This car was parked long before sundown, and then left out all night. The next morning, it is all covered with frost. Why is there a disturbance in the ice crystal pattern right over the engine?

The car's engine is hot when the car stops, and contains a lot of thermal energy. This is shared with the environment by conduction and convection, but the engine remains warm for a long time. The spot over the engine is kept a little warmer than the rest of the car hood by air currents rising from the engine. This has several effects on the frost:

This spot may be warm enough that water doesn't condense at all.
This spot may be warm enough that water is deposited as dew, rather than frost. This dew freezes later, but in the form of a transparent layer of ice instead of white frost crystals (this is the most likely explanation for what we see in the picture).
Even when the spot is below the freezing temperature, if it is warmer than the surrounding areas, frost will be deposited more slowly, giving a thinner frost layer.
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