Definition
Liquid water droplets that exist at temperatures below 0°C (32°F) without having frozen. These droplets remain liquid because they lack a surface or particle to freeze onto, but they freeze almost instantly on contact with an aircraft, producing structural icing.
Plain English
Tiny drops of water in the air that are colder than freezing but are still liquid. The moment they touch your aircraft, they turn to ice.
Context Anchor
Seen in instrument weather and aircraft icing discussions, especially when flying in clouds or precipitation at temperatures near or below freezing.
Derivation
‘Subfreezing’ combines the Latin sub- (‘below’) with ‘freezing’ — literally ‘below the freezing point.’ The phrase highlights that the droplets are colder than 0°C even though they have not yet frozen.
Why Pilots Care
Contact with an aircraft causes these droplets to freeze instantly, forming clear ice that reduces lift, increases drag, and can block control surfaces.
Grounding Statement
Picture flying through a cloud at -5°C: the cloud looks like any other, but every droplet that hits the wing freezes on impact and starts building ice.
Intuition Check
Do not assume “subfreezing water droplets” are already ice. In this context, they are usually liquid droplets that are below freezing temperature and can freeze on impact.
Example Sentence 1
The pilot diverted around the cloud layer after the briefer warned of subfreezing water droplets between 6,000 and 9,000 feet.
Example Sentence 2
Forecasts warned of subfreezing water droplets in the clouds along the route, requiring activation of the deicing system.