Whether you call it vapor, a contrail, condensation, or one of a number of other descriptions, the whitish mist that trails an aircraft is definitely an attention grabber. What causes it? Depending what you see, there are a few different explanations. One is caused by temperature, others by air pressure.
A condensation trail, better known as a contrail, is the first example we’ll discuss. This is water vapor that is a by-product of exhaust from an engine, and is formed by temperature differences and humidity. Generally speaking, warmer air can hold more moisture than colder air. When warm exhaust enters the cooler surrounding air, if the right combination of humidity and temperature differential exist, a visible “cloud” forms from moisture condensing as the warm air cools. This is similar to the way condensation forms outside of a glass of ice water on a warm summer day, and signifies that the air has reached its dew point. Only this condensation trail forms thousands of feet above the earth, in sub-freezing temperatures. The water takes the form of miniscule water droplets, which quickly freeze into ice crystals. What you end up with could be described as a long, thin cirrus cloud. Depending upon the moisture content of the air (otherwise known as humidity), some contrails last for minutes or even hours, some are barely visible before they disperse or evaporate.
If you look closely behind the aircraft, there’s a gap between the engine(s) and the beginning of a contrail. This is the space where the air cools down from being inside-engine temperature to the point of condensation, usually in less than an aircraft’s length. Contrails can be the friend or the enemy of a fighter pilot. If “the other guy” is “dragging” a contrail, they’re easy to see. If you are dragging the contrail, almost any element of surprise is lost! Once, during a pre-flight exercise briefing I attended for a Maple Flag mission at CFB Cold Lake Alberta, I listened to one meteorologist brief pilots as to which altitudes contrails would be expected to form. Of course, the pilots were cautioned to flight plan either above it or below it. Remember Twelve O’clock High? Those B-17 bombers in World War II produced highly visible contrails too; the warm exhaust of their radial engines, when cooled, led to condensation into liquid, and then into ice.
The other type of condensation we’ll talk about is pressure driven. Byproducts of lift include high and low pressure areas on either side of a lifting surface, such as a wing, leading edge root extensions (LERX), fuselage and tail surfaces. These are called vortices. At the lower pressure areas, moisture in the form of vapor can be seen in extreme cases. The lower the pressure drops, the closer the air gets to saturation at its dew point. One way to think about this involves everyday weather… under the effects of a high pressure system, it is usually clear and sunny; under a low pressure system, it is usually overcast with rain or snow. The Boeing F/A-18 has particularly long LERXs at its wing root, lending to some spectacular vapor production during the Navy’s Blue Angels flight demonstrations. The dark blue aircraft contrasts with the white vortices as they come off of the lifting surfaces.
Vortices can be likened to horizontal mini-tornados coming off of an aircraft’s wing tips. Again, with everyday weather, tornados are super-low pressure storms; a vortex is a horizontal tornado. When the air is particularly close to its dew point, airliners and other civilian aircraft can give off these visual vortices too. Common areas include the wingtips and edges of flaps. If you ever watch a jetliner landing, it’s vortices will stop when it is firmly on the ground. This is due to the lift of the aircraft’s wings ending and there’s no more low pressure areas over the wings that create the vortices.
One of the most exciting and sought-after examples of vapor for air show fans is created by an aircraft at highly subsonic, or even transonic (there are very few air shows that supersonic flight is allowed) speed. This is the physical sight of a sonic shock wave forming around an aircraft – usually a jet fighter. Condensation forms as a visible ring around the aircraft as it approaches the speed of sound. Low pressure again forces air to approach and even drop below its dew point, creating the cloud. A scientific name for this effect is the “Prandtl-Glauert transformation” (go ahead, look it up on Wikipedia!). Examples of this effect are greatly enhanced on humid days near large bodies of water – like oceanfront air shows.
The photographers here at Photorecon.com have multitudes of “vapor shots”; here are some of our favorites!