(WHTM) — Sometimes on fall or spring mornings, you may wake up to look out your window and find you can’t see anything due to the fog.

But why does fog happen and where does it occur in Pennsylvania the most?

Fog, by definition, is a thick cloud of tiny water droplets suspended in the atmosphere at or near the earth’s surface which obscures or restricts visibility. Fog forms when water vapor condenses.

According to The Weather Guys, fog can occur in two ways. One way is that the air is cooled to the dew point, which leads to condensation on tiny particles floating in the air. The other way fog can form needs water to evaporate from the surface into the air. This raises the dew point enough until the condition occurs.

The Weather Guys also says that during the fall months, we tend to have more clear skies which help the air near the ground to rapidly cool during the longer nights.  As the air cools during the longer nights, the relative humidity increases, which can result in fog formation.

In Pennsylvania, we see fog near many of the large rivers and valleys throughout the state.

The National Weather Services says that valley fog moisture evaporating from streams or rivers at the bottom of the valleys adds to the fog formation, especially when the air temperature falls below the river water temperature.

The good news is that fog will usually evaporate a few hours after the sun rises, which is a process called ‘burning off’.