Ach, fog. It gives landscape this mysterious, magical atmosphere and makes fatal road accidents so much more likely to happen. But where exactly is this milky white, smoke-like phenomenon is coming from? How is fog formed? And why is it so common in some places, while almost doesn’t occur in the others?

What is fog?

Fog is defined as a low-laying cloud, composed of tiny water droplets suspended in the air. Because of its high density, it often obstructs visibility to 1 kilometer (0.62 mi) or less, heavily affecting industries like transportation or travel. Even aviation which nowadays relays mostly on auto-landing computers is affected, as control tower must be able to see if aircraft, to properly the runway.
Cases of very cold temperature can lead to the formation of freezing fog. In this state, water droplets of which the fog is composed, will freeze instantly when come to contact on any surface, creating either soft or hard rime (depending on the temperature).

What causes fog? How is fog formed?

But since we know what fog is, it’s time to ask: where does fog come from? The first condition for the fog to form is the temperature – the difference between dew point (temperature below which water droplets begin to condense into dew) and air temperature has to be less than 2.5 °C (4.5 °F). Second condition is the humidity, which needs to be around 95% or higher. This severely reduces air’s ability to hold moisture, causing condensation, which in turn leads to the formation of the fog. This type of circumstances are commonly occurs overnight and as the sun rises in the morning, the air is gradually warmed up, causing the fog to evaporate.