Definition
A small, free-rising balloon released into the atmosphere and tracked visually or by instrument to measure wind speed and direction at various altitudes.
Plain English
A small balloon let go from the ground so observers can watch how fast and which way it drifts. The way it moves shows what the wind is doing higher up.
Context Anchor
Seen in weather observation discussions, especially when winds above the surface are being measured manually or described from a balloon observation.
Derivation
The word 'pilot' here comes from the older sense of 'guide' or 'one who leads the way' (from French pilote, originally referring to a ship's navigator). The balloon 'pilots' the observer to an understanding of the wind aloft -- it goes first and reveals the conditions.
Why Pilots Care
Supplies accurate upper-wind information needed for safe takeoff, climb, and route selection when no other wind data are available.
Intuition Check
Do not read pilot balloon as a balloon carrying a pilot. Here, the balloon is an observing tool used to show wind direction and speed above the ground.
Example Sentence 1
Before the introduction of radar wind profiling, weather observers released a pilot balloon each morning to measure winds aloft.
Example Sentence 2
Using pilot-balloon data, the pilot selected an altitude that avoided the strongest headwind on the route.