Microscopic particles, or aerosol, that act like magnets for water vapor are the sources of water droplets in a cloud. These droplets bond to each other and when they are heavy, fall to the ground as rain. And under certain conditions, these droplets freeze and fall to the ground as snow. But scientists have not figured out why only certain aerosol can become water droplets and ice crystals. The specific chemical fingerprints of these varieties of aerosol (dust, sea-spray, smoke, pollen) observed in the atmosphere are not understood. A very small subset of ambient aerosol carries the required cloud-nucleating substance. This poor understanding has led to the largest sources of uncertainty in predicting Earth’s climate. In this presentation, Earth scientist Gourihar Kulkarni will discuss the size and chemistry of these special aerosol and their potential effects on cloud droplets. Such studies provide an improved representation of clouds and precipitation in a climate model, and these computer simulations have a profound impact on predicting weather and climate change accurately.