Terpenes are a major component of many plant resins, cannabis being no exception. Most terpenes are strong smelling and often serve the purpose of attracting pollinators or repelling predators. Furthermore, it is thought that certain tree species release terpenes, which react with atmospheric compounds to produce aerosols, which in turn encourage water vapor to form clouds. During hot weather, these trees release higher levels of terpenes, doubling cloud cover and providing a strong cooling effect.