During late 1944 and early 1945 over 9,000 of these terror balloons, known as Fu-Go, were launched by Japan, with the goal of starting massive forest fires over the western U.S.. Only about 300 were documented to reach western North America and they proved to be ineffective weapons. Regrettably, six people lost their lives when they happened upon and explored an unexploded Fu-Go.
There are several books about these fire balloons, one I just finished reading: Fu-go: The Curious History of Japan's Balloon Bomb Attack on America by Ross Coen. Interestingly, this story begins with Japanese meteorological studies during the 1920s.
During the 1920s, a Japanese meteorologist, Wasaburo Oishi, began probing atmospheric structure from a site near Mount Fuji by launching a series of weather balloons called pibals (pilot balloons). By tracking these balloons with a small telescope and doing some simple trigonometry, it is possible to determine the wind speed variation with height (assuming the balloons move with the surrounding air).
Wasaburo Oishi, often considered the discoverer of the jet stream,
Wasaburo quickly found something of great interest during 1923 and 1924: during the winter the winds often increase with height, with winds of great speed located 25,000 to 35,000 feet above sea level, something that later became known as the jet stream (see a figure from his famous 1926 paper).
His 1926 paper did not get a lot of attention because it was written in Esperanto, but today many people give him credit as the discoverer of the jet stream, the current of westerly strong winds (reaching 150-250 mph at placea) in the upper troposphere and lower stratosphere. The Pacific jet stream is known as the strongest in the Northern Hemisphere and is often located over Japan. To illustrate, here is the average upper tropospheric winds (300 hPa, around 30,000 ft) for Dec-Feb 1981-2010. The winds are in meters per second. Roughly double for knots or mph. South/central Japan is jet stream heaven; Ooishi had a big advantage!
During the 1930s, German meteorologists documented the jet stream over Europe (much weaker) and during WWII the jet stream had a huge impact on aircraft operations, sometimes causing westbound aircraft to lose virtually all their ground speed.
As World War II went increasingly against the Japanese, their military decided to try a last-ditch effort to terrorize the U.S. and to force the Americans to divert their resources to fighting fires: the Fu-Go fire balloons. The work of Ooishi and other Japanese meteorologists delineated the existence of the jet stream and the military believed that weaponized balloons could reach the U.S. in 3-5 days.
The fire-balloons were made of paper, 30 feet in diameter, and had an ingenious ballast mechanism (with dropping sand bags!) to keep the balloons at altitude. At the end of its flight, each balloon could drop a collection of incendiary bombs (see image)
Sand bags and fires bombs hand from the lower portion of the Fu-Go fire balloons.
Although most of the balloons crashed in the Pacific, hundreds made it to Alaska and the western U.S. (see map), some getting as far as Iowa. Quite a few landed in the Pacific Northwest, but none were effective in starting fires.
Landing sites of Fu-Go balloons
The Japanese had meteorology against them: the jet stream is strong during winter (November through March) when the western U.S. is not vulnerable to fires. During the summer, when our forests and rangeland are dry, the jet stream is too weak to serve as reliable cross-Pacific transportation.
As I have noted in previous blogs, although fire balloons are not coming from Asia anymore, smoke, dust, and other pollutants are frequent, unwelcome, hitchhikers on the strong Pacific jet stream winds.