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by Mark Michaelsen
This edition will focus on the severe weather phenomenon known as thunderstorms. I grew up much of my life in the Piedmont of North Carolina which believe it or not is one of the HOT SPOTS for thunderstorms in the US. My incredibly understanding parents recognized early on that I had a passion for sever weather and allowed me to go out into our front yard and get down in the three foot deep ditch and watch as the storms approached like clockwork from the West each late spring and summer day. As the rumble grew louder and the sky rapidly blackened the once absolutely still pine trees began to move with the approaching wind. First up top with just a wiggle in the branches and within minutes a screaming wail and the occasional loud "POP" as a branch or two succumbed the torrent of wind and rain my heart felt like it was going to leap from my chest. As the bolts of lightning selected their random targets my survival intuition quickly began to count the seconds between lightning strokes and the sometimes wrenching "CRACK and then THUD" as the air rapidly expanded around the bolt and created what we perceive as thunder...You knew you probably shouldn't be outside but the smell of the air as it becomes supercharged with static is intoxicating. That fresh odor of rain just before the torrent hits still makes my hair stand on end with anticipation every time I am back east and get to enjoy one of natures most spectacular displays...The THUNDERSTORM Before we get into the mechanics of what makes for a thunderstorm lets look at some interesting facts as supplied by the National Severe Storm Forecast Lab located in my favorite spot on earth- Norman, Oklahoma.
Lightning Causes Forest Fires. Can Forest Fires Cause Lightning? Yes, smoke and carbon micro-particles, when introduced into the upper atmosphere, can become the initiators of static. Sufficient atmospheric static can spark discharge as lightning. Reports of massive lightning storms in coastal Brasil, Peru and Hawaii have been linked to burning of sugar cane fields. The late 90's Mexican forest fires resulted in unusual lightning activity in the USA High Plains area (Lyons, et al.) So too can dust in an enclosed grain elevator create a static discharge. Recent reports (Orville, et al) show the Houston TX petrochemical industry, discharging copious amounts of hydrocarbons into the upper atmosphere, may be responsible for higher-than-normal lightning activity in that area. And from the UK research center there is this about forked lightning. A High Percentage of Lightning Flashes Are Forked.
There are several types of thunderstorms ranging from the garden variety storm where some rain and moderate winds are accompanied by occasional strokes of lightning to the Super Cell phenomenon where straight line winds and downdrafts can exceed 100 mph and spawn tornados with winds over 300 mph capable of tearing the pavement right off the highway! So what do we need to create our Thunderstorm?
Thunderstorms in general tend to occur during the Spring and Summer months but are possible year around. Thunderstorms can bring with them a variety of dangers, including strong winds, hails, heavy rain and lightning. Not all storms bring rain, and many areas often experience ‘dry’ thunderstorms, particularly here in Southern California which pose the risk of forest fires. Typical thunderstorms last for around thirty minutes and measure around twelve to fifteen miles in diameter. You can usually predict when a thunderstorm is likely to occur. Very warm and humid days may begin clear or partly cloudy but as the clouds thicken and rise with the afternoon heating the skies grow darker and you may notice lightning strikes or hear thunder in the distance. As the thunder draws closer it may bring with it heavy rains, hail or very strong winds. Civil defense warning systems vary. The Emergency Broadcast system in your area will provide radio and television advisories if the National Weather Service places your district in a watch or warning mode. Some areas have local sirens or public address systems that allow them to warn of an impending sever weather event. Severe thunderstorms can be dangerous. About ten percent of those in the Unites States each year are classified as severe. They with them hail and winds of over 58mph. The lightening produced from thunderstorms can be a killer, and is actually responsible for a higher mortality rate than tornadoes. Lightning and tornados are not the only danger posed by thunderstorms though. The flash flooding created by a slow moving thunderstorm can be just as deadly and cause mudslides and torrents of water capable of sweeping away people and property.
THE DEVELOPMENT STAGE where rising air is indicated by the cumulus cloud with a darker flat base and then a "pillowy" section above as the storm grows. Little if any rain falls in this stage and it is usually devoid of lightning. This portion of the life cycle can last for as little as ten minutes but more often 15-20 minutes in duration. MATURE STAGE This is the most energetic and dangerous portion of the life cycle. This generally lasts twenty to forty minutes but can go on much longer. During this stage strong downdrafts, frequent lightning and even tornados are formed accompanied by heavy rain and hail. DISSIPATING STAGE As the thunderstorms life cycle begins to wind down the heavy rain tapers off, winds slow down and the frequency of the lightning strikes scales back. Caution should be taken though and shelter should be maintained as long as there is lightning present in the system.
Shelter for the Modern Mariner is often unavailable though while at sea. For big boat sailors a simple grounding system may help protect some of your equipment against a direct strike but depending on the amp value of the lightning stroke these may or may not save the hardware. As a Mariner your main interest should to reduce your chances of receiving a direct hit. Make sure you are wearing good insulated boots on deck. Keep your hands off the mast and any metallic riggings. Set your sails to the appropriate angle and reduce sail area to a minimum well ahead of an approaching storm. If you have an autopilot and you are not near shore, use it and get below if you are certain there is nothing to have a collision with and once again, keep your hands off the mast or any electronics. But what about the small boat sailor caught in a storm? Various theories have been put forth. Here is a good article on the subject: Satellite interpretation-It has only been about 35 years now that we have had useful satellite data on thunderstorms. Satellite data is quite useful in identifying areas where storm are likely to develop and also when the actually do develop. Here is an article on the subject: While thunderstorms represent a true threat to the Modern Mariner they can also provide one of natures most awesome displays of power. Given a safe location to observe storms from, thunderstorms can make for better entertainment than just about anything I can think of. Make sure you have batteries, flashlights and food and water stores and then go make a bag of pop corn and enjoy the show the next time a thunderstorm comes to a town near you. I'll cover SUPER CELLS, Tornados and waterspouts in the next edition of Weather and the Modern Mariner. 11/20/06 |
Many cloud-to-ground lightning flashes have forked or multiple attachment points to earth. Tests carried out in the US and Japan verify this finding in at least half of negative flashes and more than 70% of positive flashes. Many lightning detectors cannot acquire accurate information about these multiple ground lightning attachments. 
The life cycle of a traditional thunderstorm can be broken down into three simple stages:
Heavy rain can reduce visibility as well and actually be painful to the skin in the heaviest downpours.