The Hazards of Thunderstorms

2016-06-28 12:11:45.000 – Tim Greene, Intern


Lighting Presidentials Ryan Knapp

          As most know, the summit of Mount Washington is in the clouds with visibility limited to several hundred feet at best most of the time; today is no different. After spending the morning in and out of the clouds, it looks like we’re destined to return into the fog later this afternoon. Unlike most days though, today the National Weather Service’s Storm Prediction Center (SPC) has placed us under a marginal threat for severe weather, and less than fifty miles from the border of the slight threat polygon. This makes anywhere above treeline an extremely dangerous place to be today. Lightning takes the shortest path available down to the surface of the earth in order to discharge the massive amount of electrical charge it builds up, so naturally the closer something is to the base of the cloud the better the chance it gets struck by lightning. Of course, the best place you can be during a thunderstorm is indoors (being below treeline, or for that matter anywhere outdoors, is still dangerous). Last year, there were 27 lightning fatalities in the United States; some of what became the last activities of these victims were: fishing, playing volleyball, walking the dog, working in the garden, and hiking. Several of these unsuspecting people figured they had time to finish up what they were doing before the thunderstorm was dangerously close. Others, like the hikers, were stranded outside with little to no shelter to take refuge in/under.

          It then should not be that much of a surprise that hiking is one of the most hazardous activities to be caught doing in a thunderstorm. Not only are you stuck outside, but often times you are climbing in elevation and thus reducing the amount of atmosphere a lightning bolt has to travel to hit you. Though air is an effective insulator (the opposite of a conductor) of electricity, it is no match for the up to one billion volts of electricity in a bolt of cloud-to-ground (CG) lightning (for reference, the outlets in your house provide a paltry 120 V). It is up to hikers to seek out information ahead of time as to the potential for thunderstorms in order to avoid being caught in a life-threatening situation. Plan ahead!

How Lightning Works

          Thunderstorms require three main ingredients in order to develop: instability, moisture, and a source of lift. Larger severe thunderstorms, known as supercells, require a fourth component in wind shear (a condition where winds differ in speed and/or direction at various heights of the atmosphere). If, and only if, all three (or four) of these ingredients are present can thunderstorms form.

           The stability of the atmosphere can be defined as the degree of resistance a layer has to vertical motion (upward motion is the crux of all ‘weather’ while downward motion is associated with clear, benign conditions), so instability is simply a case where the atmosphere wants to move around vertically. Because warm air is less dense than cool air, it is also more buoyant and would rise if not for the heavier cool air sitting on top of it. This is where the source of lift comes into play; warm fronts and cold fronts (to name a few of the usual suspects) play the role of the lifting mechanism as they sweep through an area of instability and give that warm, positively-buoyant air the nudge it needs to begin its ascent into the atmosphere. When this low layer of instability has the third component, moisture, clouds are able to form through the process of condensation (the same that forms on the outside of an icy beverage on a muggy day). These clouds, which build vertically in nature, are called towering cumulus or cumulonimbus and resemble cauliflower or cotton balls. In environments of high instability, they can build so quickly you would think you are watching a time-lapse.

          Although (believe it or not) the entire process of electrification is not yet fully understood, we know that lightning forms in cumulonimbus clouds when super-cooled water droplets, ice crystals, and soft hail called graupel (think of Junior Mints) collide inside the updraft (the column of rising air in the thunderstorm) and create static electricity that builds over time. Positively-charged ice crystals rise to the top of the cloud while the negatively-charged liquid droplets though still suspended in the updraft settle toward the base of the cloud, making a natural battery. Directly underneath the cloud base, a positive charge is induced over the ground that attracts (in electricity and magnetism, opposites attract) the massive charge overhead. While most assume lightning bolts go from the cloud down to the ground, the ground sends barely visible “positive streamers” up while the cloud sends many jagged “stepped leaders” down in hope one will connect with a positive streamer and complete the circuit, draining the charge in the cloud (temporarily).



          As a quick side note, the sound of a clap of thunder is caused by the rapid expansion of air around the three-inch channel that is heated to five times the temperature of the surface of the sun. The further away you are from the source, the more the sound reverberates and echoes off of surface features the more it turns into a deep rumble.


Tim Greene, Intern

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