Basically, lighting conditions have to be just right to ... basically, allow you to actually see your own white blood cells, in your own eyes, against the ... background/everything you are seeing.

So my guess would be that in the summer, where you are, the ... ambient light of the sky is too bright, it overwhelms this effect, but in the winter, maybe its mote generally humid, or the light is coming through more atmosphere , at more oblique angles, and is thus less intense.

Though if you are also seeing a... black tail, like... they're followed by a black smear or a motion blur or something... that could be something else?