Definition
Humidity is the amount of water vapor present in the air. Water vapor is water in its gaseous (invisible) form, mixed in with the other gases that make up the atmosphere. Because warmer air can hold more water vapor than colder air, humidity varies with temperature as well as with the actual amount of moisture present.
Plain English
Humidity is how much moisture the air is carrying. That moisture is in the form of an invisible gas called water vapor — not droplets, not mist, just water mixed into the air as a gas.
Context Anchor
Seen in weather reports, forecasts, and upper air observations that describe how moist the air is at different heights.
Derivation
Humidity comes from the Latin humidus, meaning 'moist' or 'damp.' Vapor comes from the Latin vapor, meaning 'steam' or 'exhalation.' Together they describe the dampness of the air caused by water that has evaporated into a gas.
Why Pilots Care
High humidity lowers air density, which reduces engine power, lift, and climb performance while increasing the risk of carburetor icing.
Grounding Statement
On a hot, sticky summer day, the 'heaviness' you feel in the air is water vapor mixed in with it — even though you can't see it, it's there.
Intuition Check
Do not assume water vapor means visible mist, cloud, or rain. In weather discussions, water vapor is the invisible gas form of water; clouds and rain form when that water changes into tiny liquid drops or ice.
Example Sentence 1
High humidity on a warm day reduces aircraft performance because water vapor makes the air less dense.
Example Sentence 2
The upper-air sounding showed increasing water vapor with altitude, signaling possible cloud layers ahead.