Clouds are an essential part of Earth’s climate system. They play a big role in weather patterns, rain, and regulating global temperatures. A recent study titled “Supersaturation and Critical Size of Cloud Condensation Nuclei in Marine Stratus Clouds,” published in Geophysical Research Letters, gives us new information about how clouds form, especially over the ocean.

What Are Cloud Condensation Nuclei (CCN) and Supersaturation?

Clouds form when moist air rises and cools. This cooling leads to supersaturation, where the air has more water vapor than it can hold. This extra water vapor condenses onto tiny particles called cloud condensation nuclei (CCN), which then grow into cloud droplets. The size and number of these CCN are crucial for determining what the clouds will be like.

Traditionally, scientists thought that only larger particles (around 60 nm in diameter) could become CCN at typical supersaturation levels (0.2%–0.3%). However, this study by Henrik Svensmark and his team shows that much smaller particles can also act as CCN if the supersaturation is higher than previously thought.

Main Findings of the Study

1. Higher Supersaturation Levels: The study found that marine stratus clouds, especially those off the coast of California, often have supersaturation levels higher than 0.5%, and sometimes even up to 1%. This is much higher than what scientists used to believe and suggests that smaller particles (25-30 nm) can act as CCN in these conditions. The authors wrote, “On average, supersaturation in marine clouds is significantly higher than the conventional view of 0.2%–0.3%.”
2. Smaller Critical Size of CCN: By using satellite data and scientific theories, the researchers created maps showing global supersaturation levels and the sizes of particles that can act as CCN. They found that marine clouds are more sensitive to changes in the number of small particles because of these higher supersaturation levels. This means that even smaller particles than previously thought can form cloud droplets. The study notes, “Higher supersaturation implies smaller activation size for CCN making cloud formation more sensitive to changes in aerosol nucleation.”
3. Effects on Cloud Properties: Higher supersaturation levels mean that cloud formation is more dynamic and sensitive to changes in the particles that act as CCN. This can change cloud properties, like how many droplets they have and how thick they are, which affects how much sunlight they reflect and how much heat they trap. The study explains, “Due to the higher supersaturation, much smaller aerosols get activated into cloud droplets.”

How Did the Researchers Study This?

The researchers used data from different sources, including satellite observations and airplane measurements. They analyzed these data to understand the relationship between the number of CCN and supersaturation.

1. Satellite Observations: They used satellite data to estimate the number of droplets in marine stratus clouds. This, along with measurements of cloud thickness and water content, helped them analyze cloud properties over the oceans.
2. Airborne Measurements: Measurements from airplanes off the coast of California were crucial in discovering the higher-than-expected supersaturation levels. These measurements provided direct evidence that smaller particles could act as CCN. The authors noted, “Observations of marine stratus clouds in clean air off the Californian coast reveal a functional relationship between the number of cloud condensation nuclei (CCN) and supersaturation.”
3. Simulations: The researchers used a computer model to simulate how cloud droplets form under different conditions of supersaturation and vertical movement. These simulations supported their observations, showing that smaller particles could form cloud droplets at higher supersaturations. The study states, “Independent support for such high supersaturation in the marine cloud is obtained from CCN measurements provided by the ‘Atmospheric Tomography Mission.’”

Why Is This Study Important?

This study is important because it changes our understanding of how clouds form. Clouds play a key role in Earth’s climate by reflecting sunlight and trapping heat. Small changes in cloud properties can have a big impact on weather and climate. By showing that cloud formation is more sensitive to smaller particles and higher supersaturation levels, this study helps us better understand how clouds and aerosols interact.

1. Aerosol-Cloud Interactions: The study shows the complexity of how aerosols (tiny particles) and clouds interact. Understanding these interactions better is crucial for improving weather and climate predictions.
2. Cloud Microphysics: The research highlights the importance of supersaturation levels in determining cloud properties. This has implications for studying and modeling different types of clouds and their roles in weather systems.

Conclusion

The study “Supersaturation and Critical Size of Cloud Condensation Nuclei in Marine Stratus Clouds” by Svensmark et al. provides new insights into how clouds form. By revealing higher-than-expected supersaturation levels and the activation of smaller CCN, this research challenges what we thought we knew and opens new doors for understanding Earth’s atmosphere. As we continue to learn more about cloud formation, studies like this are essential for understanding the complexities of our climate.

Cloud formation is a complex process influenced by many factors. This study sheds light on some of the key aspects of this process, especially in marine environments, and highlights the importance of continuous observation and research to uncover the mysteries of our planet’s atmosphere.

The study is open access and can be read here.

H/T Ken Gregory, Friends of Science