Fog and Mist how they are formed : Metlink Weather Climate for schools
There is a little difference between fog and mist. Dew basically By cooling – In this process, air is cooled to its saturation point (dew point). By evaporation – In. There isn't a direct relationship between dew point and relative humidity in When you see fog, the real pressure of water gas was higher than the vapour. It forms when the temperature of an object drops below the dew point temperature. Fog: a suspended tiny water droplets or ice crystals in an air layer next to the . clouds are found at altitudes between and m ( and 23, ft.).
Definition[ edit ] The term "fog" is typically distinguished from the more generic term "cloud" in that fog is low-lying, and the moisture in the fog is often generated locally such as from a nearby body of water, like a lake or the ocean, or from nearby moist ground or marshes.
Six examples of ways that water vapor is added to the air are by wind convergence into areas of upward motion;  precipitation or virga falling from above;  daytime heating evaporating water from the surface of oceans, water bodies, or wet land;  transpiration from plants;  cool or dry air moving over warmer water;  and lifting air over mountains. Fog commonly produces precipitation in the form of drizzle or very light snow.
Drizzle becomes freezing drizzle when the temperature at the surface drops below the freezing point. The thickness of a fog layer is largely determined by the altitude of the inversion boundary, which in coastal or oceanic locales is also the top of the marine layerabove which the air mass is warmer and drier.
The inversion boundary varies its altitude primarily in response to the weight of the air above it, which is measured in terms of atmospheric pressure. The marine layer, and any fogbank it may contain, will be "squashed" when the pressure is high, and conversely, may expand upwards when the pressure above it is lowering.
Types[ edit ] Fog can form in a number of ways, depending on how the cooling that caused the condensation occurred.
What is the dew point, and how does fog form?
Radiation fog is formed by the cooling of land after sunset by infrared thermal radiation in calm conditions with a clear sky. The cooling ground then cools adjacent air by conductioncausing the air temperature to fall and reach the dew point, forming fog.
In perfect calm, the fog layer can be less than a meter thick, but turbulence can promote a thicker layer. Radiation fog occurs at night, and usually doesn't last long after sunrise, but it can persist all day in the winter months especially in areas bounded by high ground. Radiation fog is most common in autumn and early winter. Examples of this phenomenon include the Tule fog. Advection fog layer in San Francisco with the Golden Gate Bridge and skyline in the background Advection fog occurs when moist air passes over a cool surface by advection wind and is cooled.
It is most common at sea when moist air encounters cooler waters, including areas of cold water upwellingsuch as along the California coast see San Francisco fog. A strong enough temperature difference over water or bare ground can also cause advection fog. The advection of fog along the California coastline is propelled onto land by one of several processes.
A cold front can push the marine layer coast-ward, an occurrence most typical in the spring or late fall.
Temperature, Dewpoint and predicting fog
During the summer months, a low pressure trough produced by intense heating inland creates a strong pressure gradient, drawing in the dense marine layer. Also during the summer, strong high pressure aloft over the desert southwest, usually in connection with the summer monsoonproduces a south to southeasterly flow which can drive the offshore marine layer up the coastline; a phenomenon known as a "southerly surge", typically following a coastal heat spell.
However, if the monsoonal flow is sufficiently turbulent, it might instead break up the marine layer and any fog it may contain. Moderate turbulence will typically transform a fog bank, lifting it and breaking it up into shallow convective clouds called stratocumulus. Evaporation fog or steam fog forms over bodies of water overlain by much colder air; this situation can also lead to steam devils forming. Lake effect fog is of this type, sometimes in combination with other causes like radiation fog.
It tends to differ from most advective fog formed over land in that it is, like lake-effect snowa convective phenomenon, resulting in fog which can be quite a bit denser, deeper, and looks fluffy from above.
Most other fog is stratiform; steam devils, which look like their dust counterpartsare often seen in this situation. Frontal fog forms in much the same way as stratus cloud near a front when raindrops, falling from relatively warm air above a frontal surface, evaporate into cooler air close to the Earth's surface and cause it to become saturated.
This type of fog can be the result of a very low frontal stratus cloud subsiding to surface level in the absence of any lifting agent after the front passes. Ice fog forming in very low temperatures can be the result of other mechanisms mentioned here, as well as the exhalation of moist warm air by herds of animals. It can be associated with the diamond dust form of precipitation, in which very small crystals of ice form and slowly fall.
This often occurs during blue sky conditions which can cause many types of halos and other results of refraction of sunlight by the airborne crystals. Freezing fog, which deposits rimeis composed of droplets of supercooled water which freezes to surfaces on contact. Precipitation fog or frontal fog forms as precipitation falls into drier air below the cloud, the liquid droplets evaporate into water vapor.
The water vapor cools and at the dewpoint it condenses and fog forms. Hail fog sometimes occurs in the vicinity of significant hail accumulations due to decreased temperature and increased moisture leading to saturation in a very shallow layer near the surface. It most often occurs when there is a warm, humid layer atop the hail and when wind is light.
This ground fog tends to be localized but can be extremely dense and abrupt. It may form shortly after the hail falls; when the hail has had time to cool the air and as it absorbs heat when melting and evaporating.
Freezing conditions[ edit ] Freezing fog occurs when liquid fog droplets freeze to surfaces, forming white soft or hard rime. It is equivalent to freezing rainand essentially the same as the ice that forms inside a freezer which is not of the "frostless" or "frost-free" type.
The term "freezing fog" may also refer to fog where water vapor is super-cooled, filling the air with small ice crystals similar to very light snow.
It seems to make the fog "tangible", as if one could "grab a handful". In the western United States, freezing fog may be referred to as pogonip. In his anthology Smoke Bellew, Jack London described a pogonip which surrounded the main characters, killing one of them. The Columbia Plateau experiences this phenomenon most years due to temperature inversions, sometimes lasting for as long as three weeks.
Frozen fog also known as ice fog is any kind of fog where the droplets have frozen into extremely tiny crystals of ice in midair. Urban ice fog can become extremely dense and will persist day and night until the temperature rises. Extremely small amounts of ice fog falling from the sky form a type of precipitation called ice crystalsoften reported in UtqiagvikAlaska.
Ice fog often leads to the visual phenomenon of light pillars. In fog the distance you can see known as visibility is less than 1km, but in mist the visibility distance can be 1km-2km. Radiation Fog When does this type of fog form? This kind of fog forms when the sky is clear and the wind speeds are low kilometres per hour.
This type of fog usually forms at night and dissipates disappears during the day. In mid-winter, however, particularly in latitudes where the sun is low in the sky e. It commonly forms in the dips with sources of moisture such as streams and rivers.
Radiation fog is particularly common in autumn and winter in the UK. What is the science behind this type of fog? When the sky is clear at night, land surfaces radiate heat to space and therefore cool.
- Difference between Fog, Mist and Dew
If the air in contact with a surface is cooled to its dew-point temperature, small water droplets form condensation. If there is no wind, droplets of dew form on, for example, grass. If there is a very gentle breeze, the tiny water droplets are stirred upwards to form a shallow layer of radiation fog, as in the picture of fog at Cardiff shown.
Fog can reach 30m high in some cases. Most of the rays are actually reflected from the top of the fog but some reach the surface, otherwise it would not be daylight in the fog! The ground is gradually heated until the dew-point temperature is exceeded. The fog then dissipates disappearsoften very quickly. Steam Fog When does this type of fog form?