Fog: A Ground and Air Transportation Hazard

Even if you are not a basketball fan, you no doubt heard the news of the passing of LA Laker legend Kobe Bryant, his daughter Gianna, and 7 other people in a helicopter crash in southern California last week. The tragic news sent shock waves across the nation. As the parent of a teenage daughter who is a basketball player nearly year-round, I found this tragedy personally very difficult to process, especially upon learning that there were teammates, coaches, and parents who were scheduled to play a game that day on the doomed flight. The children, families, and coaches left behind will be dealing with this loss for a long time to come. As I sat down to write this blog post today, I was going to write on an entirely different topic, but given the thoughts in my mind, I thought it would be timely to use the opportunity to talk a bit about the hazard that fog presents to both ground and aviation travel.

What is fog?

Very simply put, fog is a cloud which is located very close to or at the surface of the earth. Generally composed of liquid water droplets, fog dramatically reduces visibility. How low the visibility gets depends on the size of the droplets, and due to how it forms, fog can often be patchy in nature. In order for fog to form (or any cloud), the air temperature must equal the dew point temperature, such that the relative humidity of the air reaches 100%. This can happen in two ways: moisture can be evaporated into the air, such as by sitting over a lake or ocean, or the air can be cooled down- cold air has less capacity to contain water vapor, and so cooling air also causes its relative humidity to increase.

Four types of fog

Radiation fog forms as the air temperature cools overnight. As I mentioned above, cooling the air increases its relative humidity and brings the temperature closer to the dew point. If moist air is present and the dew point is relatively high, fog will form as the air temperature cools after sunset. Clear, calm conditions are most favorable for radiation fog.

Courtesy: National Weather Service.

Advection fog forms when warm moist air is moved, or advected, over a cooler surface. Along the Pacific coast, this type of fog is very common during the summer. Due to the cold ocean currents near the coast, the surface water near the coastline tends to be much colder than the water offshore. Thus, when warm air from the Pacific Ocean is advected to the east over the colder water, the temperature of that air cools to its dew point and fog forms. In fact, according to the Farmer’s Almanac, three of the five foggiest places in North America are on the west coast (the other two are in Maine and Newfoundland)!

Courtesy: National Weather Service.

Upslope fog is formed when moist air flows up a mountain. When air rises, it expands and cools. If it cools to its dew point, fog will form.

Courtesy: BYU.

Evaporation, or mixing fog occurs when moist air meets colder air and mixes with it. A common example of this process is when you exhale on a cold day and a cloud forms from your breath, or when your hot coffee makes steam on a cold day.

Flying in fog

Flying in fog can be dangerous even for experienced pilots. The National Weather Service routinely issues Terminal Aerodome Forecasts, or TAFs, which are valid for 30 hours and forecasts for key parameters such as wind speed, wind direction, visibility, ceilng height, and precipitation type. The National Weather Service updates these forecasts four times per day. Additionally, the FAA issues mandated guidelines and flight rules for these flight categories:

  • Visual Flight Rules (VFR): ceiling over 3000 feet and visibility over 5 miles
  • Marginal Visual Flight Rules (MVFR): ceilings 1000-3000 feet and/or visibility 3-5 miles
  • Instrument Flight Rules (IFR): ceiling less than 1000 feet and/or visibility less than 3 miles

These guidelines vary depending upon the instrument rating of the pilot and the aircraft, as well as the capability of the particular airport.

Courtesy: Koen van Week/AFP/Getty Images.

It has been widely reported that the pilot of the helicopter on which Bryant and the others were passengers requested special visual flight rules (SVFR) clearance from local air traffic control. This allows operations in conditions which are below the normal VFR criteria and generally allow for more close contact with air traffic control. Weather conditions at the time of the crash were foggy.

While the full NTSB report on the crash will take months, it is expected that the weather at the time of the crash will be closely examined as a possible factor in causing the helicopter to go down.

Driving in fog

While it is possible that we may encounter air traffic delays as a result of fog, it is likely a much more common occurrence that we will find ourselves having to navigate in an automobile through foggy conditions. Fog can result in rapid decreases in visibility and result in near-zero visibility driving conditions. Conditions can be worse on bridges which traverse bodies of water and can result in pileup accidents on interstate highways.

If you must drive in foggy conditions, be sure to

  • slow down and allow extra following distance
  • use your low beam headlights, and fog lights if you have them
  • never use your high beam headlights
  • follow the road paint lines to help you stay in your lane
  • if you MUST pull over, turn on your hazard lights, and pull into a safe location such as a traffic stop or parking lot. If there is no such spot, pull off the road as far as you are safely able and then turn off all your lights except your hazard lights, and take your foot off the break so your tail lights are not on

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