Weather balloons and radiosondes
Every day at 00Z and 12Z, roughly 900 weather balloons launch simultaneously worldwide, each carrying a small transmitter called a radiosonde. This is the backbone data every forecast is built on.
What a radiosonde does
A radiosonde is a lunchbox-sized instrument package weighing about 250 grams. As the balloon rises, the sonde continuously measures and transmits back to the ground:
- Temperature (thermistor)
- Humidity (capacitive sensor)
- Pressure (either measured or GPS-derived from altitude)
- GPS position — which gives wind speed and direction from drift
The balloon rises at about 5 m/s until it pops around 30-35 km altitude (near the top of the stratosphere), then the sonde falls back on a small parachute.
Why they still matter in the satellite era
Satellites see the atmosphere from above and infer profiles indirectly. Radiosondes measure it directly, top to bottom. Even with modern satellites, radiosonde data is the ground truth that anchors every model.
Without the twice-daily worldwide launch, forecast skill would drop dramatically.
The launch schedule
Every NWS office launches at 00 UTC and 12 UTC. Extra launches (06Z, 18Z) happen during severe weather.
- Continental US: about 70 launch sites
- Canada: about 30 sites
- Worldwide: about 900 sites
- All roughly simultaneously — a global atmospheric snapshot
Where they land
Radiosondes drift downwind. Depending on jet stream conditions, they can land 50-200 miles from the launch site. Each has a tag: 'Harmless scientific instrument. If found, drop in mailbox, postage prepaid.' NOAA recovers, refurbishes, and reuses.
If you find one and want to keep it, you can — the tag is a request, not a legal requirement. Many hobbyists chase and recover radiosondes as a hobby. Search 'radiosonde chasing' on YouTube.
How to see the data
- SPC soundings — spc.noaa.gov/exper/soundings/
- University of Wyoming — weather.uwyo.edu/upperair/sounding.html — archive going back decades
- Iowa Environmental Mesonet — mesonet.agron.iastate.edu/archive/data/
Reading a sounding
Radiosonde data is plotted on skew-T log-P diagrams — the standard visualization meteorologists use to assess atmospheric stability, cloud potential, and storm environments.