Definition
Upper-air weather measurements made by a radiosonde, a small instrument package carried aloft by a free-flying balloon that transmits temperature, humidity, and pressure data back to a ground station as it ascends through the atmosphere. Wind speed and direction at various altitudes are derived by tracking the balloon's path. The data is used to build a vertical profile of the atmosphere from the surface up to roughly 100,000 feet.
Plain English
A weather balloon carries a small sensor package up through the sky, sending back readings of temperature, humidity, and pressure at every level it passes. Tracking where the balloon drifts also reveals the winds aloft. The result is a top-to-bottom snapshot of the atmosphere.
Context Anchor
Seen in weather and preflight planning discussions that explain how pilots get information about winds and weather conditions above the surface.
Derivation
Radio refers to the wireless signal the instrument sends back to the ground. Sonde comes from the French word for a probe or sounding device, originally used for measuring depth at sea. Together, radiosonde literally means a probe that radios its readings back. Observations is used here in its scientific sense -- recorded measurements taken at specific times and places.
Why Pilots Care
Supplies the actual upper-level wind, temperature, and pressure values used to calculate performance, choose altitudes, and anticipate turbulence or icing.
Grounding Statement
Picture a weather balloon climbing through the sky while a small box underneath it sends back live readings of the air it is passing through.
Intuition Check
Do not read “observations” here as someone simply looking at the weather. In this context, it means measured weather data sent back from an instrument carried by a balloon.
Example Sentence 1
The forecaster used the morning radiosonde observations to identify a temperature inversion at 4,000 feet.
Example Sentence 2
Radiosonde observations showed a sharp temperature inversion above the departure airport, prompting the crew to plan a higher initial climb.