As a kid, late May skies back home in northeast Ohio seemed impossibly blue. A proud member of my high school marching band, I marched in our small town’s Memorial Day parade every year in the late 1990s.

A few years later, I was a young Air Force lieutenant on my first deployment to Afghanistan, forecasting weather for combat missions. The spirit of Memorial Day for me now is one of reflection; to consider the servicemen and women I knew, men and women I’ll never know — and the very human relationship to weather and war.

For strangers and friends, weather or sports are often the first step in exchanging pleasantries. But weather and war — who talks about that? Combat meteorologists do. They spend most of their time discussing this nexus with military planners and commanders. Their sole charge? Leverage battlefield weather conditions to solve problems for allies and create problems for the enemy.

Around the sixth century B.C., a Chinese military general named Sun Tzu penned his thoughts known today as “The Art of War.” He appreciated the part played by weather, famously writing: “know the enemy, know yourself; your victory will never be endangered. Know the ground, know the weather; your victory will then be total.”

Weather conditions determine the effectiveness of a commander’s plans, the accuracy of an archer’s arrow, the grip of a tank’s tread and the efficacy a logistician’s supply chain. It is the first information briefed to most U.S. joint task force commanders and their staffs in preparation for combat.

Unexpected changes in weather can render a leader’s best plans dust in the wind — which is why good commanders and great sergeants train as they fight — preparing for mission accomplishment in the conditions they will be asked to achieve it.

“If it ain’t rainin’, it ain’t trainin’!” is a popular mantra among empathetic sergeants in the field who see the discomfort on their soldier’s faces with every raindrop soaking into their uniforms, onto their weapons and over their rucksacks. Woe to the squad that maneuvers through rain into high elevations to temperatures below freezing — multiplying problems as buttons, zippers, charts, and clothing begin to freeze.

Frostbite and hypoxia were bitter enemies of 10-man bomber aircrew during World War II. The average attack elevation was around 21,000 feet, and temperatures inside and outside the unpressurized aircraft as cold as minus-40 degrees. Airmen would be issued just enough oxygen and sheepskin insulation to stay alive. Bodies shivering with cold and adrenaline endeavored to align their bombsites on factories and rail yards, their machine gun on incoming fighter planes, their wing tips safely offset and noses in the direction of the enemy.

There may be no more famous example of the pivotal role of weather in combat than the Allied invasion onto the beaches of Normandy on June 6, 1944. Fair winds, following seas, high tides and full moon were among necessary conditions for mission success. Otherwise ships would sink, landing craft would fall short and drop zones would be obscured by fog.

Not unlike today, forecasts differed with each meteorologist; those advising Axis commanders anticipated lingering unfavorable weather for weeks, thereby shaping estimates that the Allies would not invade earlier. Forecasts even differed among the Allied weather teams.

The British officers responsible for supporting the assault phase of Operation Overlord nailed the forecast. Their briefing recommended the supreme commander of Allied Expeditionary Forces, instead of launching the invasion on June 5, wait 24 hours. The fate of the free world rested in a weatherman’s judgment and a five-star general’s trust in him.

On June 6, small amphibious open-shoe-box-shaped “Higgins boat” troop carriers moved at 12 knots toward the beaches of Normandy. Swaying induced seasickness. Pounding hearts matched the sharp thunder of exploding artillery from the enemy. A lower tide forced early evacuation — many troop carriers were stuck on sandbars at a distance from the beach — and the infantry had to descend into the cold waters of the English Channel.

Just above 50 degrees Fahrenheit, the sea was cold enough to take anybody’s breath, and deep enough to slow their attack speed to a teeth-grinding plod. Axis machine gunners dug-in to the rocks could more easily target them.

The orders to the Allied troops were straightforward: Attack directly into the jaws of the enemy. Survive the sea slog and hit the beach. Survive the 300-yard sand and shrapnel onslaught and hit the rocks. Survive the climb and destroy the nearest enemy firing positions. Liberate the nearest town and triage the wounded. Secure the bridges and cut off the Axis supply chains. Establish a foothold into Europe where troops of the free world could pour in at will.

The accurate forecast ensured wave after wave of Allied units could do just that. All told, 156,000 Allied troops stormed 50 miles of beaches on D-Day. Planning, heroism, tactics and teamwork changed the tide of the war.

Back on the home front, when the grill is fired up and ice-cold beverages are in the cooler, consider the warm breeze as it moves the smoke, leaves, petals and flags. Whether a handheld stars and stripes is planted in a potter among the wildflowers, into a spare tire at the corner of a dirt driveway, or atop a brushed nickel pole in cement, when you see the wind ripple down the fabric of the flag this Memorial Day, remember those who gave the ultimate sacrifice, the battles they fought and the storms they weathered.

The author is an active duty Air Force officer with a bachelor’s degree in atmospheric sciences from the University of North Dakota.