Met Mailbag is your chance to have a Weather Plus Meteorologist answer your weather question. Every other Thursday, our NBC Weather Pulse Blog will publish the answers to questions you send us. This week's question was answered by NBC Weather Plus meteorologist Jackie Meretsky.

Question: What causes fog?
Submitted by D. Allen, Chicago, Illinois

Answer: Residents of the beautiful Island of Newfoundland (originally called Terra Nova) are no strangers to fog. In fact, the foggiest place in the world is the Grand Banks off Newfoundland where the warm gulf stream meets up with the cold Labrador current from the north.

Across the continental U.S., we have some very foggy regions as well, namely Charleston, West Virginia because of the terrain and air flow patterns, and San Francisco. Coastal cities out West have night and evening fog which can continue well through the morning's commute.

PHOTOS: Classic Fog Snapshots

Quite simply, fog is a cloud at the ground level and typically occurs when relative humidity at the ground level reaches 100%. It has many of the same properties as the cumulus and stratus clouds we see up above, but the formation is different.

Similar to the clouds in the sky, fog is made up of condensed water droplets but with sky clouds, air particles rise, cool and expand. Air particles cool in fog as well, but for different reasons and with different consequences. For example, rain can moisten and cool the air at the ground level until fog forms. Or, sometimes a warm and moist air mass coming in contact or moving over a colder surface (sometimes snow) can cause a type of fog called advection fog.

Fog is still fog in higher elevations as long as it comes in contact with the hilltop or mountain ridge. There are actually over a dozen types of fog - it all depends on how the cooling that caused the condensation occurred.

To watch the video, please click the play icon in the video box to the right.