At low latitudes, cirrus are ubiquitous and can be in excess of 100¿C colder than the surface, limiting the amount of sunlight absorbed by the earth's atmosphere and surface, and reducing its loss of heat. Here we present aircraft measurements within cirrus over southern Florida indicating that ice crystals have smaller sizes and are more reflective than is assumed in most current climate models. If the measurements are generally representative of low-latitude cirrus, they point to a first-order correction to representations of how these clouds affect the earth's climate.