Happy #WeatherWednesday everyone! I hope you liked the astronomy infused refresher lesson on solstices in our last discussion. This time we’ll go back to qualitative meteorology today and talk about fog.

Fog development and sustenance follow simple thermodynamic principles. This means if conditions are ideal for fog, there will be fog. Fog forms the same way clouds higher in the sky do. It happens when air is cooled to its dew point. The dew point is defined as the temperature in which a sample of air will reach condensation (become visible suspended water droplets). The dew point is governed by the amount of moisture in the air parcel, quantified by grams of water vapor for every kilogram of air (mixing ratio). This is always variable based on altitude, location, topography, etc.

Every night the surface temperature cools after the loss of direct solar radiation. If it cools rapidly, typically on a calm clear evening, it can reach the dew point if it is high enough. If the wind is not calm, the surface temp has a hard time cooling to dew point because of boundary layer mixing. If the sky isn’t clear or mostly clear, then clouds can trap longwave radiation and prevent cooling to dew point. If conditions meet the requirements for fog, it can last all night and into the morning until enough solar radiation penetrates the fog to heat the surface temperature a few degrees above the dew point.

Fog is most typical in the northeast and northwest corners of the USA where the temps stay cooler but enough moisture is in the air due to influences of the big oceans. Fog also likes valleys because the colder nocturnal air settles into them due to gravity/density gradient. Since the mixing ratio is relatively constant, the air in the valley is more likely to reach the dew point. Fog also occurs a lot after hail because the ice is absorbing a tremendous amount of thermal energy in order to melt. This chills the air around it causing it to reach the dew point. This usually applies to just a shallow 1-2 meters or less above the surface where this occurs.

Fog can cause numerous accidents on roadways and make aviation more difficult. Do you think there are any benefits to fog, or is it just one of those pesky things we have to live with?