- Meteorologists (weather forecasters) did not have computers or data from satellites during World War II (1939-1945). They used instruments on the ground, at sea and in the air to collect weather data. Temperature is measured with a thermometer. Wind speed is measured with an anemometer. Direction of wind is determined by a wind vane. Precipitation in the form of rain is measured with a rain gauge. The force with which the atmosphere pushes down on a specific location on Earth is called atmospheric (air or barometric) pressure. It is measured with a barometer and reported in millibars or inches depending on the instrument used.
- During World War II, the Weather Bureau in the United States worked closely with the U.S. military and England, America’s ally in World War II. The Weather Bureau became the National Weather Service (NWS) after World War II.
- The D-Day invasion was planned for June 5, 1944 along the coast of Normandy, France. Its purpose was to liberate Europe.
- Six months before the invasion, teams of U.S. and British meteorologists prepared by studying weather charts from the past.
- U.S. Gen. Dwight D. Eisenhower was the Supreme Allied Commander for the D-Day invasion. Group Captain James Stagg was a British meteorologist. He was appointed Eisenhower’s chief meteorologist. Col. Donald Yates was the U.S. Army Air Forces chief meteorologist. He worked closely with Stagg. They each had teams of meteorologists working under them.
- As June 5, 1944 drew near, the teams disagreed about the forecast. Ideal weather conditions were calm seas, clear skies, and light winds.
- There would not be ideal conditions on June 5th. Some meteorologists predicted the weather would not be too bad. But Stagg was worried.
- Stagg reported to Eisenhower that the forecast for June 5th would be stormy and unsettled. Eisenhower postponed D-Day for 24 hours.
- As Stagg studied weather data on Sunday, June 4, he noticed a break in the weather for June 6th. He reported better weather conditions to Eisenhower. The D-Day invasion was on.
- In 1971, Stagg wrote a book (Forecast for Overlord) about his D-Day weather forecasts, which many people believe are the most famous weather forecasts ever made.
STORY by Judith Stanford Miller, Redwood Learn editor
Oct. 12, 2020 – During World War II (1939-1945), meteorologists (weather forecasters) did not have computers or satellite data. But they had weather data. They collected data from instruments on the ground, on ships at sea, and from kites and balloons in the air. Teams of meteorologists from the United States and England worked closely together before the D-Day invasion (June 6, 1944). The invasion took place along the Normandy, France coast on the English Channel. Their weather forecasts for June 5th and June 6th are the most famous weather forecasts in world history.
The United States Weather Bureau during World War II
In 1941, Hawaii was not yet a state in the United States. However, the U.S. Navy had a naval base in Pearl Harbor, Hawaii for its Pacific fleet.
On Dec. 7, 1941, the Japanese military attacked the U.S. Naval Base at Pearl Harbor. As a result, the United States entered World War II. The Allies (United States, England, Canada, the Soviet Union and many other smaller countries) were fighting the Axis Powers (Germany, Italy and Japan).
After the Pearl Harbor attack, U.S. President Franklin D. Roosevelt (FDR) issued an executive order about the Weather Bureau. FDR’s executive order defined the role of the Weather Bureau during the war. The primary work of the Weather Bureau was to give meteorological information of value to the military. The Weather Bureau was told to work closely with the War Department and the U.S. Navy.
Collecting weather data in 1941
Operation Overlord: D-Day (June 6, 1944)
A major turning point during World War II was the Allied invasion, called D-Day, along the Normandy, France coastline. It began on June 6, 1944. Its purpose was to liberate (free) France and other countries in western Europe. The countries had been forcibly taken over by Germany in 1940.
The D-Day invasion was the largest air and sea invasion ever attempted. Thousands of soldiers landed on the beaches of Normandy from boats on the English Channel. And thousands of paratroopers dropped behind enemy lines inland from airplanes.
The English Channel is the body of water that separates England from Europe.
Clear skies, low tide, calm seas, and calm winds were ideal for a successful D-Day landing.
To prepare for the invasion, teams of meteorologists began working together six months before the landing. Some of the weather experts were from the United States. And some of the weather experts were from England. The American and British meteorologists studied weather charts from the past. They thought studying these charts would help them predict the weather for early June 1944.
Gen. Dwight D. Eisenhower of the United States was the Supreme Allied Commander for the D-Day invasion. He appointed James Stagg, a British meteorologist, as his chief meteorologist. Col. Donald Yates was the chief meteorologist for the U.S. Army Air Forces. Yates and Stagg worked closely together. They had many other meteorologists helping them.
They studied daily changes in temperature, wind speed, height of clouds, and air pressure.
Air Pressure and Weather Fronts
The National Weather Service (NWS) is the new name for the Weather Bureau. The NWS explains that Earth’s atmosphere is a jacket of gases (mainly nitrogen gas and oxygen gas) that surrounds the planet. Although it seems like these gases could easily float away into space, gravity is constantly pulling the atmosphere toward Earth’s surface. The force with which the atmosphere pushes down on a specific location on Earth is called atmospheric (air) pressure.
The NWS also explains that atmospheric pressure depends on two things: 1) the weight of the atmosphere in a specific location and 2) the temperature of the air. At a low elevation – such as in a valley – there is a lot of atmosphere above and the weight is very heavy. That means there is higher atmospheric pressure at lower elevations. At higher elevations, such as the top of a mountain, there is lower atmospheric pressure.
Atmospheric pressure is measured with an instrument on the ground called a barometer. Pressure data is collected at many locations across the U.S. by the NWS. On weather maps, pressure readings are seen as a blue “H” for high pressure or a red “L” for low pressure.
What are “fronts?”
A “warm front” is a mass of warm air that takes the place of cold air.
A “cold front” is a mass of cold air that takes the place of warm air.
A “stationary” front is a mass of warm air that meets cold air but neither mass moves. The result is a period of rainy weather.
An “occluded front” occurs when a cold front catches up to a warm front since cold fronts move faster than warm fronts. When this happens the result is most likely dry air (no precipitation/rain).
D-Day Weather Forecast
The weather forecasts for June 5th and June 6th were the most famous forecasts ever made. The stakes were very high. Thousands of soldiers’ lives depended on having weather that allowed them to land. Rough seas, high wind, rain, and cloudy skies would make an air and sea landing next to impossible.
D-Day was first planned for June 5, 1944. Stagg and Yates, along with their teams, studied weather charts as the date drew near. At the time, British meteorologists had begun collecting data from high altitude balloons. Stagg poured over data from these balloons.
All of the meteorologists studied data from the past to predict the next few days. They did not agree about the forecast for June 5, 1944. Some of the teams predicted the weather would be favorable. Others disagreed. Talk about pressure!
Stagg had to decide what he would report to Gen. Eisenhower. He was uneasy because the data he had from weather balloons did not match what he was seeing on the ground. He reported to Gen. Eisenhower on Saturday, June 3rd that the forecast for June 5th would be unsettled and complex with high winds and low clouds. So Gen. Eisenhower postponed the invasion. But he said he would wait for Stagg’s early Sunday (June 4th) forecast to make a final decision. Stagg’s forecast early Sunday for Monday’s (June 5th) weather did not change a lot from his Saturday forecast. Gen. Eisenhower postponed D-Day for one day. They would meet again for an update late Sunday evening.
As Stagg was studying updated weather charts on Sunday afternoon, he noticed something unexpected. After a cold front passed, there would be a break in the bad weather. At the Sunday evening update, he gave Gen. Eisenhower and other military leaders his forecast for better weather. The D-Day invasion was on for June 6th.
Sure enough, June 5, 1944 turned out to be a stormy, windy day. What they could not have known at the time was how unusual the weather was that day. In his 1971 book, James Stagg wrote:
“When the depression L5 moved across northern Scotland during the night of June 4th/5th it produced the lowest atmospheric pressure (976.8 millibars or approximately 28.85 inches) that had been recorded in any June this century [20th century] anywhere in the British Isles.” (Forecast for Overlord by James Stagg, p. 124)
If the invasion had taken place, many lives would have been lost at sea before the soldiers could have even reached the beaches of Normandy.
Stagg reflected later that weather in the English Channel that summer acted more like winter weather than summer weather. It was very odd. Studying past weather charts was helpful but not the entire story because of the unusual summer weather. Stagg’s training, experience and patience allowed him to correctly forecast the weather.
Forecast for Overlord by James M. Stagg (1971)
In 1971, Stagg wrote a day-by-day account of his D-Day forecasts in his book, Forecast for Overlord.
“The storms of June 1944 were abnormal in their severity. It is also now a matter of record that disturbed weather dominated the whole summer and autumn of that year. The only interval [time period] of reasonably quiet settled weather occurred in the first half of August.” (Forecast for Overlord, p. 126)
In his final paragraph, Stagg states:
“I cherish the privilege of having worked with them [teams of American and British meteorologists] and of having had a part in framing and presenting the forecasts; I treasure also the letter which General Eisenhower sent me from France. And as a bonus I also benefited from what I learned in those Overlord days of the strange and unexpected ways of our atmosphere even in summer and their effects on the weather around our Islands and on the seas of the English Channel.” (Forecast for Overlord, p. 128)
- How did the U.S. Weather Bureau collect data during World War II (1939-1945)?
- What was the purpose of the D-Day invasion that was first scheduled for June 5, 1944?
- Who was the Supreme Allied Commander for the D-Day invasion?
- Where did the D-Day invasion take place?
- Who was the chief meteorologist for the D-Day invasion?
- How did the chief meteorologist and those working with him prepare for the D-Day forecasts?
- What was the final forecast for June 5, 1944? Based on that forecast, what happened?
- Why was the D-Day invasion able to go on June 6, 1944?
EXTRA! EXTRA! (Related links and Inquiry Questions)
2. What would have likely happened if the D-Day invasion had launched on June 5, 1944?
3. Compare the weather chart shown at the top of the story (June 6th) to the weather charts for June 5 in the story. How do these charts support the text of the story that Stagg correctly predicted weather conditions for June 6th would improve?
3. Why was James Stagg under a lot of pressure as he reported his weather forecasts to Gen. Eisenhower in the days leading up to D-Day?