Clouds are a big source of uncertainty in computer simulations used to study Earth systems. To reduce that uncertainty, researchers study the formation of ice in clouds. This formation influences precipitation rates, large-scale cloud motions, and cloud optical properties. This research helps to address a long-standing mystery. For more than 50 years, scientists wanted to know why water droplets will freeze at a higher temperature when they are impacted by a particle. It’s the pressure of the impact, according to new observations. A new study suggests the warmer freezing point is due to pressure changes resulting from the contact.