Weather balloons are released several times a day across the world. They carry a radiosonde that is an instrument package to take and report weather data during the flight. They transmit their status and readings via radio. The data-stream isn't encrypted and can be received and decoded by anyone with the proper receivers and software. In some cases they can even be recovered when their flights end.

In the US they will typically transmit around 1.7 GHz or 400 Mhz. Some radiosondes in use are shown below

• Lockheed Martin LMS-6 → 1676, 1678, 1680, 1682 MHz
• Vaisala Radiosondes RS92-SGP, RS41-SG, RS92-KL → 400-406 MHz
• Graw DFM-17 → 400-406 MHz

In this area most of the radiosondes you will see are Vaisala RS41-SGs transmitting in the UHF band around 400MHz which our antennas and SDRs can easily receive.

There is free software that can be used to decode and track weather balloons. One example is radiosonde_auto_rx.
This free software is available on github  🔗 Automatic Radiosonde Receiver Utilities

References:

• "Chasing Weather Balloons With Software-Defined Radio", IEEE Spectrum
    🔗 Hunting for weather balloons

• The Sondehub is an outstanding website that provides lots of information and a real-time map of weather balloon activity worldwide.  🔗 Sondehub