Contrails

The Inland Northwest Region sits in an excellent location for the viewing of high altitude planes, and their associated condensation trails. With many east and west bound coast to coast flights, the paths often bring them across the region. This photo taken on a warm, clear spring day, gives the weather forecaster some indication that moisture is increasing at the higher elevations of the atmosphere. Sure enough, by that evening, clouds had moved into the region with scattered showers by the next morning. So what are Condenstation trails, frequently called Contrails? In short, contrails are cloud-like streamers frequently observed to form behind aircraft flying in clear, cold, humid air. These condensation trails may form by either of two distinct processes. The first method occurs when water vapor that accompanies the exhaust from a jet engine is added to the atmosphere. If the humidifying effect of this moisture addition overcomes the the heat of combustion, then exhaust trails will form. The thermodynamics of this process is such that the effect will only become noticible at very low temperatures such as those found high in the atmosphere. Thus, this type of condensation trail was not frequently observed until the advent of high altitude flight. This is the process which formed the contrails in the picture above. The second process for forming a condensation trail occurs in air that is clear, but almost fully saturated with water vapor. The aerodynamic pressure reduction resultant of air flowing around a propeller or wing tips can cool the air to induce condensation, thus forming 'aerodynamic trails'. This sort of trail is often seen when a jet plane is landing or taking off and close to the surface.