This writing is continuation of a whole article about the haze as an important weather factor. Previously I have described the basic types of haze and I drawn attention on impact the atmospheric aerosols on human’s health. Now, in terms of the continuation I would like to describe the impact of haze on the light scattering in the atmosphere and the visual range.
1. LIGHT SCATTERING IN HAZY ATMOSPHERE
The presence of the haze in the atmosphere impact on the light scattering. Haze makes usually milky white color for the atmosphere (Pic. 1), while against a bright background it will become yellowish, grey or orange-red at the sunrise/sunset (Pic. 2); likewise against darker background it will be light blue (Pic. 3). This is due to optical effect caused by the haze particles; i.e. the scattering of light by the haze particles. The type and direction of scattering depends critically on the size of the particle (Ackerman, Knox, 2007). The presence of haze is strongly related to the Beer-Lambert Law, described in earlier article. It means, that as cloud is further from observer, its color turns from dark to bluish, when shaded and from yellowish to reddish, when illuminated.
There are 3 main sources of particular matter in the terrestial atmosphere (Kokhanovsky, 2008):
– surface (e.g dust or sea salt),
– gas to particle conversion
– cosmic aerosols
Important is also the orientation and shape of the aerosol particles, which can slightly modify the route of light through the hazy environment. These shapes are different depends on the kind of environment. In arid environment, like desert, semi-desert as well as in continental air mass these shapes can be really various and irregural due to local factors. One of this is a local soil composition and thus aerosol source composition, which can vary dramatically from one region to another (Reid et al., 2003). Another factor is the time of atmospheric residence (see details in previous article) these particles, which for all this tame falls into reactions with other particles suspended in the air. These reactions change a chemical composition, often along with the physical shape of aerosol (Khalizov et. al., 2009). In the result a dust particles often consist of agglomeration of different materials and tends to have more complicated shapes. These particles can contain sharp edges, points, chains or internal voids.
In a wet environment, aerosols interact with the ambient water vapour. This process affects their size, shape, chemical composition and consequently their optical properties. The shape of aerosols depends on the relative humidity. As more water is coated on the surface of the particle, the overall shape would become more spherical, changing the scattering properties. Water is absorbed by the volume of particle. In effect the particle changes its shape, size and optical properties. In humid environment, where most of particles has a regular shape i.e. cubic or spherical, the light scattering will be more effective.
Next to the shape, also the size of atmospheric aerosols is important in light scattering. For arid conditions a main role in the size of aerosols plays wind, which keep them suspended for a long time (Hinds, 1999).
As the final, the pivot factor, determining the degree of light scattering and changes of visual range is a relative humidity. We can treat a relative humidity somewhat as a “distance” from the dew point, which is a normal fog, and observer is able to see nothing. Relative humidity says us about the presence of smallish water droplets in the air, which is next absorbed by aerosol particles or causes their coagulation or condensation. This is a strong relationship between relative humidity and light scattering. Depends on the aerosol hygroscopic fraction the increase of light scattering may start from 50% of relative humidity, whereas at about 70% a rapid light scattering changes are observed (Pic. 6).
Basically there are 2 types of light scattering depends on the illumination source location. This is forward and backward scattering (Pic. 7).
Forward scattering involves a changes of radiation less than 90 degrees. In our case it will refer to section of sky being on the same side, where the primary illumination source – the Sun is.
Backward scattering (backscatter) will involve a changes of radiation higher than 90 degrees, thus occur on the antisolar section of sky (Pic. 8).
The effect of light scattering depends on the size of the particles (Pic. 8). The weakest light scattering occurs for air molecules and the smallest aerosols, that is typical for near-Rayleigh conditions. The most effective scattering occurs for water droplets or extremely dense haze particles suspended in the atmosphere, when non-selective scattering plays a main role.
We used to treat the Earth’s atmosphere like a place of multitude of simple particles, where each particle is exposed to, and also scatters, the light that has already been scattered by other particles (Liou, 2002). This is a multiple scattering, which is accompanied by absorption. A large absorption occurs for mineral, dustlike and water soluble particles and particularly for soot (Liou, 2002).
We have to take into account also the effects of the angle between the source of illumination and part of sky observed. This is easy to spot when cloud layer cover the sky and some clear gaps appear. Then a rays of sunlight (crepuscular rays) can be seen from the point of the sky where the Sun is located. That happens under hazy conditions. When big aerosol concentration take place under a clear sky, the observer is able to spot a very bright area when look close to the Sun. The sky is appear to be darker towards the simetric point on the sky or antisolar point (when observer is above the layered haze) (Pic. 9). Knowing, that apparent illuminance of the Sun varies from around 10000 Lx at the 10 deg altitude to 129000 Lx at zenith we can simply calculate the illuminance level of the certain regions of sky (Pic. 10).
Even during the day free of haze in near Rayleigh scattering conditions the light dispersing throughout the sunlighted blue sphere can be described as per below:
- The brightest area is near the ilumination source (Sun or Moon),
- The darkest area is located on the opposite side on the sky. Its presence above horizon depends on the location of the Sun. For example, when Sun shines in zenith, then the darkest area of the sky will encircle the sphere at around 40-50 deg altitude. In case as per picture below, the darkest region of sky is located more or less on the opposite part of the sky at 50 deg altitude.
- Light areas of the sky are near the horizon due to thickness of the atmosphere, whilst the brightest part of near horizon sky is to be observed on the Sun direction.
Under a free of haze conditions the sky brightness (and diffuse sky radiation) is much more various. The brightest area is located obviously near the Sun, and corresponds around 100% of solar illuminance, however is not widely developed likewise in misty conditions. Even near the edge of the Sun the sky is blue, but its colour is less pronounced due to close vicinity to the Sun and forward scattering of air molecules. On the contrary of the solar point, the antisolar direction is much less brighter; the range of illuminance level corresponds between less, than 10% to about 20% of the illuminnance of incident light. Considering it for solar altitude 50 deg (Pic. 11, 12) the incideng light level is going to reach 87000 lx, whereas the sky region in antisolar direction should reach the illumination level about 8 – 18000 lx.
During the free of haze conditions a main role plays air molecules, that scatter light about equally in the forward and backward direction and scatter more blue light than red, hence is the blue sky (Malm, 2016). Due to haze sky appears to be brighter than during the near Rayleigh scattering conditions.
2. RELATION WITH PLANETARY BOUNDARY LAYER
Planetary boundary layer also know as the atmospheric boundary layer is the lowest part of the atmosphere. Its behaviour is strongly influenced by Earth’s surface on the temperature, moisture and wind through the turbulent transfer of air mass. As a result of surface friction winds in the planetary boundary layer are weaker than above and tend to blow toward areas of low pressure. The planetary boundary layer reacts for temperature changes quickly so its thickness is various within the day. Usually the planetary boundary layer reaches from few several meters up to 5 kilometers (above the desert). Above this layer is the “free atmosphere” where wind is parallel to isobars unlike to inside this layer where is affected by drag over the surface.
The planetary boundary layer behaviour differs between flat surface and mountainuous area, where is more complicated (Pic. 16, 22). There it usually adjust to the local altitude. The land roughness influences on the wind system. Big role plays also the effect of landcover. Whereas in the valleys is summer on the top can be still wintry weather with snow cover. These factors makes the planetary boundary layer more unpredictable. Moreover planetary boundary layer changes throughout the day (Pic. 17) due to changes of irradiation.
During the daytime hours the planetary boundary layer comprises with the surface layer, that is the lowest part. It features a superadiabatic lapse rate, moisture decrease with height. In windy conditions, the surface layer is characterized by a strong wind shear caused by friction. Above a surface layer the mixed layer is located, where next to a horizontal air mass movements the vertical, turbulent updrafts play a main role. The air is risen up very quick (with up to 40km/h velocity). The convective mixed layer has a nearly constant distribution of the potential temperature, wind speed, humidity and haze concentration, because of strong buayancy generated convective turbulent mixing. Once the supersaturation of the air is sufficient, then clouds start to form. It happens at the upper part of planetary boundary layer, which forms the cloud cover or entrainment zone during the day. The condensation of moist air, containing aerosol relases energy, creates buoyancy and clouds rises until they reach their level of neutral buayancy.
Rising air particles are replaced by drier air from free atmosphere, that penetrates down the planetary boundary layer. Hence during the day, when some cumulus clouds start to appear on the sky, visual range appears to be better, than at the morning. The air, that penetrates down from the free atmosphere, doesn’t contain as much aerosols as the rising air do.
Between the upper edge of cloud layer, reliant of the highest thermal plume and the deepest part of the sinking free air is called the entrainment zone. This is the layer of intermittent turbulence and overshooting thermals at the top of convective boundary layer. The thickness of the entrainment zone depends on the vibrance of the turbulent movements. When turbulence and thermals are more vigorous, then the entrainment zone is thicker, unlike to stronger temperature inversion caps, where is thinner. In this case, the cloud base will be a capping inversion layer during the day. When Sun is going to set, then turbulence decays. In the result the mixed layer is turned into a residual layer, which is neutrally stratified. Then turbulence is in near the same intensity in all directions. It means, that for instance a emitted smoke plumes tends to disperse at equal rates in vertical and lateral directions, creating a cone-shaped plume (Stull, 1988). Above the resitual layer the capping inversion exist, on which level the cloud deck is produced, likewise in daytime conditions, when turbulence is weak. Sometimes the capping inversion can count about 40% of whole planetary boundary layer. The capping inversion is an elevated inversion layer, that caps a convective boundary layer. The capping inversion is caused by warmer air being above a region of cold air. Cloud formation from the lower layer is “capped” by the inversion layer. If the capping inversion is too strong, then prevents thunderstorm from developping, unless is forced by strong updrafts. A strong cap can result in foggy conditions. Last issue, that I would like to add here is the general height of planetary boundary layer. It is assumed, that a standard height is between 1,5 – 2 km above ground. Practically the height of planetary boundary layer changes due to the weather conditions and may vary between 500 – 2500 m range.
Now let’s considerate how it works with haze issues. Planetary boundary layer can be denoted by a thin layer of haze or cloud deck that can be seen from the plane just after start and before landing (Pic. 21, 22). This layer or clouds can also be seen directly from the surface, especially when a lot of pollutants is trapped inside (Pic. 18) or after sunset, when is not directly illuminated (Pic. 19).
To rundown this section I can say, that haze is always bigger within the planetary boundary layer because:
- The air mass movement inside the planetary boundary layer is slower than in above “free atmosphere” space so the ventilation of the area is also lower,
- Planetary boundary layer gathers the water evaporated into the air from biosphere, hence the humidity level is higher than in “free atmosphere”,
- This is the part of the atmosphere, where the temperature inversion layer forms as a stable layer. Due to this a lot of air pollutants and aerosols concentrates there and deteriorates the visibility.
- Another meteorological conditions like wind direction. Wind can transport the aerosols from different region featured with another kind of surface, like e.g sand. Other unfavourable weather conditions can be stability stratification
The structure of planetary boundary layer varies within the season, weather condition and time of the day. The depth of this layer is bigger during the night and winter and smaller in the summer and during daytime hours. The biggest haze will occur in surface layer.
3. RELATION WITH INVERSION
The inversion phenomenon is very important in haze conditions because it traps air pollution, such a smog close to the ground. An inversion can also suppress convection by acting as a “cap”.
An issue with capped inversion has been explained in he secton above. In this section I would like to focus about the inversion layer, which is a subsidence and radiation inversion. There are a two common types of temperature inversion, as occur mainly in the surface layer. As we know from previous section, the inversion play an important role in determining cloud forms, precipitation and visibility. Both the capping inversion with the subsidence and radiation inversion affects diurnal variation in air temperature. The principal heating of the air is caused by its contact with a land surface with has been heated by the Sun’s radiation. Then this warm air is transferred to the atmosphere by conduction and convection. Once evening approaches (roughly when Sun is about to set) this proportion changes. The air just above the ground, which was warmer during the day, now become colder, because ground cools off. A heat is radiating towards the sky, thus the radiation inversion is produced. The radiation inversion traps the moisture and pollution. It results a morning fog or thick smog, depend when you live.
When the widespread layer of air descend, what happens under a deep high pressure area, the subsidence inversion develops. The layer is compressed and heated by the resulting increase of atmospheric pressure. If the air mass sinks low enough, the air at higher altitudes become warmer, than at lower altitudes; an temperature inversion is produced.
Subsidence inversions are common especially during the winter. It can also occur on the lee side of mountain range. This inversion also traps the haze and pollutants. Depends on the pressure and weather conditions inside the anticiclon the thickness of the subsidence inversion can vary up to several hundreds meters.
4. HAZE IMPACT ON VISIBILITY
Visibility can be definied as a degree of atmospheric clarity (Malm, 2016). Haze can be treated like kind of bad weather, because it can significantly degrade the visibility of a scene. This phenomenon tends to produce a distinctive gray hue, which effect atmospheric transparency. Optically, this is due to the substantial presence of particles in the atmosphere, that absorb and scatter light (Tan, 2008). Light from the atmosphere and light reflected from an object are absorbed and scattered by these particles, causing the visibility of a scene to be degraded (Pic. 25 – 26).
The visibility conditions differ mainly in the types and sizes of the particles involved in their concentration in space (Nayar,Narasimhan, 1999). The manner in which a particle scatters incident light depends on its material properties, shape and size (Nayar, Narasimhan, 1999). The exact form and intensity of the scattering pattern varies dramatically with particle size (Minnaert, 1954). Extremely high concentration of these small particles can appreciably diminish the horizontal visibility. The transparency of the atmosphere increases when the aerosols are removed from the atmosphere. These particles can be removed by two mechanism: deposition at Earth’s surface (dry deposition) and incorporation into cloud or rain droplets during the formation of precipitation (wet deposition) (Pandis, Seinfeld, 2016).
The concentration of haze nucleic depends on wind and humidity. More information about this has been shown in the 1st section of this article. Wind disperse and transports the areosols, whereas the humidity causes a hygroscopic growth of haze particles, especially a high near-surface humidity. Another factor for the development of the haze is dynamic mechanism. The boundary layer is controlled by descending movement, that suppress the vertical transportation of the aerosols. Tropospheric aerosols vary widely in concentration and composition over the Earth. This is caused by nonuniformity of the geographic distribution of particles and kind of depositions being reliant of the local weather conditions.
In the result the horizontal visibility in one area can differ from horizontal visibility in another (sometimes even adjacent) region. Haze impact on visibility is weather-dependant. Before the short term forecast has been developed people used to predict the weather by changes of the visibility. For instance before cold or warm front visibility is getting worse unlike after passing the cold front, when is very good, because the descent airflow facilitate to decreasing of aerosols particle concentrations (Marmureanu et all, 2017).
In European conditions usually polar and arctic air mass brings the best horizontal visibility unlike to tropical masses with high level of dust or moisture.
In meteorology, visibility is a measure of distance at which an object or light can be clearly discerned. In extremely clean air the visibility can be more than 320 km (200 miles) where there are large markers such as mountains or high ridges. The visual range is described in 10-degree scale where 0 means thick fog with visibility lower than 50 m and 9 means extremely good visibility with distance longer than 50 km.
The arctic (and antarctic) air mass is the clearest, because it contains the lowest amount of particles coming from local basement. Hence when is no products of water vapour condensation this air is almost perfect. The polar continental air mass and arctic maritime air mass is believed to have one of the highest transparency level due to lack of the antropogenic factor and dust. Under haze-free atmospheric condiotions the light is scattered on air molecules only. In this case a diffuse sky radiation restricts only to near-Rayleigh light scattering conditions, where short wavelengths are scattered more than longer ones during the day. It doesn’t mean, that under haze-free weather conditios our remote horizon will be clearly visible. Always, even on clearest day, the remote horizon will have a blue appearance. There are basically two reasons of it:
– air-tiny hydrocarbon particles relased by vegetation and next chemically react with ozone molecules, scattering a blue light selectively,
– air molecules, which scatters only short wavelengths according to Rayleigh scattering.
Generally the maritime air masses provides better visibility conditions than continental ones, because they are clear. However these masses provide favorable moisture conditions for the occurrence of a haze event. The worst visibility is to observed during the tropical continental air mass advection, where a lot of dust raised by wind from the ground. Much more details about the visual range changes will be provided in forthcoming articles soon.
5. WEATHER FORECAST
Most of weather forecast services does not focus about the haze prediction itself. It remains a second order weather element. The best way is to check the relative humidity or visibility forecast. It should bring a relevant information about the haze density. In terms of the visibility forecast helpful can be a dew point forecast. In the other hand the air pollution forecast is widely accessible, which has been raised in previous article.
Aforementioned numerical weather patterns shows only a general situation in a few countries at once, which can be helpful for plan a long distance observations. There is another way to check the visual range forecast. An example you can see in the picture below, but I will explain it in the forthcoming article about visibility.
Haze is important weather factor, which can affect for many issues from atmospheric light scattering even to impact for human health (air pollution). For the long distance photography purposes haze results in the visual effect of a loss of contrast in the subject, due to the effect of light scattering through the haze particles. For these reasons sunrise and sunset colors appear subdued on hazy days and stars may be obscured at night. In some cases atenuation by haze is so great that, toward sunset, the sun dissappear altogether before reaching the horizon. Big haze comcentration deteriorates the human health causing the eyes, nose and lungs afflictions. Air polluted is also a hazard for expecting mothers. Big haze concentration modifies a light scattering in the lower part of the atmosphere making sky more brighter than it really is. The biggest haze concentration is related to the planetary boundary layer (unified haze) within the inversion layer may exist (layered haze). This text provides the general knowledge only. Is a lot of things to develop in the next articles.
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1. Scattering of light – graphs
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5. Planetary boundary layer
6. Atmosphere boundary layer structure
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