17th April 2006
Cloud-to-Ground Lightning
Source: Excerpt from The Book " Weather "
Distribution: Worldwide, except Antarctica; common in the tropics. Height: Usually from cloud base to ground; rarely from cloud top. Cause: Electrical discharge between cloud and ground. Associated Weather: Heavy rain or hail, strong winds from associated thundercloud. Hazard Warning: Strikes may damage property and cause serious injury or death. Lightning occurs when there is an electrical discharge within, or around, a thunderstorm. Cloud-to-ground lightning occurs when the electrical charge travels between a negatively charged cloud base and the positively charged ground. This is the most spectacular variation of lightning, forming brilliant, jagged bolts between the sky and the ground. Each lightning stroke lasts a fraction of a second. Sometimes a number of strokes is needed to discharge the electrical build-up, giving the lightning a flickering appearance. Often the main stroke combines with smaller offshoots that discharge into the air or inside the cloud.
The ground, and almost any solid object in connection with the ground, will conduct electricity more effectively than air. This means that elevated landmasses and tall objects such as buildings and trees are prone to strikes.
Most cloud-to-ground lightning occurs from the base of a cloud. However, a rarer form, known as a positive flash, occurs when positive charges higher up in a cloud react with negative ones on the ground, sending a mighty lightning bolt from the top of the cloud to the ground. Since the path of this type of stroke is much longer, the charge has to be far more powerful.
The color of lightning indicates the content of the surrounding air. The flash will appear red if there is rain in the cloud, and blue if there is hail. The presence of a significant amount of dust in the atmosphere will produce yellow lightning. White lightning indicates low humidity; as a result, this is the form of lightning most likely to generate fires on the ground.
Striking Out
Although only about 20 percent of lightning reaches the ground, strikes occur somewhere on Earth over 100 times every second. In North America, about 400 people are struck by lightning each year, and about one in four of these strikes is fatal.
There are a number of precautions you can take to minimize your chances of being hit by lightning during a storm. If possible, move indoors. When lightning strikes a building, it tends to run along plumbing and electrical circuits, so you should avoid touching metal pipes or using any electrical equipment, including telephones and computers. One of the safest places to be is inside a car, as the car's tyres provide insulation. Aircraft are also safe, because they are not in contact with the ground and therefore cannot conduct electricity. If you are caught outdoors, do not shelter beneath isolated trees, as they are favorite pathways for the lightning's leader strokes. Keep clear of metal objects such as wire fences, which can conduct electricity over considerable distances.
Should your hair begin to stand on end, this may mean that you are within the area of positive charge below the cloud and that a strike is imminent. If this happens, crouch on all fours at once and keep your head low. Do not lie full-length on the ground, as this will increase your contact with any charges that may be conducted through the ground by wet soil.
If someone is struck by lightning, expert medical attention should be requested at once and cardiopulmonary resuscitation attempted. The greatest myth associated with cloud-to-ground lightning is that it never strikes the same place twice. The top of the Empire State Building is struck about 500 times a year and was once struck 15 times in just 15 minutes.
Acknowledgement due: John W. Zillman, William J. Burroughs,
Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot and Richard Whitaker.
