and which offers the most favorable cost/benefit to your operation’s budget.