Chemtrails are a conspiracy theory that claims that long-lasting condensation trails left in the sky by high-flying aircraft are actually "chemtrails" consisting of chemical or biological agents, sprayed for nefarious purposes undisclosed to the general public. However, there is no scientific evidence to support their existence, and experts on atmospheric phenomena say that the characteristics attributed to chemtrails are simply features of contrails responding to diverse conditions in terms of sunlight, temperature, horizontal and vertical wind shear, and humidity levels present at the aircrafts altitude. The term chemtrails is a popularized expression, and the House leader has stated that "it is our belief that the petitioners are seeing regular airplane condensation trails or contrails". If there really were a large-scale program dumping material from aircraft at the scale described, there would have to be a large operating program to manufacture, load and disperse materials. If such a program existed at the scale required to explain the claimed amount of chemtrails, it would require thousands or perhaps tens of thousands of people. In summary, chemtrails are a conspiracy theory with no scientific evidence to support their existence.