Jonny was here <a href=" http://www.bellavistaar.gov/where-to-buy-ivermectin-for-humans-uk.pdf ">where to buy ivermectin for dogs</a> Patrick Smith, a pilot, also dismisses the theory in his book about air travel, Cockpit Confidential. &ldquo;Contrails are formed when humid jet exhaust condenses into ice crystals in the cold, dry, upper-level air ­ it&rsquo;s not unlike the fog that results when you exhale on a cold day,&rdquo; he says. &ldquo;Contrails are clouds, you could say. Water vapor, strange as it might sound, is a byproduct of the combustion within jet engines, which is where the humidity comes from. Whether a contrail forms is contingent on altitude and the ambient atmospheric makeup - mainly temperature and something known as vapour pressure.&rdquo;