Definition
An upper-air weather observation made by a small instrument package (a radiosonde) carried aloft by a free balloon. As it rises through the atmosphere, the radiosonde measures pressure, temperature, and humidity at successive altitudes and transmits the data by radio to a ground station, where it is decoded and used to build a vertical profile of the atmosphere.
Plain English
A weather reading taken by sending a small instrument up under a balloon. As it climbs, it radios back the pressure, temperature, and moisture at each level, giving forecasters a picture of what the air looks like up high.
Context Anchor
Seen in aviation weather discussions, winds-aloft information, freezing-level forecasts, and weather analysis that depends on conditions above the surface.
Derivation
Radio (the means of transmission) plus sonde, from the French word meaning a probe or sounding instrument used to measure something at a distance. So a radiosonde is literally a probe that reports back by radio.
Why Pilots Care
Supplies winds-aloft and stability data that directly affect route selection, turbulence avoidance, and icing forecasts.
Grounding Statement
Picture a small instrument hanging below a weather balloon, rising through the atmosphere and radioing back what it measures at each height.
Intuition Check
Do not read observation here as a visual sighting. In this term, an observation is a measured weather report collected by instruments.
Example Sentence 1
The morning radiosonde observation showed a strong temperature inversion at 4,000 feet, which explained the smooth air and trapped haze below that level.
Example Sentence 2
Before departure the pilot checked the morning radiosonde observation to confirm expected cloud bases along the route.