In the sky, tiny particles are needed for clouds to form. These particles can come directly from the ground (for example, from volcanoes or pollution released by industry) or they can form in the sky when sticky molecules clump together. More particles in the sky make clouds whiter. Whiter clouds reflect more of the sun’s rays. This tends to cool down Earth’s climate. So particles in the sky affect clouds and this in turn has an effect on global warming. Therefore, it is important to understand how these particles form. To study this, we measured how fast particles were made when we added controlled amounts of sticky gases to a tank at the CERN research lab. With computer simulations, we used the results to estimate which gases are most important for making particles in different parts of the atmosphere. This research will help scientists understand how particles affect Earth’s climate.