Definition
Water present in the atmosphere, existing in any of three states: invisible water vapor (a gas), liquid droplets (clouds, rain, drizzle), or solid ice crystals (snow, hail, ice fog). The amount of moisture in the air at any time depends on temperature, since warmer air can hold more water vapor than colder air.
Plain English
Water in the air. It can be an invisible gas, tiny liquid drops, or frozen crystals, depending on temperature and conditions.
Context Anchor
Seen in aviation weather discussions when explaining clouds, fog, rain, snow, visibility, and ice on the aircraft.
Derivation
From the Latin 'mucidus,' meaning damp or musty, by way of Old French 'moisteur.' The original sense was simply 'dampness' -- the presence of water in something that isn't obviously wet. That fits the aviation use, where moisture is often present in air that looks perfectly dry.
Why Pilots Care
Moisture content controls cloud development, visibility, icing risk, and precipitation, all of which affect flight safety and route decisions.
Grounding Statement
Air can hold water you cannot see, and when conditions change, that water can show up as clouds, fog, rain, snow, or ice.
Intuition Check
Moisture does not only mean something is visibly wet. In aviation weather, moisture can be invisible water in the air that later becomes clouds, fog, precipitation, or ice.
Example Sentence 1
Warm air rising off the ocean carried significant moisture aloft, which condensed into the cumulus buildups along our route.
Example Sentence 2
High moisture near the surface often leads to low ceilings and reduced visibility during the approach.