As days become shorter and nights grow colder, the city is planning its annual census to send volunteer teams into the city who will seek out and count, one by one, the winter weather’s most tried and tired victims — Boston’s homeless.

After last year’s census found almost 7,000 homeless, the city has set aside a period of four hours to conduct this year’s count.

City officials say the census, which will take place from 9 p.m. Dec. 18 to 1 a.m. Dec. 19, aims to find trends and better understand how the economy, cost of housing and government programs affect the homeless.

“That cumulative information can be useful in determining cause and effect and planning support services,” said Karen LaFrazia, director of St. Francis House, a Boston homeless shelter that helps conduct the study. “You can’t resolve a problem until you understand its size and scope.”

Groups of five to 10 volunteers from neighborhood and community groups will cover 33 areas of the city while others will consult with homeless shelters to report the number of full beds, said St. Francis House spokeswoman Elizabeth Lund.

Lund said the homeless shelters help the city in “looking for places that the homeless are likely to frequent.”

Though Lund said the organization helps to reach out to as many people as it can, she said it is impossible to provide a precise figure.

“The final number is not definitive, because it doesn’t take into consideration people who have been homeless earlier in the year or those who will be without shelter in the coming weeks,” she said. “The census is subject to people’s ability to find and count the homeless.”

Most shelters keep a nightly count and report their numbers to the city, said homeless shelter Pine Street Inn spokeswoman Alicia Ieneri.

“We still count each bed per night, even if the same person is filling that bed,” Ieneri said.

Last year’s census found 6,636 homeless people, a 4 percent increase since 2005.

Additionally, there were 311 more homeless families in Boston in 2006 than in 2005, the number of women on the streets jumped 85 percent from 34 to 63 and there are 36 percent more homeless in Boston than there were 10 years ago.

The census blames this growth on the loss of “state rental housing subsidies . . . and the long-term failure of Federal and State agencies to adequately fund public housing operating and capital costs.”

“We have not built enough affordable housing,” said Boston University Human Development Program Director Deborah Belle. “People can’t afford a place to live. . . . Many people have been traumatized by violence, many men from wars and women by sexual and physical assaults, causing tremendous emotional distress in their lives as a whole.

“The answer to homelessness isn’t jobs, because many homeless people have jobs,” Belle added. “The real answer is in providing affordable subsidized housing and economic incentives.”