It has always been a sign that the weather is about to become unbearable in New York City when they start showing the fall clothes in the stores. This past weekend the Fendi fur coats showed up in the windows of Bergdorf Goodman's, and sure enough, the temperature hit 99 degrees, the hottest day of the summer, and right in the neighborhood, three horses dropped dead.

The National Weather Service may claim this has to do with a heat wave sweeping the eastern seaboard, but in New York (where it was even hotter than in Washington) we know better--it has become insufferable because the fall line has arrived in the stores.

There is aggravation, in these parts, about the horses, though. A group of 15 people in Coney Island may have blithely watched a rape Sunday, (even rapists, it seems, go to the beach in this weather) but when news of the horses' demise broke, the switchboards at the American Society for the Prevention of Cruelty to Animals (ASPCA) went wild. To deal with that, today, with the temperature in the high 90s, the ASPCA held a sidewalk press conference to confront the issue of horse abuse, near Central Park, where the city's 200 or so carriage-horses operate.

It was hot, indeed. Rivulets of sweat ran off the faces of the reporters, falling on their own and each other's shoes. Sweat soaked the shirts of the outraged and outspoken ASPCA officials. No sweat fell off the horses, however, the ASPCA, at midnight Sunday night, having ordered all horses off the street, under the auspices of the the Rental Horse Licensing and Protection Law.

Wasn't it shutting the barn door after the horses died to wait to act until equine mortalities struck, a reporter asked?

"We have no hard and fast rules," said the ASPCA's director of humane law enforcement. Henry Ulrich, a hefty man, who was somewhat peevish in the heat, said, "A horse goes down, we do whatever we can to get him back up, cool him off. It's just an extremely hot spell. I guess people were dying all over the place yesterday. I could die here right now, and if I did, tough luck."

He was wrong, it turned out, about the deaths. There were no heat-attributed deaths in the city this past weekend, though it was the belief, of all participants of the past few days, that the weather could certainly justify murder.

Ulrich was also wrong about the information that all horses were off the street. Just as the ASPCA was finishing its spiel this afternoon, a horse-drawn carriage came slowly across Central Park South, two blond women of middle age inside. Their curious looks at the wonders of New York turned to raw terror as five teams of TV reporters descended, asking them, in effect, why they were contributing to the demise of a perfectly nice horse.

Naturally, given the circumstances, they refused to give their names. They did, admit, however, that they were from out of town, and this was the very first day of their very first visit to the city. That explained everything. Only a tourist--a breed that is the last to know anything--would come to visit, July or August.

It's an unpleasant city, in summer. The heat inversions dull the sun, the sidewalks seem to shimmer, the subways--never the pleasantest ride in town--are likened to the sauna. ("I feel like the Wicked Witch of the West," an unidentified but typical woman told a reporter from United Press International today. "I'm melting.")

Sidewalk strollers, in an attempt to replenish body fluids, surround the street vendors, buying fresh squeezed orange juice and egg-creams. Downtown, where the older buildings will not sustain air-conditioning, or will not sustain it for very long, people still gather on the stoop, to share a six-pack, or, if they are young and poor, meet the sun directly, taking a blanket and some lotion and the radio and heading up to Tar Beach--the roof.

That is not to say that air conditioning fails only in the older buildings. Yesterday, the city steaming, there was a problem with the air-conditioning at the World Trade Center--a 110 story, twin-tower complex the windows of which, you should know, are entirely sealed.

There was a problem with air-conditioning, World Trade spokesmen say, because they thought they saw a body floating in the Hudson River where the Trade Center discharges water from its cooling system, and they wanted to stop the discharge while the river was searched. The shut-down of air conditioning lasted 45 minutes, and no body was found. Spokesmen insist the discomfort during the interval was minimal and that many inside the twin towers were not even aware the air conditioning had been shut off. This is fortunate, the Twin Towers lacking a stoop.