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CONTRAIL COMPUTATION ON THE SKEW T DIAGRAM

Condensation trails, abbreviated contrails, are elongated, tubular-shaped clouds composed of water droplets or ice crystals which form behind an aircraft when the wake becomes supersaturated with respect to water. There are two types of contrails: aerodynamic contrails and engine exhaust contrails.

Aerodynamic contrails form by the momen-tary reduction of air pressure in the airfoil vortex. If you have ever seen the Navy Blue Angles or the Air Force Thunderbirds during a low-level demonstration, then you have probably noticed these contrails trailing the wing tips, especially during high-speed turns. These contrails appear as the vortex creates a partial vacuum, lowering the air pressure sufficiently to bring it to satura-tion. They dissipate rapidly as the pressure in the vortex returns to normal behind the aircraft. We will not be concerned with this type of contrail.

The second type of contrail is the engine exhaust contrail, formed by exhaust water vapor, a by-product of the combustion process, bringing the air to saturation. These are the conspicuous contrails you probably have seen crossing the sky, especially during the winter. Proper analysis of the temperature, humidity, and pressure will allow you to determine the heights at which contrails will form.

Why are we concerned with contrail analysis and forecasting? Contrails can make an otherwise inconspicuous high flying aircraft very noticeable.

Figure 6-2-20.-Surface hail size correction diagram.  

In a warfare situation or while conducting covert high-altitude photo-reconnaissance, a key to the success of the mission may be the element of surprise. Our aircraft may wish to avoid flight altitudes conducive to contrail formation to decrease the probability of their detection. Stealth technology aircraft will especially wish to avoid producing contrails. Shipboard, the CIC officer and the OOD can use your analysis of contrail formation altitudes to aid visual detection of high-flying hostile aircraft.

We have already identified the contrail scales and their values on the Skew T diagram earlier in this lesson. Given a specific level, you can use these scales, the temperature, and the humidity to determine if contrails will form.

When the free-air temperature is to the right of the 100 percent line, contrails will not form, regardless of the humidity.

When the free-air temperature is to the left of the 0 percent line, contrails will form, regardless of the humidity.

When the temperature is between the 0 percent and 100 percent lines, contrail formation depends on the humidity. The relative humidity must be equal to or greater than the value indicated by the line for contrail formation to occur. If the temperature at 300 millibars is 45C, for instance, the temperature is just to the right of the 60 percent line. I would call it about 67 percent. Contrails will form only if the relative humidity at that level (evaluated with respect to ice) equaled or exceeded 67 percent. Right about now you are probably thinking, "So how do I figure out the relative humidity when the moisture cut-off level is at 40C? There arent any moisture reports at this level."

When the humidity data is unknown, empirical data indicates good results are achieved if 40 percent humidity is assumed where there are no clouds reported at the level, and 70 per-cent humidity is assumed when clouds are reported at a level. See figure 6-2-21 for an example of a contrail formation analysis where clouds are present but relative humidity data is missing.

Figure 6-2-21.-Contrail computation.

Learning Objective: Describe the computa-tion procedures for icing level analysis on the Skew T.

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