Lightning


There are more than 3 million lightning flashes worldwide per day. Recent satellite data suggest that there are more than 3 million lightning flashes worldwide per day, or more than 30 flashes per second on average. This includes flashes within or between clouds as well as flashes extending from cloud to ground. These worldwide numbers are considerably less than scientists thought existed before satellite observations. Across the United States, lightning detectors show an average of about 20 million cloud-to-ground flashes per year.

A cloud-to-ground lightning bolt is an exchange of electrical energy between charged regions on Earth and within a thunderstorm. Storms typically have an area of negative electrical charge at their base and a positive region at the top, with smaller pockets of charge elsewhere. Scientists aren't sure how these develop; they do know the charges are carried by water droplets and ice crystals.

The negative charge at the cloud's base causes a "shadow" of positive charge in the ground below. At first, an insulator—the air—impedes the connection between the two regions. Eventually, the negative charge within the cloud grows too great for the air to restrain it. An electrical impulse, called a leader, extends downward from the cloud in steps, each covering about 150 feet (50 meters). These steps are responsible for the jagged quality of a visible lightning bolt.

When the leader nears the ground, streamers arise to meet it, and the circuit is complete. A bright streak of electricity—the visible lightning stroke—ascends along the same course the leader took. Several more strokes may follow this same path over a period of a few tenths of a second. Extreme heat generated by the electrical current generates the powerful, outward-flowing sonic wave known as thunder. People on the ground may hear thunder as a sharp, brief crack or a long, rolling rumble, depending on the location and orientation of the lightning strokes.

About 10% of lightning bolts bring positive instead of negative charge to the ground. Usually more intense than the average bolt, these positive cloud-to-ground bolts often descend from the icy cirrus clouds ahead of a thunderstorm. Thus, a positive bolt can strike a point well before the first raindrop falls there.

Research


This Schweizer 2-32 sailplane was used by NCAR for many years to study lightning and other aspects of thunderstorms.To study lightning, scientists analyze electric fields using ground-based and airborne instruments. In the 1980s and 1990s, a specially equipped sailplane gathered data from inside electrical storms over Florida, New Mexico, and Colorado. More recently, satellites from NASA and NOAA have provided global estimates of lightning frequency. A 1996 study across northeast Colorado, called STERAO, gathered some of the first three-dimensional profiles of lightning strikes. And a network of detectors established in the 1980s now tracks cloud-to-ground strikes across the United States. Data from this network often appear on television weathercasts as a map where dots or x's indicate each flash.

Lightning is known to produce nitrogen oxides within thunderstorms. These chemicals can react with others in the presence of sunlight to produce ozone. Since most lightning occurs inside a storm, the added ozone tends to show up several miles high rather than near Earth's surface, so it doesn't add significantly to ozone pollution at ground level. However, the resulting ozone has a long lifetime in the upper troposphere (a few miles above the ground), so it could be carried over long distances. According to an NCAR analysis, ozone from storms across southern Africa is being transported by the subtropical jet stream eastward to Australia, where it causes significant rises in ozone levels in the upper troposphere.

Safety

Because it usually kills people one at a time, unlike floods or tornadoes, lightning tends to be underrated as a hazard. Some simple tips could mean the difference between life and death. Even when lightning doesn’t kill, the physical, neurological, and psychological effects after a strike can be severe and long-lasting.

thunderstorms • hail • lightning • tornadoes • floods • hurricanes • winter storms