Hi, I'm Bryan Yeaton and this is the Weather Notebook.

There is no single definition for drought, because it's so multifaceted.

The most obvious way to think of drought is a lack of rain. In a desert climate, the rainfall extremes from year to year can be huge: one summer could be a soaker, and the next may be bone-dry. But in a wetter regime like the mid-Atlantic, the changes from year to year tend to be more subtle. Even a slight drop in rainfall over months or years can be enough to make a dent in reservoirs and ground-water supplies. That's when you start to shift from a meteorological to a hydrologic drought. If it's only been dry for a little while, plants with deeper roots may survive. But after a longer period without much rain, the deeper soil dries out, and a hydrologic drought gets under way.

Once dry weather starts to affect crops, you might call it an agricultural drought.

If you're an economist, you could define drought by its cost to the nation. Or maybe you're just craving a single index that pulls together lots of ways to think of drought. You can find just such an index on the Web. Each Thursday a team of government and university scientists updates the U.S. Drought Monitor. It's a map showing areas where drought ranges from moderate to exceptional, based on a variety of measures, including rainfall and ground water.

To look at the U.S. Drough Monitor, go to our website, www.weathernotebook.org, for a link. Writer and meteorologist Bob Henson helped us gauge this drought story. The Weather Notebook is a production of the Mount Washington observatory and is supported by Subaru and the National Science Foundation.