Signs of Tropical Summer: Heat Lightning
by Tara A. Spears
As I sat last night watching the pyrotechnic show of heat lightning, I thought how amazing that something so beautiful could be so deadly. One needs to know that lightning is the visible form of energy transfer and needs to be respected as a potential dangerous weather phenomenon.
Heat lightning is the term for the flashes of lightning on the horizon or within clouds that do not have accompanying sounds of thunder. The reason for this silent lightning is that it is very far away and the sound waves dissipate before they reach the observer. Generally, heat lightning occurs as an early warning sign that thunderstorms are approaching, or it can appear after a thunderstorm has moved past an area. In coastal Mexico, heat lightning is most often seen during August and September out over the ocean at night- this is the remnants of storms that formed during the day along a sea breeze front coming in from the interior mountains.
Another reason for the name of heat lightning is because it usually happens on hot summer nights and to distinguish it from lightning accompanied by audible thunder and cooling rainfall. Since this summer in the Riviera Nayarit we have had the heat and high humidity but not much rainfall, there has been an abundance of heat lightning.
In addition to the heat lightning, we have experienced what scientists call dry lightning: lightning strikes with thunder but without significant precipitation. There have been several dramatic thunderstorms that boomed and flashed for hours, but the official San Pancho weather station recorded only .08 of measured rainfall!
Believe me, the humidity did not dip noticeably afterwards. Dry thunderstorms are common in the western portion of the US and Mexico. This type of storm results when the rain produced by a thunderstorm falls through a substantial layer of very dry air which evaporates the precipitation before it reaches the ground. According to the National Weather Service, less than a tenth of an inch of rain is the threshold for characterizing a thunderstorm as dry.
Potential for Damage: Don’t let sunshine fool you into thinking there is no danger for lightning strikes. One week this summer we had three afternoons in a row where it was full sun with a storm cloud-free sky yet there was thunder. A friend returned home to find that lightning struck her house electrical connection and fried/burnt up EVERYTHING that was plugged in: appliances, TV and computer modems included. Very expensive situation. During the intense thunderstorm season I keep everything but the refrigerator (on a heavy duty surge protector) unplugged; yes, it is inconvenient as hell, but I have had to replace several microwaves, computer systems and printers over the years of tropical living so I take precautions now. Knowing that thunder indicates lighting is happening in the area, I get out of the pool even though the storm mass isn’t visible. The NASA map shows lightning frequency for 2011. The places with the largest number of lightning strikes are deep red. Much more lightning occurs over land than ocean because daily sunshine heats up the land surface faster than the ocean.
The heated surface heats the air, and more hot air leads to stronger convection, thunderstorms, and lightning. The map also shows that more lightning occurs near the equator than near the poles. This pattern is also due to differences in heating. The equator is the warmest spot in the world, creating widespread convection, thunderstorms, and lightning across the tropics every day.
It’s the physical composition of Mexico that attracts thunderstorms en mass: interior mountains bordered by oceans and sub-tropic and tropical latitudes. Add the huge population of tourists and residents and it is not surprising that Mexico has the highest incidence of lightning strikes in the world. The US National Lightning Safety Institute estimates that 24,000 people are killed by lightning strikes around the world each year and that an additional 240,000 are injured. A reported 223 people were killed in Mexico last year, 50 in the US, and only 3 in Canada. Lightning strikes can produce severe injuries, with a significant mortality rate from a direct hit; of those that survive, 80% will have long-term injuries. These severe injuries are not usually caused by thermal burns, since the current is too brief; instead nerves and muscles are directly damaged by the high voltage producing holes in their cell membranes-ugh.
Take Precautions: The safest way to avoid injury from lighting strikes is to get indoors or inside a vehicle as soon as you hear thunder or see lightning of any type. Remember that it doesn’t have to be raining for lightning to hit. The risk remains for up to 30 minutes after the last observed lightning or thunder. Even when you are in a safe shelter, avoid using telephones, modems, and other electric devices, warns the National Lightning Safety Institute. A harmful overcurrent can reach a person through the phone jack, Ethernet cable or electric outlet. I personally know an individual who felt the overcurrent while taking a shower during a thunderstorm.
The National Lightning Safety Institute recommends using the F-B (flash to boom) method to gauge distance to a lightning strike. It’s ok to take the time to compute this once you are sheltered, but don’t stay outdoors as you do it. The formula approximates the distance by timing the interval between the visible lightning and the audible thunder it generates. So, a flash preceding thunder by three seconds is about one kilometer (0.62 mi) (3×340 m) distant.
Consequently, a lightning strike observed at a very close distance will be accompanied by a sudden clap of thunder, with almost no perceptible time lapse, and a distinctive smell.
The ancient people, Aztec and Mayas, had the right attitude by viewing lightning as a deity or achieving its power from a deity. We might have the scientific explanations in the 21st century, but viewing the natural phenomenon of lighting still brings a tremble and awe for the power of nature.