Q: Why is it so much hotter on the subway platforms than it is outside?

A: Cement and asphalt absorb heat from the summer sun, which soaks into the ground and radiates into the subway station and has the unfortunate effect of leaving the stations hot and uncomfortable even when the heat breaks.

On top of that, the subway cars’ brakes and air-conditioning units throw off waves of heat that get trapped underground. The physics of air circulation are ridiculously complex, but suffice it to say that even with an entrance at either end of the station, there isn’t nearly enough of a cross breeze to circulate the warm air out and the cooler air in.

A couple of stations seem to have fans installed, though New York City Transit wasn’t sure why some, like Union Square, got blowers and others didn’t. But the fans just circulate stale air and so are not very effective.

I do recall an occasion four or five years ago when NYCT was doing some work at the Borough Hall Station in Brooklyn. The workers had set up an enormous fan – four or six feet in diameter – to get rid of the sawdust they generated. The fan hauled the hot air up and out of the station and really made it pleasant to stand on the platform. Because passengers spend a lot more time on the air conditioned subway cars than they do on the platform, the city doesn’t deal with the stuffiness factor, but all it would take is big fans, and at least some of the stations – the ones near the surface – could be a whole lot more comfortable.

Questions about getting around the city? E-mail transit@nysun.com.