The formation of ice in the lower atmosphere occurs on rare ‘ice-nucleating particles’, or INP. The origin and chemical properties of these particles is now understood sufficiently well from lab experiments that we can begin to build global models of INP distribution and use them to understand the formation of ice in the atmosphere. In this presentation I will show results from our new global model of INP and how we have used it to understand the cause of large biases in cloud reflectivity in climate models.