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This makes the air denser and increases relative humidity, condensing more water vapor, and resulting in a phenomenon known as radiation fog.

The Central Valley’s thick tule fog is a type of radiation fog that forms on calm, clear nights, usually after a soaking rain, when the Earth gives up its heat — radiates its heat like a radiator — into space.

The sort of fog we've been experiencing is known as radiation fog.

BBC News website readers have been sending in their pictures of fog that shrouded parts of England earlier. described it as radiation fog which occurs when thermal radiation overnight cools the land and the air close to the ground's surface.

Moreover, because ice has only half the specific heat of water, and is much more transparent to heat, such a "radiation fog" composed of ice crystals is a much less effective retainer of heat than clouds or fog made of unfrozen water particles.