The Richter Scale
Devised by Charles Richter in 1935, the scale measures the energy released by the earthquake (other factors are involved in how damaging a quake might be, such as its depth). The scale is logarithmic, that is each successive whole number represents a ten-fold increase in power. A magnitude 7 earthquake, equivalent to 179,100 metric tons of TNT, is ten times more powerful than a magnitude 6. For comparison, a one-megaton nuclear bomb can measure 7.5 on the Richter scale.

There are about 35 earthquakes worldwide each day, but only 18 major and one great earthquake per year. Recent U.S. quakes include Northridge (1994), a 6.7, and Loma Prieta (1989), a 7.1. The 1906 San Francisco quake is estimated to have been an 8.25.

Tsunamis

Tsunamis (the name is from Japanese tsu, harbor, and nami, wave) are generated by earthquakes. Because of this they occur mostly in the Pacific (80 percent are related to the circum-Pacific seismic zone). On the high seas tsunami wave crests may be only a foot or so in height (the amplitude) but 600 miles (965 km) apart (the wavelength). As the wave comes into shallower water near the coast, its wavelength decreases and amplitude increases--up to 200 feet (61 m). Tsunamis move very quickly, often at speeds of 150 mph (241 kph) and up to 180 mph (300 kmph).

Hurricanes

Hurricane Mitch
According to the USGS, Hurricane Mitch (category 5) was the most destructive hurricane in history of the Western Hemisphere. From October 27 through November 1, 1998, it hammered parts of Honduras, Nicaragua, El Salvador, and Guatemala with 180+ mph winds, dumping from one to six feet of rain. Widespread flooding and landslides left 11,000 dead and destroyed 50+ percent of the infrastructure in Honduras and Nicaragua. More than 2 million were left homeless.

The Saffir-Simpson Hurricane Damage Potential Scale

1	74-95 mph/119-153 kph winds, minimal damage
2	96-110 mph/154-177 kph winds, moderate damage
3	111-130 mph/178-209 kph winds, extensive damage
4	131-155 mph/210-249 kph winds, extreme damage
5	over 155 mph/249 kph winds, catastrophic damage

Concerned by frightening reports in popular media about possible celestial roadwreck including the earth and comets, asteroids, and the like, MIT's Richard Binzel presented a scale for conveying to the public the gravity of any possible threat, like that used for hurricanes, to a meeting of colleagues at Turin (Torino), Italy, this June. The "Torino scale" was adopted by the International Astronomical Union in July. It measures the kinetic energy of an asteroid approaching earth against probability of a collision, giving a rating--from 0 to 1--of how serious the thing really is.

If you want to risk it, the best meteor showers are the Quadrantids (January 3-4, 100 per hour), Perseids (August 12, 80 per hour), and Geminids (December 13, 100 per hour). Be certain to wear a hard hat or carry an umbrella, just in case.

No scale has yet been suggested for rating the danger posed by bizarre human behavior inspired by astronomical phenomena, for example the mass suicide of Heaven's Gate cult members when comet Hale-Bopp was visible in 1997 or, more recently, the death of a baby, born during the solar eclipse in August in a remote village in northern Romania, whose mother killed it, apparently fearing an ancient superstition that monsters are born during eclipses. Such behavior may be a more immediate threat than catching an asteroid.