New York City on Tuesday experienced wind gusts between 60 and 70 mph, as Tropical Storm Isaias raced up the East Coast. Wind speeds this high haven’t been seen in New York since Superstorm Sandy.
Gusts recorded at JFK airport afternoon reached 70 mph, which would beat the largest gust from Superstorm Sandy at 69 mph. High winds during that storm in 2012 caused the collapse of a crane on the top of a skyscraper in Manhattan.
Cranes and other tall structures are at particular risk due to their height. At 100 stories, wind gusts were much stronger.
Wind speeds increase with altitude due to an absence of friction. The higher you go in altitude, the less boundary friction is experienced by the wind, meaning that wind moves quicker.
“The tallest buildings in NYC will see about 10% higher gusts above the 60th floor,” CNN meteorologist Chad Myers said before the winds came through. “This is due to the lack of friction compared to all of the buildings and trees at lower levels.”
“The wind and flooding impacts from Isaias will be similar to what the city has seen from some of the strongest coastal storms,” says Ross Dickman, the meteorologist-in-charge of the NWS in New York City, “but we haven’t seen one this strong in many years.”
Changes in altitude aren’t the only thing that can make tall structures dangerous in a wind storm. The wind tunnel effect refers to how skyscrapers can change wind speeds and patterns on their own.
When wind hits the face of a building, it spreads outward in all directions. This increases wind speed going outward from the building, and can cause projectiles to be picked up and thrown easily.
“Some flat top buildings use tar and stones as the last layer of waterproofing,” says Myers. “The smaller loose stones can also be blown off the building and into other highrises near them creating a glass-break risk or even a danger to pedestrians walking below.”
City building codes generally take wind load into account. Building codes in Miami were revised after Hurricane Andrew in 1992 to require skyscrapers to withstand wind speeds of 175 mph.
New York City building code requires any building to be able to withstand wind pressure of 15 psf (pounds per square foot) at a minimum. Wind speeds from Isaias were only expected to yield 15.2 psf of pressure at a maximum.