What is a radiosonde, and why search for one?

An automatic sonde launcher seen in Salem, Oregon, July 2021

A radiosonde is the technical term for a weather balloon. It’s a small wireless sensor package used to gather data about the atmosphere such as temperature, humidity, and the wind’s speed and direction. These readings are a key component of weather prediction and research. At over 1,300 sites worldwide, sondes are launched twice a day, hoisted into the upper atmosphere by large hydrogen-filled balloons. Over the course of several hours, they climb up above 100,000 feet (30,000 meters).

When the balloon bursts, the sonde falls gently back to Earth under a small parachute, often never to be seen again. The sondes are low-cost and designed to be used only once. For the agencies that launch the sondes, that’s where the story ends. But for our hobby, that’s where it begins!

Sonde-hunting enthusiasts around the world collaborate to track the sondes and find them after they land. Every second that they’re aloft, the devices transmit data back to their launch point. The transmissions include their current GPS-derived position (latitude, longitude and altitude). The data is considered public information and is not encrypted, so hobbyists have learned to receive and decode the transmissions, typically with a cheap software-defined radio. Volunteers have set up hundreds of listening posts worldwide. The next step is to get out into the field and try to find the sonde based on its tracking data. It’s a great way to touch grass!

SondeHub is the largest public repository of radiosonde data collected by hobbyists. Over the past couple of years, I’ve built tools that help track radiosondes based on the data from SondeHub. My site makes all those tools available to the public. You can also find all the source code on GitHub.

You may still be wondering: but why search for sondes?

Why not? It’s fun. It’s like a high-tech treasure hunt, similar to Geocaching.

More information can be found at:

  • Upper Air Data—explains the purpose of radiosonde launches from one of the agencies that launch them, the U.S. Government’s National Oceanic and Atmospheric Administration (NOAA).

  • SondeHub—the largest repository of crowdsourced balloon tracking data. It includes live maps and an API for retrieving both live and historical data. People also report which balloons they’ve found here. The tools on this site use data from SondeHub.

  • RadioSondy—another repository of volunteer-tracked balloon data.

  • Balloon Path Predictor—type in the launch time and location and the site will use a model of winds aloft and terrain to determine where the balloon is likely to land.

  • Radiosonde auto-rx is software for Linux that can decode transmissions from many common models of radiosondes and upload the data in real-time to SondeHub. It is typically used with an RTL-SDR radio.

  • Bryan Klofas’ Radiosonde Antenna—a nice example of how to build an antenna tuned to 404 MHz, the frequency of many radiosondes launched in the United States

  • MySondyGo is a portable sonde tracker; a useful self-contained device for picking up transmissions from nearby balloons while in the field.

  • Radiosonde North America—an active, friendly Facebook group where U.S.-based sonde hunters post photos, stories from the hunt, and questions.