It almost feels like autumn: thick, damp fog hangs over the Pacific Ocean off the west coast of California, enveloping San Francisco.

Just a few kilometres inland, bright sunshine promises a perfect summer's day.

The phenomenon occurs on the US west coast when warm air passes over areas of water cooled by cold ocean currents. This causes the air in a layer a few dozen metres thick to cool down and excess moisture to condense into fog.

This coastal fog can linger for long periods during the day, and temperatures remain low: while it rarely exceeds 20 degrees in the fog, even in summer, inland temperatures often rise well above 30 degrees.