Barometers have been around for hundreds of years, but they still are an effective way to monitor the approach of severe weather. It’s difficult to think of air as having weight, but it does, and the weight is expressed as atmospheric pressure. In the seventeenth century, Italian physicist Evangelista Torricelli discovered that changes in atmospheric pressure had an odd effect on mercury. Torricelli invented the first barometer by filling a long glass tube with mercury and then turning the tube upside down and placing its open end into a small cistern of mercury. When bad weather caused the atmospheric pressure to decrease—that is, when the air became lighter—there was less pressure exerted on the mercury in the cistern. The mercury in the cistern would rise, and this allowed the mercury in the tube to drop. By marking the glass tube to measure inches, Torricelli could accurately monitor changes in atmospheric pressure.

When the weather is calm at sea level, the column of mercury in a barometer will stay at 29.92 inches on the glass tube. Barometers quickly became standard equipment on sailing ships when mariners discovered that the approach of severe weather at sea caused the mercury in the glass tube to fall to a lower level.

The hurricanes that form over the ocean, marked by extreme low pressure, have evoked fear and respect since prehistoric times. Indians living near the Atlantic called the powerful storms of late summer “hurracan,” which also was their word for “evil spirit.” The powerful storms have sent countless ships to the bottom of the Atlantic. The barometer gave European sailors a chance, at least, to detect and avoid these terrible storms.

The always cautious Sam Cutler—who, like the ancient philosopher Socrates, was more focused on what he didn’t know than what he knew—wanted to know more about hurricanes. While at the Weather Bureau’s Miami office, he dropped in on meteorologist Ernest Carson. Cutler, true to his training as a safety officer, wanted to understand the threat hurricanes posed to the vets on the Keys. He also told Carson he wanted to receive any hurricane warnings issued by the Miami office.

Carson’s Miami office was responsible for monitoring conditions from the northern end of Key Largo to West Palm Beach. Conditions in Islamorada, where the work camps were headquartered, were monitored by the Weather Bureau office in Key West, and hurricane warnings for Islamorada were issued by the Weather Bureau’s office in Jacksonville. But Carson told Cutler that he’d be glad to give him copies of any warnings received by the Miami office.

Carson also passed along a weatherman’s rule of thumb—a hurricane does not have to strike the Keys to cause hazardous conditions on the islands. Anytime a hurricane approaches the Keys or passes through the Straits of Florida—the narrow body of water that separates the Keys from Cuba and The Bahamas—conditions are very dangerous on the islands, Carson warned.

The information undoubtedly made an impression on Cutler, who had spent his career devising ways to minimize risks in the workplace.

The reason for the danger is simple—the Straits of Florida are only about 90 miles wide, and a hurricane can be several hundred miles wide, with hurricane-force winds of at least 74 mph extending 50 miles or more all around the eye of the storm. A hurricane in the northern hemisphere rotates counterclockwise, and the winds on the right side of the hurricane’s eye are even more dangerous because they have the added force of the hurricane’s forward speed. Thus, if a hurricane has 100 mph winds at its center and the storm is moving at 15 mph, the winds on the right side of the eye will be 115 mph. Sailors refer to this part of a hurricane as the dangerous semicircle. A ship at sea will go hundreds of miles out of its way to avoid this part of a storm.

The winds on the left side of the eye are less dangerous because the storm’s forward motion reduces their force. This means that the storm with 100 mph winds at its center traveling at 15 mph will have winds of only 85 mph on its left side. Sailors refer to this side of a storm as the navigable semicircle, and if a ship’s captain must sail near a hurricane, he will try to position his vessel so that it is on the left side of the eye’s forward motion.

A hurricane moving westward through the Straits of Florida subjects the Keys to the situation that sailors go to great lengths to try to avoid. The islands are squarely within the hurricane’s dangerous semicircle. Unlike a ship at sea, however, the Keys can’t maneuver away from the storm.

A hurricane in the straits is “one of the worst situations that you can get on the Florida Keys,” Ivan Tannehill, assistant chief of the U.S. Weather Bureau’s Forecasting Division, said in 1935. The stronger winds on the hurricane’s dangerous semicircle pile up water against the islands, he said.

Weather Bureau officials had long ago realized that one of their biggest problems was crafting hurricane advisories that alerted people to danger without creating panic. It was a difficult line to walk—so difficult, in fact, that the Weather Bureau reminded meteorologists of the agency’s policy of issuing warnings in bulletins sent out every summer before each hurricane season. The bulletin reminded meteorologists that when they ordered the red-and-black hurricane warning flags to be hoisted at coastal stations, they were telling residents that “there is immediate need of precautionary measures to save life and property within the area encompassed by the warning.”

In other words, if you see the hurricane warning flags flying, you are in danger and it’s up to you to protect yourself.

Storm of the Century: The Labor Day Hurricane of 1935