U.S. weather experts know exactly what would officially end this year's killing drought: nine to 15 inches of rain falling in one month over the hardest-hit parts of the country.

Numbers don't tell the whole story, though. It will take the right kind of rain - slow and steady over a period of weeks - to cure the worst U.S. drought in more than half a century. The wrong kind could only make matters worse, according to the National Oceanic and Atmospheric Administration (NOAA).

The nine-to-15-inch estimate (23 to 28 cm)is derived from the Palmer Drought Index, which considers temperature and precipitation to project what it would take to end a dangerous dry period. The index does not consider how quickly the rain falls.

Richard Heim, a climatologist at NOAA's National Climatic Data Center, reckons that half an inch (about 1 cm) of rain, every other day for a month, would soak into the parched soil and go a long way toward ending the extreme dry conditions that have blighted the corn and soybean crops, slowed traffic on the Mississippi and threatened to send food and fuel prices soaring.

But if the requisite amount of rain fell in a single day, Heim said by email, it could cause flash floods that would run off sun-baked ground without seeping in, doing little to end the drought.

Much of the U.S. corn and soybean crop is already lost due to the drought, and even if it started raining now, that would not restore the crops to normal levels.

Without numerous days of steady rain, this could be a repeat of the drought year of 1988, said Brad Rippey, a meteorologist at the U.S. Department of Agriculture. That year, drought devastated corn, soybeans and winter wheat, causing problems that carried over into the next growing season, he said.

Sterling Smith, a commodity strategist for Citigroup, suggested that might happen to this year's soybean crop, which he said "might take two growing cycles to straighten out." Soybean prices could be high well into spring 2013, Smith said on August 10, as the U.S. Agriculture Department forecast soybean inventory could shrink to a scant two-week supply before next year's crop is ready for harvest.

It's not just a U.S. issue. The United Nations Food and Agriculture Organization warned on August 9 that the world could face a food crisis like the one seen in 2007-08 if countries restrict exports as they worry about rising grain prices fueled by drought. Global food prices surged in July, FAO reported.

EL NINO TO THE RESCUE?

Leading members of the Group of 20 nations said Monday they are prepared to call an emergency meeting to address high grain prices caused by the U.S. drought and poor crops around the Black Sea.

Autumn rains, which tend to be steadier than summer downpours, could help in the United States. So could El Nino, a weather pattern spawned by warm surface waters in the equatorial Pacific that tends to send significant precipitation across the southern tier of U.S. states.

The odds of El Nino developing this year are greater than 75 percent, Rippey told Reuters. If this pattern develops, southern states from California through the Gulf Coast and parts of the Southeast would get a good soaking. But El Nino also could well send dry, warm weather from the Northwest through the drought-hit northern Plains and parts of the Ohio Valley.

Beyond agricultural drought, some parts of the United States are experiencing hydrologic drought, with rivers, lakes and the underground sources of water known as aquifers at low levels because of increasing demand as populations expand into dryer areas.

These subterranean lakes were filled (recharged is the hydrologic term) thousands of years ago when what is now North America was a much wetter and more humid place, said Van Kelley, a hydrogeologist at INTERA, a Texas-based geoscience and engineering consulting firm.

"Most of the country is using groundwater that's been recharged thousands and thousands of years ago," Kelley said in a telephone interview. "And so in most aquifers, we're pumping what some people call fossil water."

Florida's groundwater system can be swiftly recharged with heavy rains, Kelley said. But many U.S. aquifers - including the giant Oglala aquifer that lies under a swath of the American midsection from South Dakota to Texas - have been depleted.

Aquifers like the Oglala took thousands of years to fill and even heavy rains won't refill them after years of pumping water out, Kelley said. But aquifers can be replenished by diverting river water into them when supplies on the surface are plentiful.

(Reporting by Deborah Zabarenko, Environment Correspondent; additional reporting by Charles Abbott; Editing by Marilyn W. Thompson and Dan Grebler)