1. Weather Balloons Carry Radiosondes Up Twice a Day
The Japan Meteorological Agency launches a rubber balloon carrying a radiosonde at exactly 09:00 and 21:00 Japan Standard Time every day. This happens at 16 weather stations across Japan and at the Showa Base in Antarctica [1]. The radiosonde measures air pressure, temperature, humidity, wind direction, and wind speed up to an altitude of about 30 km [1].
On the ground, the balloon has a diameter of 1.6 m. As it rises, the air pressure outside decreases, causing the balloon to expand. At an altitude of about 30 km, the balloon reaches a diameter of about 8 m and bursts [2]. This means the diameter has grown about 5 times larger. The balloon rises at a speed of 300 to 400 m per minute [2]. At this speed, it takes about 75 to 100 minutes to reach that height. Once the balloon bursts, the radiosonde separates and begins to descend. Where will it land?
2. Why Winter Balloons from Akita Cross the Mountains to the Sea
After the balloon bursts, the radiosonde descends slowly using a parachute [1]. The Akita Local Meteorological Observatory explains that in winter, most radiosondes launched from Akita cross the Ou Mountains and the Kitakami Mountains. They land far away in the Pacific Ocean along the coast of Iwate Prefecture [3]. From Akita, this means crossing the entire mountain range to the east and reaching the sea on the other side.
However, depending on atmospheric conditions, they may also land on land [3]. This is specific to the winter pattern in Akita; it does not mean balloons land in the sea everywhere in Japan. Even if the wind carries them that far, how strong must that wind be?
3. The Jet Stream: Strong Westerly Winds High in the Sky
The jet stream is a strong flow of westerly winds high in the atmosphere. It can be thousands of kilometers long, hundreds of kilometers wide, and several kilometers deep. Its wind speed can exceed 100 m per second [4]. To put that in perspective, 100 m per second is equivalent to 360 km per hour.
It is said that the jet stream often becomes stronger near Japan during winter [4]. The Akita Local Meteorological Observatory explains that the radiosondes are carried across the mountains to the sea because they ride on this westerly wind [3]. It is important to remember that the wind high in the atmosphere can have a completely different strength than the wind felt on the ground. If the radiosonde is carried by the wind, tracking its path can reveal information about the wind itself.
4. How a Radiosonde's Path Measures Wind
The Japan Meteorological Agency uses "GPS radiosondes." These devices use GPS signals to obtain data on altitude, wind direction, and wind speed [5][1]. By examining the direction and speed at which the radiosonde drifts, scientists can determine the wind direction and speed at high altitudes.
In the example of winter in Akita, the drift direction was from west to east. The change in position during the ascent serves as a wind observation. The landing spot itself is not the observation value; rather, the movement during the flight is what provides the data. So, how far do radiosondes drift in other countries? Let us look at explanations from the United States.
5. How Far Balloons Can Drift in the United States
According to the US National Weather Service (NWS), balloons can exceed 115,000 feet (about 35 km) in altitude during flight. They expand as they rise and eventually burst. The flight can last more than 2 hours, and the balloon may drift more than 125 miles (about 200 km) from the launch site [7].
This connects to the explanation for Akita in winter: both involve being carried by high-altitude winds. If a radiosonde drifts more than 200 km over 2 hours, the landing location can be surprisingly far from the weather station that launched it.
6. What to Do If You Find a Radiosonde
According to the Fukuoka Regional Meteorological Observatory, if you find an instrument, you should contact the phone number written on the device's label or the nearest weather station. They will arrange for recovery. Recovered instruments are used for reuse and operational management [6].
Radiosondes from the Japan Meteorological Agency are not hazardous materials. However, if you find something where the launcher is unknown, or if you find an inflated rubber balloon or a plastic bag, you are advised not to touch them and to contact the nearest police station [6]. To prevent issues if the radiosonde gets caught on power lines, the Japan Meteorological Agency designs them to be light, and the suspension cords use materials with excellent insulation properties [3].
Safety Note: The Japan Meteorological Agency states that radiosondes are not hazardous. The design of the descending instrument also prioritizes the safety of those who might find it.
7. How Enthusiasts Around the World Track Radiosondes
SondeHub is a website used by enthusiasts who search for and recover radiosondes [8]. People who find a radiosonde can report their discovery. If a report is made, other enthusiasts do not need to search for that specific balloon. Meteorological agencies can also learn how widely the landings are distributed [8]. However, this is an initiative by overseas groups; the Japan Meteorological Agency does not encourage recovery by enthusiasts. In Japan, as mentioned in the previous section, you should contact the label or a weather station.
According to US weather service figures, about 75,000 radiosondes are launched each year. Only about 20%, or roughly 15,000, are found and returned [7].
8. Try It: Watch Which Way the Clouds Move
Try this: look at the clouds outside your window. Observe which direction they are moving. Use the window frame as a marker and watch for about 1 minute. You can compare the direction in the morning and evening to see if it changes. The direction may appear different depending on the height of the clouds, so it may not match the direction the radiosonde drifted. However, you can visually sense that there is wind flowing high up, separate from the wind near the ground. Do not lean out of the window or look directly at the sun.
For more information, the Japan Meteorological Agency’s explanation of upper-air observations includes the purpose of the twice-daily radiosonde launches and the items measured [1]. Contact information for finding an instrument is summarized in the guide from the Fukuoka Regional Meteorological Observatory [6].
Safety Note: Do not lean out of windows and do not look directly at the sun. The guide from the Fukuoka Regional Meteorological Observatory on "What to do if you find an instrument" provides instructions for recovery and safety.