Ask Dr. Knowledge
Does lightning only strike in one direction?
 | Lightning can go from cloud to ground, from ground to cloud, and from cloud to cloud. (Tommy Metthe/Abilene Reporter-News/File 2005) |
The first thing to agree on is what the "direction" of an electrical current is, which is to some extent a matter of convention. For the following, let's call the direction of an electrical current the direction in which electrons go.
Normal matter has equal quantities of positive charge carried by protons in atomic nuclei and negative charge carried by electrons. For reasons that are still far from well understood, water droplets at the bottoms of clouds tend to have a surplus of electrons and thus a net negative charge. At some point, and again, we have only an incomplete understanding of what goes on in detail, electrons start to run toward the positively charged (electron-deficient) ground below. This starts in the form of "streamers," which can be up to 160 feet long.
In the meantime, the earth itself, especially near high, pointed objects, can start to pull electrons in the atmosphere downward, generating a down-going current near the ground, i.e., a flow of electrons downward, but still with no overall lightning strike. This current is sometimes called a "positive streamer."
If the two streamers connect, a huge flow of electrons is initiated, which corresponds to a proper lightning strike from cloud to ground. A single strike may repeat several times, even if the eye only sees one big flash.
There is a sort of reverse phenomenon called "positive lightning" in which current flows the other way, with electrons going from the ground to the positively charged top of clouds. It is relatively rare, but can be much more intense, last much longer, and strike miles from a cloud. Colloquially referred to as a "bolt from the blue," this sort of lightning is much more dangerous.
Lightning can also go from cloud to cloud, in which case there's no clear direction to talk about.
Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to drknowledge@ globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.
No comments:
Post a Comment