CaptainC wrote:
This is condensation. as the air leave the prop tips, it expands rapidly and as as we know, when it expands, it cools. If the temp/dew point spread is relatively close, that expansion causes the effect you see, it can be off the tips of props or wingtips. Very common is maritime environments, but can happen anywhere if conditions are right.
Many thanks for the definitive explanation! I have seen that off wing tips, so considered that might be the case in this case. Dew Point in Kodiak Alaska is definitely up there! Mystery solved.