Definition
The process by which water vapor changes directly into ice without first becoming liquid water. It occurs when moist air contacts a surface whose temperature is below freezing, causing the vapor to bypass the liquid stage and form ice crystals or frost.
Plain English
Water in the air turning straight into ice on a cold surface, skipping the liquid step. This is how frost forms on an aircraft sitting outside on a cold, clear night.
Context Anchor
Seen in aviation weather discussions about frost, clouds, and ice forming when moist air is below freezing.
Derivation
From the Latin 'deponere,' meaning 'to put down' or 'lay down.' The word describes vapor 'laying itself down' onto a surface as solid ice — useful because it captures the idea of something settling out of the air directly into solid form.
Why Pilots Care
Recognizing deposition helps anticipate frost and ice buildup that reduces lift and increases drag.
Analogy
Like frost appearing on a cold window directly from humid air, with no liquid water forming first.
Grounding Statement
On a cold, clear morning, the white frost you see on a parked aircraft did not start as dew that froze — it formed directly from water vapor in the air settling onto the cold metal as ice.
Intuition Check
Deposition does not mean a legal statement or simply “putting something somewhere” here. In weather, it means water vapor changing directly into ice.
Example Sentence 1
The pilot noticed frost on the wings caused by overnight deposition and arranged to have it removed before the first flight of the day.
Example Sentence 2
Deposition allows ice crystals to grow in high cirrus clouds even without liquid droplets present.