VOLCANOES: In the last five years, a new generation of instruments has allowed volcanologists to observe possible eruption precursors. These include satellite-controlled radar and infrared sensors that record ground swelling to within a fraction of an inch, tiltmeters planted on volcano flanks, remote sensors to measure venting of carbon dioxide and other gases, and seismic instruments that can create three-dimensional images of magma chambers 5 to 8 kilometers below the surface. But such ominous signs may build for weeks or months, then fizzle out. Or, like the rumblings that started at Mt. St. Helens in late September, they may just continue.
TORNADOES: Since the mid-1990s, the National Weather Service has covered the continental U.S. with a constantly monitored system of Doppler radar sites that detect supercells -- violently churning weather systems 3 to 6 miles across that can spawn tornadoes. The NWS depends on volunteer storm watchers to call in actual tornado sightings, and most alarms are still false. But the real ones give 10 to 40 minutes' warning to nearby people listening to the TV or radio or alerted by tornado sirens.
ASTEROIDS: Since 1998, NASA has spent about $3.5 million a year to catalog more than 500 "near-Earth objects" over a kilometer across, but only one so far is the subject of a warning: asteroid 1950DA, which has a 1-in-300 chance of hitting in 2880. The International Astronomical Union has created a formal process for evaluating newly spotted objects, but it is not set up for surprises. On Jan. 14, 2004, an asteroid appeared to be only days away from colliding with Earth, and astronomers did not know who to call, never mind what to tell them. The scare blew over within hours, when the asteroid's path was recalculated.
-- Kevin Krajick