Definition
Microscopic particles suspended in the atmosphere -- such as dust, salt, smoke, or pollen -- onto which water vapor condenses to form cloud droplets, fog, or precipitation. Without these particles, water vapor in the air will not readily condense into visible moisture, even when the air is saturated.
Plain English
Tiny specks floating in the air that water vapor sticks to and turns into water droplets. They are the seeds clouds and fog grow on.
Context Anchor
Seen in aviation weather discussions about cloud formation, fog, haze, and visibility.
Derivation
From Latin condensare, 'to make thick or dense,' and nucleus, 'kernel' or 'small core.' The name reflects what they are: small cores around which moisture thickens into liquid form.
Why Pilots Care
High concentrations can lead to rapid fog or cloud development that reduces visibility and affects takeoff, landing, and VFR flight decisions.
Analogy
A water droplet forms more easily when it has something to cling to, like moisture collecting on a small speck of dust. Condensation nuclei are those tiny cling-points in the air.
Grounding Statement
Even saturated air will not form visible moisture on its own -- it needs something to condense onto, and condensation nuclei are what it uses.
Intuition Check
Do not assume water vapor always turns into droplets by itself. In the atmosphere, moisture usually needs tiny particles to collect on before visible droplets can form.
Example Sentence 1
Coastal airports often see fog form quickly because salt particles in the air act as condensation nuclei.
Example Sentence 2
After a volcanic eruption, increased condensation nuclei in the upper air produced widespread thin cloud layers.