This phenomenon has five main forms based on how they develop, but the most problematic is usually radiation fog. It can form just about anywhere and has disruptive impacts that go well beyond being stuck in traffic on the morning commute.
This blog overviews how this fog forms, it’s economic cost, and a climatic trend that stands out from most.
What Does Radiation Have to do with Fog?
Radiation fog owes its existence to a process called radiative cooling.
Generally, Earth’s surface consists of materials that aren’t very good at holding onto whatever heat has been put into them by day. The heat radiates out, so much so that at night, the surface often becomes cooler than the air above it.
Under clear skies with very little wind, this cooler ground can in turn cool the near-surface air. Meanwhile, air higher up stays warmer. This setup is known as a thermal inversion.
If there’s enough moisture in the air, some will condense in the cooler surface layer, leading to mist or fog. This ‘surface cloud’ is unable to rise, due to the laws of thermodynamics: The cooler air is, the denser it is, so per unit volume, it weighs more than the air above.
‘This ‘surface cloud’ is unable to rise, due to the laws of thermodynamics’
With strong enough sun or the wind picking up, even dense fog can be dispersed – but otherwise, it can linger well into the day, sometimes right the way through.
Dense Fog: An Economical Downer
Dense fog spells big trouble for businesses. Delays and cancellations lead to considerable economic cost in sectors such as aviation, rail and haulage to name a few.
This usually has a negative effect on consumers as well.
For example, widespread dense fog across Europe in early Nov 2015 led to a combined nearly 137,000 minutes of delays to flights across the major airlines by the afternoon of 2nd Nov. An estimated total cost of £7,411,700 or 8,241,920 Euros* in just two days!
Delayed flights impart a cost to customers too; an estimated $35 to $63 per hour (approx. £27 to £49 or to 31 to 56 Euros), up to 3 hours**.
In the haulage sector, delays incur costs that can go both ways; for example supermarkets may miss peak selling hours for high-demand foods such as milk and bread, and some of them have been known to fine suppliers for late deliveries.
The net cost of dense fog is enough to justify its reputation. It can be among the most concerning weather phenomena to see predicted by a weather forecast.
‘…the net cost of dense fog is enough to justify its reputation. It can be among the most concerning weather phenomena to see predicted by a weather forecast.’
These Days the Picture is Clearer
Thankfully, dense lingering fog occurrence has become increasingly uncommon in recent decades.
This is due in large part to reduced air pollution following widespread action to cut emissions from industry and transport. Atmospheric moisture readily condenses onto many pollutants.
This is often well demonstrated on ‘bonfire night’ (Nov 5th), when the output of fires can lead to patches of fog developing even in breezy conditions.
Another contributing factor is rising global temperatures. For example, a 2016 study observed that in a controlled test environment, a mere 0.1°C increase in temperature had a comparable effect to reducing aerosol pollutant concentrations by 10%; an increase in visibility by 36 m.
‘…a mere 0.1°C increase in temperature had a comparable effect to reducing aerosol pollutant concentrations by 10%; an increase in visibility by 36 m’
This is down to fog’s reliance on saturation of air, which occurs when the atmospheric moisture content exceeds the air’s capacity to store it as vapour. That capacity increases with air temperature.
So, these days, it takes a greater concentration of moisture for fog formation to take place than it did even just a couple of decades ago. In many parts of the world, it has become much more likely to observe mist than fog (defined by visibility; < 1 km for fog).
Reduced incidence of fog is likely to be a rare upside among the far more numerous and in some cases more severe downsides to a continued warming of global temperatures during the coming decades.
James Peacock MSc
Head Meteorologist at MetSwift
* Based on the average cost-per-minute of U.S. delays in 2017 of $68.48 (£54.10 or 60.16 Euros).
** If the fog causes delays of 3 or more hours, some cost to customers is transferred back to the airlines. They are entitled to between £110 and £530 in compensation (approx. 125 to 603 Euros), under EU rule 261/2004.