The fog comes

on little cat feet.

It sits looking

over harbor and city

on silent haunches

and then moves on.

—  Carl Sandburg, 1916

Gothamist has been obsessing over fog this season, and for good reason. These beautiful cloud colonizations of the understory highlight our city’s skyscraper peaks and shroud our harbor in mystery and danger.

The National Weather Service, part of the National Oceanic and Atmospheric Administration, explains that what we experience is called “advection fog,” which is generated by “the horizontal movement of warm moist air over a cold surface. This means that advection fog can sometimes be distinguished from radiation fog by its horizontal motion along the ground. Sea fogs are always advection fogs, because the oceans don’t radiate heat in the same way as land and so never cool sufficiently to produce radiation fog. Fog forms at sea when warm air associated with a warm current drifts over a cold current and condensation takes place.”

Learn about the different types of fog from this National Weather Service educational page.