A pale box the size of a lunch tin, half settled into wet sand with a string trailing out of it.
Foam pitted and yellowed, algae dried on the underside, sand packed into every vent.
A teacher out walking the shoreline at Ludington picked it up on the eleventh of February, 2026.
It had left the ground in Wisconsin on the nineteenth of October, 2025, and it never flew here.
It came down in the water, off the Door Peninsula, on the Wisconsin side.
So the interesting part is not the flight. It is how a thing like that survives a winter.
Why a foam box floats for 1 whole winter
Everything about a radiosonde is built for one short use, and that is exactly what makes it durable.
The shell is closed cell foam wrapped around a sensor board, a radio and a battery, because the whole package has to stay under a couple of pounds for a balloon to lift it. Foam plus a sealed air space means it rides high on the surface instead of swamping.
Nothing in it rots on a useful timescale either. Plastic housing, foam, a printed board, and fresh water rather than salt, so even the exposed contacts come through.
Then there is how a floating object actually crosses a Great Lake, which is not how a boat does it.
Three things push it. Wind on the inch or two standing above the surface, the surface current underneath, and in winter the ice, which can freeze around it, carry it for weeks and then let it go somewhere else entirely.
Those three work on different clocks and often in different directions.
100,000 feet up and 1 office in Wisconsin
The instrument is a radiosonde, and it rides under a balloon filled with hydrogen or helium.
The balloon swells as the air thins around it until it bursts, typically somewhere above 100,000 feet, and the box comes back down under a small parachute. The whole ascent runs about two hours.
During that climb it radios temperature, humidity, pressure and position back continuously, which is the entire point of the exercise.
This one went up from the weather office at Green Bay, which is the only office in Wisconsin that launches them.
Its parachute put it into the lake near Washington Island, at the tip of the Door Peninsula.
From there the box was no longer an instrument. It was debris.
20 percent of 75,000 ever come back
The scale of the program is the part that makes a beach find ordinary rather than freakish.
The weather service puts the national total at roughly 75,000 radiosondes a year, with one office page giving about 76,000, and reckons perhaps 20 percent are found and returned.
That leaves something like 60,000 a year coming down in forests, fields, rooftops and water with nobody the wiser.
What a finder is told to do has changed, and the change is worth stating carefully. One weather office page now says there is no need to send one back, and asks only that the string be cut off and discarded.
Older coverage describes postage paid return envelopes and most finders using them.
So the current instruction is office by office, not a single national rule.
What 115 days of no tracking leaves out
Here is the honest limit on this story.
Nothing aboard records anything once the flight ends. The data went out live during the ascent, so none of it was waiting in the box, and a test buoy or a green fireball gets reconstructed the way this cannot.
Which route it took, how long it sat in ice, whether it crossed in one push or in a dozen, none of that is recoverable, and the local report says only that wind pushed it to the Michigan shore.
The flight numbers are solid because they were measured. The crossing has no numbers at all.
Typical figures put a radiosonde above 100,000 feet and more than 100 miles downwind, and the launch office describes the two hour climb.
Everything after the splash is inference.
2 finds on one shoreline
She had found one before, years ago, and thinks she threw that one out.
This time she kept it, and a friend turned up another one nearby, which is the detail that says most about how many of these are lying around.
She works in a classroom, and what interests her is the teaching piece, the question of what the thing is and what it is doing on a beach.
That is the whole recovery program now. Not a return envelope, but somebody looking down.
A weather balloon payload is designed to be thrown away twice a day, every day, from every office.
Most of them are still out there.
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