It's that one telltale sign that autumn has arrived.

Waking early for a surf, then walking outside to be greeted by crisp, clear air.

Every year it's the same. After months of oppressive humidity, the first proper intrusion of cold air clears it all away, while grooming the surf into smooth, defined lines.

And while that's the best of it, it's not the least of it: The stars also twinkle brighter and that pre-dawn glow becomes more vivid.

All this pagenatry may lead you to wonder: Why does colder weather make the air clearer and visibility greater?

The answer lies with water. Or more accurately humidity. And with even greater accuracy, relative humidity.

If absolute humidity is the measurement of how much water vapour is in the air, then relative humidity is how much water vapour is in the air compared to how much it can hold. And this measurement is more useful for the general public to identify with.

The calculation goes like this: The warmer the air, the more water it can hold - which also explains why relatively humidity differs from absolute humidity.

During the summer months, the air is warmer and loaded with more water vapour thanks to evaporation off the warmer seas and onshore winds blowing the air mass onto the coast.

This increase in marine aerosols - a fancy name for water vapour, though it also includes salt - scatters light, which reduces visibility and creates the summer haze that makes distant objects indistinct. This effect ramps up the more the relative humidity rises, increasing two-fold from 80% relative humidity and above.

The below image graphs the scattering of light as a function of humidity. You can see that from about 50% relative humidity, the scattering ratio starts to increase exponentially, doubling once it rises above 80% relative humidity.

Aerosol Light Scattering Measurements as a Function of Relative Humidity (Derek E. Day, William C. Malm & Sonia M. Kreidenweis)

Looking deeper into the chemistry, and the marine air mass contain lots of salt particles which are the nuclei for condensation to occur on.

The marine aerosols change from solid to liquid when the relative humidity rises above 70%, which results in the salts being disproportionally responsible for the scattering of light.

In practical terms, this creates more haze and distortion when peering through the atmosphere, and is the key reason visibility is lower during the spring and summer months, when the coastal air mass is marine in origin.

As we transition to autumn, the land cools below that of the adjacent ocean, creating local land breezes that draw out drier air from over the continent. Air that has considerably less moisture, less marine aerosols, and a lower relative humidity.

In short, clearer air.

The increased frequency of cold fronts passing over the Great Dividing Range or any other land barrier, dries the air mass out even further. Something known as the rain shadow effect.

There's one more thing to note as you wake early and walk outside for a surf.

While bringing clearer weather and skies, the drop in humidity also feels cooler, thanks to sweat evaporating more effectively into the drier air (evaporative cooling). The opposite is true when it's hot and humid, with the saturated air being less effective at evaporating sweat off the body, reducing the evaporative cooling effect.

So even if the Bureau forecasts a balmy temperature, the lower humidity will take more heat away from the body. That temperature will feel different than it did back in February.

Take a vest or a wetsuit - make it thicker as the season goes on.