Below are responses to some common questions I get about fog.
Why fog?
Fog, in addition to being beautiful and theatrically mysterious (unless it is really thick, then it is just a bit depressing!), is also an important part of the water cycle. In some places, such as the Atacama or Arabian deserts, it is a crucial source of water for local flora and fauna. It can even be harvested to make beer!
Why fog?
Fog, in addition to being beautiful and theatrically mysterious (unless it is really thick, then it is just a bit depressing!), is also an important part of the water cycle. In some places, such as the Atacama or Arabian deserts, it is a crucial source of water for local flora and fauna. It can even be harvested to make beer!
The main reason I study fog, however, is that it presents a hazard for human navigation due to the reduction in visibility. Air, land, and marine transport are all impacted. Better prediction of fog events will help to reduce the negative impact.
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What is Fog?
Fog is essentially a cloud in contact with the Earth's surface. Meteorologically, it is defined as when visibility at the surface falls below 1 km.
Fog is essentially a cloud in contact with the Earth's surface. Meteorologically, it is defined as when visibility at the surface falls below 1 km.
What is the difference between fog, mist, haze, and smog?
The difference is predominantly semantic, and varies, related to the reduction in visibility that occurs with each phenomenon. Where fog is when visibility is less than 1 km, mist is when visibility is less than 2 km (but greater than 1 km), while haze is a reduction in visibility to around 7 km or less. Haze and smog different from fog and mist as well, as they are not only composed of water droplets, but also different aerosol particles and gases (e.g., ozone smog seen over cities). Smog is a combination of the words "smoke" and "fog".
Interestingly, in many languages, there is no distinction between the phenomena. Also, in Dutch, 'mist' is the translation of 'fog', which can cause a bit of confusion!
The difference is predominantly semantic, and varies, related to the reduction in visibility that occurs with each phenomenon. Where fog is when visibility is less than 1 km, mist is when visibility is less than 2 km (but greater than 1 km), while haze is a reduction in visibility to around 7 km or less. Haze and smog different from fog and mist as well, as they are not only composed of water droplets, but also different aerosol particles and gases (e.g., ozone smog seen over cities). Smog is a combination of the words "smoke" and "fog".
Interestingly, in many languages, there is no distinction between the phenomena. Also, in Dutch, 'mist' is the translation of 'fog', which can cause a bit of confusion!
How does fog form?
Fog forms when moist air reaches saturation (relative humidity of ~100%), resulting in condensation of water vapour into water droplets. There are essentially two ways this can occur: either more water is added (e.g., through evaporation or advection and mixing), raising the dew point temperature, or the air is cooled to the dew point (e.g., radiatively, or by advection/mixing of cold air with warmer, moister air).
This is analogous to filling a glass with water. A glass can only hold so much water before it overflows, so if you continue to pour water in, it will eventually become "saturated" as in the case of adding more water. Likewise, if you have a large glass filled with water (but not overflowing) and pour all of the water into a smaller glass, it is like cooling the air: the new glass can no longer hold all the water.
Condensation doesn't always occur right at saturation. The process relies on the presence of small aerosols (condensation nuclei) for the water molecules to come together in droplet form. If these are abundant, condensation can occur before the dewpoint temperature is reached. Conversely, if the air is particularly "clean", with few particles present, the water will need to be supersaturated (well below the dew point temperature) before droplets will begin to form.
Fog forms when moist air reaches saturation (relative humidity of ~100%), resulting in condensation of water vapour into water droplets. There are essentially two ways this can occur: either more water is added (e.g., through evaporation or advection and mixing), raising the dew point temperature, or the air is cooled to the dew point (e.g., radiatively, or by advection/mixing of cold air with warmer, moister air).
This is analogous to filling a glass with water. A glass can only hold so much water before it overflows, so if you continue to pour water in, it will eventually become "saturated" as in the case of adding more water. Likewise, if you have a large glass filled with water (but not overflowing) and pour all of the water into a smaller glass, it is like cooling the air: the new glass can no longer hold all the water.
Condensation doesn't always occur right at saturation. The process relies on the presence of small aerosols (condensation nuclei) for the water molecules to come together in droplet form. If these are abundant, condensation can occur before the dewpoint temperature is reached. Conversely, if the air is particularly "clean", with few particles present, the water will need to be supersaturated (well below the dew point temperature) before droplets will begin to form.
What are the different types of fog?
Fog is typically classified according to the way in which it was formed. Some examples:
Fog is typically classified according to the way in which it was formed. Some examples:
- radiation fog forms under calm, clear-sky conditions where the air at the surface cools radiatively toward saturation
- advection fog is when warm, moist air is blown over a cold surface, with the mixing of the air resulting in saturation
- steam fog is seen when cold air is advected over warmer lakes/bodies of water; the mixing of the saturated air over the water with the cold air results in saturation
- cloud base lowering fog is, as the name suggests, when a cloud subsides and reaches the surface