More than 85 per cent of Australians live within 50 kilometres of the ocean, but the variation in weather across any slice of this narrow coastal band is substantial.

The contrast will be particularly sharp this weekend for Western Australia, South Australia and western Victoria —  travelling from the coast to the adjacent hinterland will bring a substantial increase in maximums.

Take Perth for example, beachside residents can expect a relatively pleasant 30-degree-Celsius ocean breeze on Saturday afternoon, while eastern suburbs are trapped under a plus 40C dome of heat.

The cool relief along the coast will result from a sea breeze, but what causes this natural air conditioner? Why does it blow hardest during spring and summer? And why on some days does it fail to eventuate completely?

Not every wind from ocean is a sea breeze

As with other meteorological phenomenon, the definition of a sea breeze is sometimes misunderstood.

Onshore winds alone do not indicate a sea breeze has formed, even though a flow off the ocean can still generate a substantial temperature contrast if winds are near parallel to the coast.

This occurs across Sydney in a non-sea breeze north north-easterly (NNE), which can lead to a differential of more than 10C between the coast and western suburbs.

When the prevailing winds are just off the parallel, a similar inland/coastal temperature difference results.(ABC News)

So what is a sea breeze then?

The term refers to a temporary localised circulation of air which forms along a coastline due to uneven heating of land and water during daylight hours.

You know a true sea breeze when an offshore breeze weakens in the morning, reverses direction through the afternoon, then weakens at night and reverts to an offshore direction overnight.

The anatomy of a sea breeze

According to atmospheric researcher Deborah Abbs et al (1992), the sea breeze was initially discussed by Greek philosophers, however, the first correct description was not made until around 1700 and it wasn’t until after World War II the sea breeze was extensively studied.

The genesis for a sea breeze starts with the uneven heating of the earth’s surface. The sun’s rays from a high summertime angle warm up solid ground rapidly — as anyone attempting to walk barefoot on a hot road would attest.

While land surface temperatures fluctuate from the daily minimum to the daily maximum, sunlight — even in the peak of summer — will have a minimal impact on the ocean temperature.

The sea is also transparent and in motion, and therefore heating from the sun will be spread well below the surface, as opposed to ground heating where energy is focused through only the very top few centimetres.

The heated land then warms the slim layer of air just above the surface and since warm air expands and rises, leads to a localised area of low pressure.

The rising air cools aloft (only a few hundred metres elevation) and moves laterally back out off the coast before sinking towards the sea where an area of high-pressure forms.

The strongest sea breezes occur in late spring and early summer.(AAP: Lukas Coch )

And since air travels from high pressure to low pressure, the cooler denser maritime air pushes inland and undercuts the rising warmer air over land, thus completing a circulation and forming the sea breeze we feel on the ground.

The strength of the sea breeze and the extent of cooling is mostly determined by the magnitude of the land and sea temperature contrast.

Forty-degree air over land adjacent to a 20C body of water will generate a much stronger sea breeze than a land-sea variance of just a few degrees.

This is why the strongest sea breezes occur in late spring and early summer when the sun is at its apex position over Australia, but well before surrounding oceans reach their annual maximum warmth in February.

Why a sea breeze won’t form every day

Given a land/sea temperature gradient exists, the occurrence and timing of a sea breeze depends on the prevailing weather pattern.

A high-pressure system is ideal as winds are light which allows the strong thermal gradient to form without lateral interference from surrounding winds.

During the warmer months, high-pressure cells favour a position near Australia’s southern coastline, which, according to scientist Yifei Zhou (2022), explains why Adelaide and Melbourne experience more frequent sea breeze days— around 30 to 40 per cent, compared to Sydney and Brisbane where only 20 per cent of days have a true sea breeze.

Perth has the most frequent and strong sea breeze of Australia’s capitals.(AAP: Dave Hunt)

Surprisingly though, it’s Perth, lying further north than Adelaide and Melbourne, which has the most frequent and strong sea breeze of Australia’s capitals, occurring on nearly 60 per cent of days.

It’s so persistent and strong that it was even designated its own name, the Fremantle Doctor.

Perth’s susceptibility is due to the city’s west-facing coast, meaning the predominant summer easterly winds are offshore for the city, which sets up a sharp temperature contrast with the cool west coast currents.

When a high-pressure system is absent, winds are often too strong for a sea breeze circulation to form, most notable when very hot and dry air blows in from the nation’s interior.

On these days, proximity to the coast becomes irrelevant.

How far inland does sea breeze travel?

The cooling influence of a sea breeze is gradually diminished away from the coast.(AAP: Dean Lewins)

The sea breeze is not just a coastal phenomenon, they can spread hundreds of kilometres inland, although their cooling influence is gradually diminished away from the coast.

A typical sea breeze will develop in the late morning and reach about 50km inland by the late afternoon. But the circulation often continues expanding into the evening, which, according to Ms Abbs et al (1992), was suggested first in Australia by meteorologist Reg Clarke in the late 1940s who proclaimed an evening easterly in Canberra was a sea breeze from the coast.

Mr Clarke then proved in the 1950s, by tracking a sea breeze by car, they can even reach Kalgoorlie by midnight, 345km inland.

Sea breeze can greatly impact intensity of thunderstorms

A sea breeze boundary may initially enhance the updraft of the storm.(ABC Weather Obsessed: Ben Aboody)

A typical sea breeze is associated with fair weather, although along the leading edge of a sea breeze pockets of cumulus clouds often form.

During unstable weather, when thunderstorms are approaching the coast, a sea breeze boundary may initially enhance the updraft of the storm.

However once a storm front passes into the sea breeze circulation, the cooler air acts to suppress the storm.

This process explains why storms over inland suburbs often weaken before reaching the coast.

Occasionally, mostly in the tropics, when the atmosphere is very humid a sea breeze may even trigger a thunderstorm if the rising air becomes buoyant and continues to rise to a depth sufficient to form a cumulonimbus cloud.