What is the role of cloud condensation nuclei (CCN) in weather and climate?

• A. They act as greenhouse gases increasing atmospheric warming.
• B. They have no effect on clouds or precipitation.
• C. They seed cloud droplets, influencing cloud properties and precipitation.
• D. They only affect rainfall in deserts.

Answer: (C)

Cloud condensation nuclei are the tiny particles in the atmosphere that water vapor condenses onto to form cloud droplets. The amount and type of CCN determine how many droplets form in a cloud and how large they grow. When many CCN are present, clouds have many droplets that are smaller on average, making the cloud more reflective to sunlight and often cooling the surface below. This microphysical change also affects precipitation: smaller droplets collide less readily, which can delay rainfall or alter its intensity, though under some conditions CCN can also help produce rain as droplets grow. In short, CCN seed cloud droplets and shape cloud properties and precipitation, influencing weather and climate. They are not greenhouse gases, and their effects are global rather than limited to deserts.