In the beginning, it must have seemed like a great idea to put the official National Weather Service thermometer for downtown Baltimore on the roof of the U.S. Custom House, flat and secure and 113 feet above the harbor.
But 83 years have passed since the huge gray granite building at Gay and Water streets was new, and high-rise office towers and parking garages looming closer are raising temperature readings and forecasters' concerns about their accuracy.
"The location isn't ideal, but we won't move it," said Fred Davis, meteorologist-in-charge for the NWS forecasting office at Baltimore-Washington International Airport. "This is the second oldest weather station in the country. That long-term record is invaluable."
Baltimore has official weather service records dating to 1908 at the Custom House, and continuously back to Jan. 1, 1871, at four other locations in the immediate vicinity. The only station that has operated longer is in Charleston, S.C.
The warming dilemma is shared by most weather stations in big cities, now that their once-isolated instruments are deep within urban "heat islands" of asphalt streets and glass-and-steel buildings topped with roaring air conditioners.
"Urbanization is a contributor to what some people see as climate warming," said Grant Goodge, quality assurance chief at the Climatic Data Center in Asheville, N.C. "It might be giving us a false indication of how much, how soon."
As a result of a 1988 study of this "urban bias" in temperatures at hundreds of stations around the United States, large-scale climate research by the weather service now includes a correction factor applied to readings based on population.
For urban areas with 10,000 people, 0.2 degree is subtracted from the average daily temperature; for 100,000 people, 0.6 degree; for 500,000 people, 1.0; for 1 million people, 1.6 degrees.
But the effect on the Baltimore station is hard to pin down beyond a familiar pattern that followers of local weather know quite well.
Temperatures downtown are almost always higher in the summer and warmer in the winter than at BWI.
That pattern has existed since weather record-keeping began at the airport on July 23, 1950 -- with a precision thermometer located in a grassy open space between runways -- and it suggests that the heat island effect has been at work for much of the city's recent history.
"Baltimore's lucky," said Mr. Goodge. "Budget cuts forced some urban stations to close when outlying stations were started at airports. Those records were lost, and it's very unfortunate. They're needed now more than ever."
The Custom House setup features a ventilated, white-painted enclosure standing some 5 feet above a low wooden platform. Inside is an antique electronic thermometer that transmits readings every 15 seconds by telephone line to a recording device at the BWI office.
"It better never get hotter than 110 or colder than minus 10, 'cause those are the limits," laughed Barney Roberts, an electronics technician who works for the weather service.
The heat record in downtown Baltimore is 107 degrees, set on July 10, 1936.
The cold record is minus 7 degrees, set on Feb. 10, 1899, and tied on the same date in 1934.
And the instrument had better never break. Mr. Davis recalls that a 10-day shutdown to repair the aging temperature gauge in the mid-1980s prompted a public outcry, including an angry editorial in a local newspaper.
Mr. Roberts visits the little station "about once a month, or more often if the readings don't seem quite right." Such a mystery showed up this past January, when he discovered that a huge satellite dish installed close to the platform lowered the temperature readings.
It was quickly removed.
The biggest jolt to the integrity of the station's records occurred in March 1984, when the original platform became unsafe and was replaced with a new one nearly a dozen feet lower.
"It seemed to make us more sensitive to the heat effects," said Mr. Davis.
"This is obviously most representative of the temperature on the Custom House roof," he added.
"But we think it's pretty close to what people are feeling down on the street. The whole city is a heat island, not just the corner of Gay and Water streets."
In addition to temperature readings, the Baltimore station provides precipitation records, thanks to volunteers from the Coast Guard office downstairs who regularly check the two rain gauges.
But measurements of wind speed and direction were dropped years ago, early victims of the encroaching development.
In 1921, the local weather bureau chief complained that the roof of the Custom House had been isolated from the wind by the erection of high buildings in its vicinity.
Mr. Davis, a 33-year veteran of the weather service, pointed out that the Baltimore station's future may be in doubt because of weather service plans to install automated equipment at BWI by 1995 and shift the forecasters to Washington.
"Most of us at the airport will just retire," he said. "But I hope they keep it going at the Custom House. This is what people live by, the temperature. It means a lot to the man on the street."