Armed with a thermometer, a reporter concluded what subway riders already know: When it’s hot outside, it’s even hotter below.

A decidedly sweaty and quasi-scientific survey by the New York Sun of more than 10 stations showed an array of temperatures that ranged from a low of 86.3 degrees in select parts of Grand Central station to a slow-roast of 102.7 degrees at the Chambers Street station on the nos. 1, 2, and 3 line. The results, logged by a $35 digital thermometer purchased from RadioShack, did not surprise riders like Joe Desposito.

“It’s unbearable,” Mr. Desposito, 47, said, before stepping onto a no. 3 train at Chambers Street. “Once you step out, you feel like you’re in an Easy-Bake Oven.”

Temperatures in the city reached a high of 91 degrees yesterday, according to the National Weather Service, which issued a heat advisory. The temperature alone made it one of the hottest days of the year, but with high humidity the temperature felt closer to 100 degrees. Con Edison set an electricity-production record, reaching 12,361 megawatts at 4:00 p.m. yesterday, topping the previous record of 12,207 megawatts set August 9, 2001.

Below ground, respites for riders came in two places: subway cars with working air-conditioning units, and beneath several ventilation fans spread along the platform of the Lexington Avenue line at Grand Central station, which pipes air into the station from Grand Central Terminal, using steam produced by the building and mixing it with lithium bromide. The highly evaporative combination creates 40-degree water that is pumped through copper pipes throughout Grand Central’s ventilation system, cooling the terminal and parts of the station below it.

The system is similar to one used in Washington, D.C.’s Metro, which has “air-chilled” – as opposed to air-conditioned – all of its 46 underground stations since the system opened in 1976.

Only Union Square station has another mechanism – fans – that attempts to address the high temperatures below ground. But on the platform, the fans did little to lower the 99.9-degree temperature recorded at the height of the evening rush.

For riders on the platforms of the remaining 467 stations, cool air comes only when it escapes from the open door of an air-conditioned car. The whoosh of a departing train provides the necessary circulation. The result, at least yesterday, is anywhere between a one and three-degree drop in temperature.

Reasons vary for why one subway station is hotter than another. Depth is not necessarily a major factor. The F-train stop on Roosevelt Island, located deep underground, was a balmy 92.5 degrees around 4 p.m. with refreshing breezes blowing through the station. Heat given off from a subway car’s brakes and air-conditioning unit, along with the sheer volume of trains traveling through a station, are major contributors to a platform’s temperature, a spokeswoman for New York City Transit, Deirdre Parker, said. Circulation is another issue, one reason why all three stations along Chambers Street recorded some of the highest temperatures yesterday.

Like so many riders, Carmen Aguado, who works as a messenger for UBS, the banking and financial services group, has her coping mechanisms, mainly water and patience.

“I’m a messenger, so I have to keep it up,” Ms. Aguado, 48, said before piling onto a 74-degree air-conditioned train.

A retired police officer, Dennis Gill, can take the heat; it’s the cars that aren’t air-conditioned he can’t stand.

“I’m not running for trains,” Mr. Gill, 58, said. “I just catch the next one. At my age, its not so good to be running for trains.”

So Mr. Gill waited yesterday afternoon at Queens Plaza as an E train pulled into the station and opened its doors. Though it was rush hour, the car was relatively empty.

“Talk about a non-air-conditioned car,” Mr. Gill said incredulously.

The temperature inside the car: 98.2 degrees and climbing.