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mmm... sound

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The "Humidity" you always hear about means the nothing more than the relative amount of water per unit volume of air. Water is naturally soluble in air, and its status as a soluable is dependent on temperature, pressure, and volume. Vary any of the three the appropriate way and the water in the solution may condense.

When a plane passes through the air, it introduces a substantial variation in local pressure, especially at the wing tips. The drop in pressure yields conditions which result in the water coming out of solution, resulting in localized "fog," thus the contrails at the wingtips.

When the plane goes supersonic, it introduces a much more complex phenomenon commonly known as a shock wave, wherein the planes physical surface creates a conical pressure wave which depressurizes the air around it, creating the same localized "fog."

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