The different-sized ice crystals in contrails descend at different rates, which spreads the contrail vertically. High-flying aircraft can sometimes be detected by their vapor contrails. She flies high in the intellectual skies, leaving beautiful sunlit contrails. The California Energy Commission issued a statement blaming this on "clouds, jet contrails and weather". Stealth may extend to avoiding or preventing vapor contrails. These millions of tiny water droplets and/or ice crystals form the contrails. Contrails, and other clouds directly resulting from human activity, are collectively named homogenitus. Contrails produced from jet engine exhaust are seen at high altitude, directly behind each engine. The rate at which contrails dissipate is entirely dependent on weather conditions. Contrails may also produce lobes but these are incorrectly termed as mammatus. Persistent spreading contrails are suspected to have an effect on global climate. Contrails, by affecting the Earth's radiation balance, act as a radiative forcing. The tips of rotating surfaces (such as propellers and rotors) sometimes produce visible contrails. Therefore, the overall net effect of contrails is positive, "i.e.", a "warming" effect. Contrails are composed primarily of water, in the form of ice crystals.