National Weather Service United States Department of Commerce

Expanding Heat Wave; Heavy Rainfall Across the Upper Midwest and Gulf Coast; Snow for the Northern Rockies; Critical Fire Weather for Southwest

Diverse pattern across the country with accumulating heavy wet snow across the northern Rockies into the new week. Meanwhile, deep tropical moisture is expected to move ashore across the Gulf Coast States with the threat of heavy rainfall. This threat extends into the upper Midwest where flash flooding and a few severe storms. In addition, a heat wave is building from the Plains into Northeast. Read More >


About Dense Fog and Smoke

The main period for fog formation in South Florida is from late Fall to early Spring. When fog does form in South Florida, it typically forms over the Evergldes and along the southwest coast of Florida. The western suburbs of the metropolitan areas from West Palm Beach to Miami can also see fog at times. However, fog in uncommon along the south coast of Florida.

Wildfires, as well as controlled burning for agricultural and land management purposes, can produce large areas of smoke over the rural interior of South Florida. This occurs primarily during the drier months from November to May. While most of the burning occurs in remote areas and has little impact on the population, there are rare occasions when smoke pushes close to roads or developed areas.

Fog or smoke are considered dense when the visibility is lowered to a quarter of a mile or less. The combination of smoke and fog is a very dangerous situation that can lower visiblity to zero. If dense fog is predicted or observed over a large enough area, the National Weather Service wil issue a Dense Fog Advisory. For marine interests, the potential for dense fog will mentioned in the Coastal Waters Forecast, and in Marine Weather Statements as needed.

For the purposes of the daily Hazardous Warning, the five-tiered Levels of Concern are described below. The color system shows coverage of dense fog or smoke (1/4 mile or less) over an area.

Causes of fog formation over South Florida

The Earth constantly radiates infrared energy into the atmosphere. On nights when there is little cloud cover to absorb and scatter this energy, it radiatiates into space. This is called radiational cooling, since it tends to cool the ground in the process. The cooling ground in turn cools the ground immediatly above it. Given sufficient low level moisture, this allows the water vapor in the air close to the ground to condense into tiny cloud droplets. In other words, the water vapor (an invisible gas), cools and turns into a mist of suspended drops of water (fog). This is the main mechanism for fog formation in South Florida. Winds tend to disrupt this process by allowing the cooling moist air near the ground to mix with warmer and often drier aloft. This type of fog is classified as radiational fog.

The second mechanism for fog production in South Florida is advection fog. "Advection" is what meteorologists call the movement of air along a surface. Advection fog occurs when warm. moist air off the Gulf of Mexico or the Atlantic moves (or "advects") over a cooler land surface. This happens occasionally during the Spring when maritime air is gradually warming, but air over the land is still influenced by periodic cold fronts. As with radiation fog, the cooling moist water vapor condenses into fog. This is often responsible for fog formation over the southwest coast of Florida. Sometimes fog is formed by a combination of both processes.

Dense Fog or Smoke Safety Rules

Driving Rules
  • Drive with lights on low beam.
  • Reduce speed.Allow for plenty of room between you and other cars.
  • Avoid crossing traffic unless absolutely necessary.
  • Listen for traffic you cannot see.
  • Use wipers and defroster as necessary for maximum vision.
  • Be patient! Don't pass lines of traffic.
  • Unless absolutely necessary, don't stop on any freeway or other heavily traveled road.
  • If your car is disabled or you can’t continue, pull well onto the shoulder and turn off lights. Move away from your vehicle.
  • Consider postponing your trip until the fog clears.
  • Be especially cautious in and near school zones. Watch for flashing yellow or red signals on school buses. Watch for children waiting for buses in the fog.
  • Also, be aware that smoke from grass and forest fires along roadways can combine with fog to rapidly drop visiblities to zero.
Recommendations for boaters in dense fog
  • Sound signals to let other boaters know where you are located :
    • For powerboats - one prolonged blast of the horn at intervals of not more than two minutes when underway.
    • For anchored vessels, vessels conducting fishing, or sailboats under sail alone - one prolonged blast of the horn plus two short blasts at intervals of not more than two minutes.
  • Maintain good situational awareness. Keep close track of your position relative to shoals, points of land, and other vessels.
  • Consider postponing your trip until the fog clears. If already underway, consider finding a safe, out of the way anchorage.