Understanding of clouds remains incomplete in the field of climatologyand thus constitutes the main Source of uncertainty in the forecasts of the climatic changes. A comprehensive analysis of the large-scale weather phenomena as well as microscopic processes is necessary to understand this subject.
A little reminder about cloud formation
In a simpler way, clouds form when warm, humid air rises, cools and reaches a saturation pointthat is to say, a point where it can no longer contain all its water vapor. Under the effect of variations in temperature, pressure, this water vapor then condenses around tiny particles suspended in the air called “condensation core”.
This process results in visible formations in the sky, with different shapes and heights depending on local atmospheric conditions.
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The new study published in Geophysical Research Letters, highlighted the importance of these all small particles which can also be referred to as “aerosols”, in the cloud formation, as well as their influence on future climate trends. These aerosols can be of natural origin (pollens or other organic aerosols, sea salts, etc.) or anthropogenic (carbon soot, etc.).
Smaller and smaller condensation nuclei!
Let’s start again. For a cloud to form, two main conditions must be met :
- the air must be saturated with waterwhich means that it contains more water than it can normally contain,
- there must be a small particlecalled condensation coreon which water can condense.
Exactly, researchers focused on “condensation nuclei” in Californian stratocumuluslow clouds spread horizontally above the oceans.
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Measures of marine stratocumulus clouds carried out in 2014 by Nevada scientists served as basis under study. These measurements reveal a relationship between the quantity of aerosols and the supersaturation of water in the atmosphere. These measurements, combined with global satellite measurements from the MODIS instrument, made it possible to calculate the amount of droplets in cloudsfrom which a global supersaturation map has been established.
The surprise lies in the fact that the oversaturation is generally higher than what was Assumed. In other words, the amount of water in the air was greater than expected, which means that smaller particles could serve as condensation nuclei.
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Indeed, Researchers found that extremely small aerosol particles, measuring only between 25 and 30 nanometers, can influence cloud formation, thus putting back calls into question previous climate research which assumed that a larger minimum aerosol size was necessary.
To better understand supersaturation, let’s put in the towel!
Imagine that air is like a sponge and water vapor is like the water that the sponge can absorb. When the sponge is almost saturated, it can still absorb a little more water. However, there is a point where the sponge is so full that it can no longer absorb additional water. It’s like when air is supersaturated with water vapor, it already contains much more water vapor than it can normally hold.
At this point, the slightest change in temperature or pressure can cause excess water to condense, just like the sponge overflows when you press on it. Even though technically the air shouldn’t be able to hold more water vapor, it does anyway, until it reaches a point where condensation occurs.
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If, traditionallywe thought that the condensation nuclei must have been quite big, about 60 nanometers or moreso that water condenses around them, this new research shows that in reality, much smaller particles can do the trick.
A discovery that turns everything upside down!
This discovery has important implications in climate modeling and predicting future changes. By better understanding the role of ultrafine particles in cloud formation, we can improve our climate forecasts and better anticipate future challenges.
Reference : Henrik Svensmark et al, Supersaturation and Critical Size of Cloud Condensation Nuclei in Marine Stratus Clouds, Geophysical Research Letters (2024). DOI: 10.1029/2024GL108140
