The fog catalog

Fog arrives when conditions are right, and although all fog is essentially the same, it forms in different ways. Click on "Read more" for a catalog of Rhode Island fog.

This picture was taken Sunday morning at Salter Grove in Warwick.

Sunday morning's dense fog was generated by a mild air mass that moved into the state overnight, across the deep cold snow left over from last week's storm. As the mild air reached the cold snow, the air started to cool. Cooling makes the air less able to hold moisture, so the moisture condenses into water droplets, creating a cloud near the surface, which is what we call fog.

As the day went on, a cold front was expected to bring in colder, drier air, which would dissipate the fog. But the front stalled to our south, and the fog persisted in most places into the evening.

A Fog Catalog 

Advection Fog

Advection fog occurs when warm air moves over cold land or water. As the air mass cools, the moisture it holds will condense, forming fog. This is what we got on Sunday morning.

Radiation fog

This is what we call "ground fog." You might see it along river valleys and other low, damp areas early in the morning. It forms as the air cools overnight (this is called "radiational cooling," hence the name). It usually dissipates when the air warms up as the sun climbs higher in the sky, or if the wind picks up.

Steam fog

This is similar to advection fog, but forms when cold air moves over warmer water. We can see this sometimes along the coast or over fresh water ponds, especially in fall or winter when the water is often warmer than the air. The warm water adds moisture to the cold air, causing condensation. This kind of fog is more unstable than advection fog, and may appear to twist and writhe with an upward motion, while most fog is generally quiet and stable.

There are several other kinds of fog, but these three are the most common types found in R.I.

Learn more about fog and weather ...

You can download a free PDF copy of the Aviation Weather textbook published by the U.S. government, at this link. Amazingly, this book hasn't been updated since 1975! It is overdue for an update, but it is pretty basic, and still can help you to get a better idea of how weather works. Chapter 5 explains how fog forms, and you will also find out what Relative Humidity and Dew Point mean!

You can check out local forecasts, updated several times a day, from our local National Weather Service office. The NWS doesn't have an actual office in R.I., our weather info comes from the office in Taunton, Mass. It's based on data collected from a bunch of R.I. sites, including weather instruments based at all six of our state airports.

About Salter Grove

Salter Grove is a public access point to the upper Bay, found off Narragansett Parkway. Click here for a Google map. Click here for a free online guide to more than 350 public access points in Rhode Island, you might find a place nearby that you never knew about!