Heat haze can occur during all times of year. It's not dependent on absolute temperature, but rather on temperature gradients in the air. Basically, as it's usually created, the sun warms the ground, which then heats up the air immediately above it. That air is then hotter than the air above it, so you get convection, and hence heat haze as the differing air temperatures mix.
It tends to be worse in the summer, yes, but I've seen it pretty bad in the winter too (then again, winter in LA
isn't exactly cold).
Technically, as long as you have any air between you and your subject, you can get heat haze. But it's more noticeable with greater subject distance, and with higher zoom. I've certainly noticed it at air shows at not too great a subject distance.
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