How much bigger -- or smaller -- do cities and counties become during the day as commuters come and go?

Daytime population is of increasing interest to government agencies and businesses because it reflects a reality not accounted for in the nation's official population tallies. Its uses include disaster planning, traffic management and locating new retail outlets.

In the Census Bureau's first estimates of daytime populations, released last week, the District and several suburban employment centers ranked at or near the top of communities of comparable size jurisdictions in change from night to day. (The estimates were based on 2000 Census data, which is the best available neighborhood-level detail on commuting patterns.)

The District, with an official population count of less than 600,000, swells to nearly 1 million people during the day when commuters are added. The 72 percent increase from night to day was the largest among the nation's big cities. The 410,000 net daytime gain was second to only New York's 563,000.

Upper Marlboro, the government center of Prince George's County, grows by more than 800 percent from night to day, according to the Census Bureau. Tysons Corner, with its dense office and retail centers, grows by nearly 300 percent. The daytime populations of Fort Meade and Leonardtown, one a military employment center and the other a government center, each increase by more than 200 percent.

At the other end of the spectrum, several of the region's bedroom communities shrink by more than 40 percent during the day, among them Burke, Franconia, Montgomery Village and North Laurel, according to the census figures.

-- D'Vera Cohn