Fog, Dew and Frost !!!

Fog, Dew and Frost

Evaporation or Mixing Fog
This type of fog forms when sufficient water vapor is added to the air by evaporation and the moist air mixes with cooler, relatively drier air. The two common types are steam fog and frontal fog.

Steam fog forms when cold air moves over warm water. When the cool air mixes with the warm moist air over the water, the moist air cools until its humidity reaches 100% and fog forms. This type of fog takes on the appearance of wisps of smoke rising off the surface of the water.

Frontal fog is the other type of evaporation fog. This type of fog forms when warm raindrops evaporate into a cooler drier layer of air near the ground. Once enough rain has evaporated into the layer of cool surface, the humidity of this air reaches 100% and fog forms.


Ice Fog
This type of fog forms when the air temperature is well below freezing and is composed entirely of tiny ice crystals that are suspended in the air. Ice fog will only be witnessed in cold Arctic / Polar air. Generally the temperature will be 14 F (-10 C) or colder in order for ice fog to occur.

Freezing Fog
Freezing fog occurs when the water droplets that the fog is composed of are “supercooled”. Supercooled water droplets remain in the liquid state until they come into contact with a surface upon which they can freeze. As a result, any object the freezing fog comes into contact with will become coated with ice.

The same thing happens with freezing rain or drizzle.


Fog, Mist and Haze 

– Fog (FG) is defined as being visibility of < 1,000m due to liquid particles or ice crystals suspended in the atmosphere. RH is taken to be 100%.

– Mist (BR) is defined as being reduced visibility > = 1,000m but not more than 5,000m due to the presence of water droplets in the atmosphere. RH may be assumed to be around 95%. 

– According to ICAO, haze (HZ) or smoke (FU), is reduced visibility due to the presence of solid particles (lithometeors) in the atmosphere to a value of < = 5,000m. 

– MIFG (Shallow Fog) is 5 feet deep. 

– The type of obscuration – Mist, Haze, Smoke, Dust or Sand – is not normally quoted if the visibility is more than 5000m. 

– Radiation fog is generally 500 feet thick, need winds of about 2-8 knots. 

– For advectin fog, 15 knots wind is needed. High winds can lift advection fog into low stratus or clear it altogether by turbulent mixing. It depends on the gap between the surface temperature and the dew point temperature of the air mass. If the surface temperature is well below the air mass dew point the advection fog will persist in winds as high as 30 to 40kt. 


You can see further in mist than in fog. By international convention, visibility in fog is less than 1 km, which is the definition used in shipping and aviation forecasts. However, in forecasts for the general public in Britain, fog refers to visibility of less than 200 yards. Both mist and fog are caused by microscopic water droplets suspended in the air. The visibility depends on how far light can travel before too much of it is randomly scattered by hitting droplets. The bigger and closer together the droplets are the smaller this distance is. 

In mist the droplets are very tiny, in fog they are bigger. When droplets exceed about 200 micrometres in diameter they tend to fall earthwards and are called drizzle. Mizzle is a mixture of mist and drizzle, also known as Scotch Mist. Water droplets in mist, fog or clouds do not evaporate because the air is saturated or very close to it, (the relative humidity is between 100% and 95%). 

Haze is another term for reduced visibility but the particles in suspension which cause haze are dry and the relative humidity is below 95%. It is possible, though rare, to get dry fog which is haze with visibility below 1km. 


Advection Fog 

Advection fog is the name given to fog produced by air in motion or to fog formed in one place and transported to another. This type of fog is formed when warmer air is transported over colder land or water surfaces. Cooling from below takes place and gradually builds up a fog layer. The cooling rate depends on the wind speed and the difference between the air temperature and the temperature of the surface over which the air travels. Advection fog can form only in regions where marked temperature contrasts exist within a short distance of each other, and only when the wind blows from the warm region toward the cold region. It is easy to locate areas of temperature contrast on the weather map, as they are usually found along coastlines or between snow-covered and bare ground. 

Sea Fog — Sea fog is always of the advection type and occurs when the wind brings moist, warm air over a colder ocean current. The greater the difference between the air temperature and the ocean temperature, the deeper and denser the fog. Sea fog may occur during either the day or night. Some wind is necessary, not only to provide some vertical mixing, but also to move the air to the place where it is cooled. Most advection fogs are found at speeds between 4 and 13 knots. Sea fogs have been maintained with wind speed as high as 26 knots. They persist at such speeds because of the lesser frictional effect over a water surface. Winds of equal speed produce less turbulence over water than over land. Sea fogs, which tend to persist for long periods of time, are quite deep and dense. Since the temperature of the ocean surface changes very little during the day, it is not surprising to hear of sea fogs lasting for weeks. A good example of sea fog is that found off the coast of Newfoundland. 

Land Advection Fog — Land advection fog is found near large bodies of water; that is, along seacoasts and large lakes. Onshore breezes bring maritime air over a land surface, which has cooled by radiation at night. Also, fogs may form over the ocean and be blown over the land during either the day or the night. Another situation favorable to fog formation is one in which air flows from warm, bare ground to snow-covered ground nearby. Land advection fog cannot exist with as high wind speed as the sea type because of the greater turbulence. It dissipates in much the same fashion as radiation fog. However, since it is usually deeper, it requires a longer time to disperse. 




If the dew point temperature is below 0 deg C the air will meet the frost point first as it cools and the moisture will probably deposit (sublimate) directly to ice forming a heavy frost on the surface rather than radiation fog.


Arctic Smoke or Sea Smoke 

It is formed when cold air passes over a warm surface usually occurs when very cold polar air flows off the ice cap or from a glacier over relatively warm open water. 

Normally these conditions – moisture and heating from below – would produce instability and convection in the air mass. So there has to be a marked temperature inversion at low level to prevent this.