We all know that fog keeps us on the ground when we want to be flying. But are you aware that there are different kinds of fog? Do you know the conditions that are required for fog to form? Do you know about radiation fog, advection fog, frontal fog, sea fog and hill fog?
This post should refresh your memory or perhaps even enlighten you.
Mist and fog occur when tiny water droplets are suspended in the air causing a reduction in visibility. Mist is very similar to fog and differs only by definition:
When the visibility is less than 1000m – FOG exists. Relative Humidity = 100%.
When there is reduced visibility but visibility is greater than 1000m – MIST exists. Relative humidity < 100%.
Water droplets come into existance due to condensation which causes the water vapour in the air to condense out as a liquid. This normally occurs when the air is cooled by an underlying cold surface or by the interaction of two air masses.
Different Types of Fog
There are three different types of fog and they form under specific conditions:
- Radiation fog
- Advection fog
- Frontal fog
The conditions required for the formation of radiation fog are:
- Moist air – (high relative humidity) that only requires a small amount of cooling to reach its dewpoint temperature.
- A cloudless night sky – This permits the earth to cool and this cold, earth surface subsequently cools the air in contact with it.
- Light winds – (5 – 7kts) to mix the lower levels of air slightly and thereby thicken the fog layer.
These three conditions are commonly found in a high pressure system.
If the wind is absolutly calm, only a very thin layer of air will get cooled by the cold earth surface. This causes dew (or frost in the winter) to form directly on the surface. Fog will not form above it. Dew will form when the temperature remains above zero degrees Celsius and frost will form when the temperature drops below freezing point.
If the wind is greater than approximately 7 knots, the extra mixing of the air will prevent fog from forming at the surface and instead, a low layer of stratus clouds may form.
Radiation fog is much more likely to form over land than over sea. This is because the land temperature varies significantly and the sea temperature by comparison remains relatively stable throughout the year.
Dispersal of Radiation Fog
After sunrise, the earth’s surface begins to warm up. The air in contact with the surface will also begin to warm up and this causes the fog to disperse. In a spring morning in Ireland, this normally occurs at around mid morning. If the fog has not dissipated by 1400hrs, it is unlikely to clear. The thicker the layer of fog, the slower the earth surface will warm up and therefore it will take longer for the fog to dissipate.
When I hear the word Advection, I think about a parcel of air moving HORIZONTALLY across the surface of the earth. Imagine a warm, moist parcel of air moving over a colder surface. It will be cooled from below. If its temperature reaches its dewpoint, then fog will form. Fog formed in this way is called advection fog. It can occur very quickly during the day or during the night.
Sea fog is a type of advection fog. It can be caused by:
- A tropical maritime air mass moving towards the pole over a colder ocean or meeting a colder air mass: or
- An airflow off a warm land surface moving over a colder sea; or
- A warm, moist maritime air mass moving over a cold land surface. This can lead to coastal fog that lingers along the coast line.
Frontal fog forms by the interaction of two air masses in one of two ways:
- Cloud that extends to the surface during the passage of a front. This forms mainly over hills and is called Hill Fog.
- The air becomes saturated due to the evaporation from all of the rain that has fallen and the vapour condenses out into its liquid state to form fog.
These condition may form in the cold air ahead of a warm front or and occluded front. If this occurs, the fog may cover a very large area.
Now that you know the different types of fog and what conditions are required for fog to form, you are in a much better position to plan your flight. You can now read between the lines of the meteorological forecasts and anticipate the possibility and probability of fog forming. Knowing about radiation fog, advection fog, frontal fog, hill fog and sea fog will make you a safer, more responsible pilot.