Have you ever noticed how it's often foggy around Bonfire Night? There's actually a good reason for that.

Yes, it's the depths of autumn, and autumn is often associated with fog. But Guy Fawkes Night actually provides an extra reason why the atmosphere may create fog.

Fog is just cloud that's on the surface. Like cloud, it's made up of millions upon millions of tiny water droplets. Water is always present in the air around us, but most of the time, you can't see it because it's in vapor form.

As air cools, like it does during the long autumn nights, the water cools and condenses. It changes from water vapor that you can't see into millions of tiny water droplets that you can.

But here's the key: for that change to happen, for the water to condense, it requires something else. It requires tiny little particles to be in the atmosphere. These are called condensation nuclei, and without these, the water just wouldn't change from its invisible form into its tiny droplet form.

Condensation nuclei is just a fancy name for bits of stuff in the atmosphere, like dust and smoke. The more of these condensation nuclei there are, the more likely it is that fog will form.

So yes, you guessed it. On Bonfire Night, bonfires and fireworks create more of these tiny particles, and so fog is more likely on Bonfire Night and on the morning of the 5th of November in New Zealand.