What causes these vertical lines that sometimes come stretching out of the top of a flame?

Picture (imgur direct link) for reference.

I've seen this before and always found it strange. I took a picture in part to rule out it being something to do with my eyes/brain. A few minutes of googling didn't get me any information at all. The lines don't always appear, don't always appear in the same size/place/configuration and more often than not seem to disappear over time.

I can't imagine what it could be - difference in local atmospheric pressures or oxygen ratios don't seem like they could explain straight vertical lines.

Thanks in advance both to mods & commenters.