At four-fifteen on the morning of June 4, 1944, Group Captain James Martin Stagg, a meteorologist for the British military, arrived at the library of a grand manor house on the southern coast of England. On the other side of the room was General Dwight D. Eisenhower, the supreme commander of the Allied Forces—the man Stagg needed to convince that D Day should be postponed.

The conditions for the launch had to be just so: a full moon for visibility, low tides to expose the underwater German defenses. That left a narrow window of just three days in June, and June 5th was the date the generals had settled on. But the Allies’ warships and aircraft would also need calm seas and clear skies, and here Stagg and his team had foreseen a problem.

Even though the skies outside promised a bright morning, the meteorologists calculated that a parade of storms was poised to barrel across the Atlantic, hampering the prospects of success. The generals were wary of any delay, but Eisenhower reluctantly agreed to hold off.

A few hours later, Stagg had better news. Allied weather stations were reporting a ridge of high pressure that would reach the beaches of Normandy on June 6th. The weather wouldn’t be ideal, but it would be good enough to proceed. Eisenhower gave the order to reschedule the invasion.

It’s hard to overstate the importance of that weather forecast, as John Ross makes clear in a book on the subject. Had the Allies gone ahead as planned, the invasion probably would have failed. Had they postponed it until the next interval with favorable moon-and-tide conditions, they would have lost the element of surprise. The German meteorologists had also foreseen the storms, but they’d missed the significance of the brief glimpse of calm. They were so confident that an Allied attack was impossible that Field Marshal Erwin Rommel, the commander of the Normandy defenses, decided to take a few days’ leave for his wife’s birthday. He’d even bought her a new pair of shoes in Paris for the occasion. Years later, when Eisenhower was asked why D Day had been a success, he reportedly said, “Because we had better meteorologists than the Germans.”

In our world, weather forecasts are so ubiquitous that we treat them as notable only when wrong. It’s easy to forget what a crucial role they play, and to overlook the monumental achievement they represent. But Andrew Blum’s new book, “The Weather Machine” (Ecco), asks us to pause and marvel at the globe-spanning networks of collaboration required to turn the weather from something we experience to something we can predict.

It wasn’t always possible to be so complacent. Wartime made the stakes of weather forecasting especially plain. Sometimes, as with D Day, visibility was important; at other times, cloud cover and fog could help conceal a position. Alongside the battle for land, sea, and air, then, a quiet war over the atmosphere was being waged. The weather war even had its own clandestine undercover missions in search of mundane treasures like data on temperature, pressure, and wind speed.

In the Northern Hemisphere, storms tend to move from west to east, so any prediction of what lay in store for Europe relied on knowing what was happening in the Atlantic. The Allies, who were in control of all the major landmasses that lined the ocean, had the upper hand. The Nazis had to use long-range aircraft and secret weather ships to gather observations. The Allies tried to sink those ships, but the Nazis also made use of radio reports of cancelled English soccer matches for hints about weather to the north.

So, as Blum explains, in 1942 the German government came up with an ingenious solution. With help from the Siemens-Schuckertwerke group (a predecessor of the modern-day Siemens) and others, it developed a series of automated weather stations: these were an intricate array of pressure, temperature, and humidity sensors, encased in storm-resistant metal containers and equipped with batteries and a radio antenna. Some would hitch rides with the Luftwaffe and transmit weather readings from remote locations on the edge of Europe. By 1943, the devices were powerful enough to communicate across the Atlantic. That year, a Nazi submarine sneaked to the northernmost tip of Newfoundland, where a team of German soldiers took ten cannisters ashore on two rubber dinghies. For the plan to work, the weather station needed to stay undetected after it had been left in the wilderness, so they labelled the equipment “Canadian Meteor Service” and scattered the site with a host of American cigarette packs. Only in 1981 was the ruse discovered.

The weather station itself was operational for a matter of months before the radio frequency jammed and the batteries died. But this mission marked one of the few known incursions of the Nazis on North American soil: getting hold of weather data was important enough to take the risk.

To anyone in Europe, the indispensability of that data hasn’t diminished over time. But what in war was treated as a weapon to be mobilized against your enemy has become, in peace, the stuff of international coöperation for the common good. That’s always been the way with weather. It has a strange ability both to unite and to divide.

Blum tells a story about the early telegraph which highlights this point. By 1848, more than two thousand miles of telegraph lines had been laid across the United States. They were a technological marvel, but they were prone to problems when it rained. Every morning, telegraph operators checked with their colleagues in the surrounding cities to see what the weather was like. “If I learned from Cincinnati that the wires to St. Louis were interrupted by rain,” one operator was recorded as saying, “I was tolerably sure a ‘northeast’ storm was approaching.”

The effect was to change people’s perception of time and space. Being able to communicate through the telegraph might have made the world seem smaller, but those weather reports also made the world bigger, creating distance between places on a map. America was no longer a collection of small communities but a coherent whole, spread out across a landscape, resting beneath a solitary sky. The weather changed from something that you experienced to a shared pattern stretching hundreds of miles which you could see coming. “Once the news could travel faster than the winds,” Blum observes, “the winds need no longer come as a surprise.” This connectivity could be ruptured: when the telegraph network was interrupted during the Civil War, the flow of weather reports stopped and the towns and villages went back to not knowing what the skies might hold for the future.

Modern weather forecasting, too, sits in the intersection of unity and division. The rockets and satellites launched during the Cold War helped provide a major breakthrough in weather prediction: the first images sent from the edge of space showed Earth wrapped in bands and whirls and vortices that stretched thousands of miles. It was a view of a world that belonged to all of us. As Blum’s detailed (sometimes overly so) chapters on satellites make clear, any snapshot of our atmosphere is something that unites the world even while enabled by technology designed to destroy it.