Condensation nuclei are which of the following?

• A. The large droplets that form clouds.
• B. The gases that promote evaporation.
• C. The surfaces within clouds that support diffusion.
• D. The small particles of solid matter in the air on which water vapor condenses.

Answer: (D)

Condensation nuclei are tiny solid particles suspended in the air that water vapor condenses onto to form cloud droplets. These nuclei provide surfaces at relatively low supersaturation, making cloud formation possible under typical atmospheric conditions. Examples include dust, sea salt, smoke, pollen, and ash. The large droplets you see in clouds are the result of water vapor condensing on these small particles and growing, not the nuclei themselves. The other descriptions don’t fit because gases don’t serve as condensation surfaces, and while diffusion happens at droplet surfaces, the defining feature is that condensation occurs on these tiny solid particles.