Above the Clouds
Predicting the weather as we know as photographers can be pretty tricky but possible. So, can we also achieve this with mist and fog? And what causes the dreaded haze? Well, it’s all down to water droplets, dust, and dew points. Prediction where and when it forms however is probably the tricky bit.
I love mist and it’s one of the conditions I relish getting out my camera. It can transform an ordinary or perhaps familiar landscape into something quite special and often a lot more interesting. I mainly use a 70-200 or 100-400 lens when I am above the fog, unless there is a wide-angle composition, but this is usually limited to my 24-70. I rarely use a focal length wider than 35mm.
What exactly is fog and mist?
Fog is caused by tiny water droplets suspended in the air. It is most commonly seen where warm, moist air meets sudden cooling conditions, like the clouds suddenly clear overnight or in the mornings. The thickest fogs tend to occur in industrial areas where there are many pollution particles (condensation nuclei) on which water droplets can grow.
The official definition of fog is a visibility of less than 1,000 m. This limit is appropriate for aviation purposes, but the likes of us, an upper limit of 180 m with severe disruption when the visibility falls below 50 m (often the stuff we photographers like to capture).
Mist on the other hand is still a form of fog. It is often used interchangeably and perhaps sounds more romantic than fog. Mist however is just less dense fog where visibility is better.
So why does it form?
Well, there is something you might have heard about called Dew point. Air contains water unless it’s a vacuum. When the air is classed as dry the saturation levels are low and when humid the air is highly saturated. The dew point is the temperature, when the air cools, when it becomes saturated with water vapor. When further cooled, the water vapor will condense to form liquid water (dew).
The measurement of the dew point is related to humidity. A higher dew point means there will be more moisture in the air. The colder the air the less water it can retain. So as water laden air cools, the amount of water it can hold drops and thus water stops being a vapour and condenses around dust particles to form droplets whish effectively either become cloud at higher altitudes or on the ground as fog and mist. Also, when air cools to its dew point through contact with a colder surface, water will condense on the surface (also called a frost point when below freezing with hoar frost).
Where and how does it form?
Fog is generally classified according to the physical process which produces saturation or near-saturation of the air.
The main types of fog are:
Radiation fog usually occurs in the winter, aided by clear skies and calm conditions. The cooling of the land surface overnight cools the air close to it. This reduces the ability of the air to hold moisture, allowing condensation and fog to occur. Radiation fogs usually vanishes after sunrise as the ground heats up (less so if the sun is shrouded behind clouds) but less so at higher elevations like in the mountains.
This is the kind of mist and fog, we as landscape photographers often like to capture. The low laying mist often means tall objects like buildings, trees and rock outcrops often point out through them to create great atmospheric images.
It is most prevalent during the fall and winter. It forms overnight as the air near the ground cools and stabilizes. When this cooling causes the air to reach saturation, fog will form. Fog will first form at or near the surface, thickening as the air continues to cool. The layer of fog will also deepen overnight as the air above the initial fog layer also cools.
As this air cools, the fog will extend upward. The most favoured areas for fog development are sheltered valleys where there is little to no wind and locations near bodies of water.
Valley fog is just a form of advection fog forms where cold dense moisture-rich air falls into the lower parts of a valley condensing and forming fog. These are most often river valleys where there is a high amount of ground moisture.
It is often the result of a temperature inversion with warmer air passing above the valley. This is what is often what is called ‘a cloud inversion’.
Valley fog is confined by the landscape around it and can last for several days in calm conditions during the winter.
It occurs when warmer moist air passes over a cool surface and is cooled. The best example of this where I live in Stirlingshire is sea fog (or Haar) when moist air from the south moves over cooler waters.
If the wind blows in the right direction (here from the east but also sometimes from the west) then sea fog can become transported over coastal land areas. This is what is also often what is called ‘a cloud inversion’. It is also common at sea when a warm front pass over an area with snow cover.
Often this is around 300m above the lowlands here but occasionally it is ground level. If you manage to climb a mountain, above say 700m, with a 300m base level inversion, you can get above it.
In the UK for example, coastal fog usually occurs in the spring and summer months when conditions begin to warm up, but the sea (which warms more slowly) stays relatively cold. The impact, location and movement of coastal fog depends upon several conditions, including wind strength, wind direction and land temperature.
If, as is common along the UK's east coast, the winds blow in from the east, the fog will often rapidly cover the coast in a blanket of fog. If the land temperature is warm the fog can quickly dissipate as the parcel of air warms, however if the land temperature is cooler, the fog can linger for a longer time.
Upslope fog or hill fog forms when winds blow air up a slope (called orographic uplift). As the air temperature decreases with height, water vapour in the air will condense into small water droplets, forming fog over the mountain. Upslope fog usually begins from the foot or middle of the mountain and covers a wider area. It becomes sea of clouds when it accumulates over valleys. Orographic cloud or a cap cloud is more likely to form near the top of a peak.
On a sunny day, upslope fog will form over the mountains without any warning. This is because high grounds tend to heat up rapidly by strong sunshine during the day, resulting in an expansion of air and lower surface pressure over the region.
Air from foothill or regions with higher surface pressure will rise along the slope, forming upslope fog. We will see clouds above us from the foothills and encounter fog going up the mountain slope.
Upslope is often seen when warm moist air drifts over cooling land, and often seen when the skies are clear, which approaches late in the evening, overnight and before sunrise. You may have been out camping on a clear evening only to wake up in fog.
Evaporation fog is caused by cold air passing over warmer water or moist land. It often causes freezing fog, or sometimes types of frost. This most often occurs in the autumn over the sea when the water temperatures are still warm, and the cooling weather brings in cold air.
The moist air which has evaporated from the waters hits the cold air and condenses into fog. This often occurs here over the lochs especially as the cold air falls from the hillsides and mountains overnight.
Supercooled (freezing fog) contains supercooled water droplets - which remain liquid beyond freezing point. One of the characteristics of supercooled fog is that rime (feathery ice crystals) is formed on the windward side of vertical surfaces such as fence posts and grasses.
The process of starting off an ice crystal is called "nucleation." So in freezing fog there is an absence of impurities (condensation nuclei) in the droplets of water (which is also supersaturated) and therefore cannot form crystals. It’s pretty geeky but explains it well.
You can also see a number of atmospheric phenomena associate with fog like Brocken Spectre's and fog bows if you are lucky enough to have the clear skies behind you.
How can we predict it?
That is the tricky bit. But the best times to see the types of mist we enjoy capturing such as upslope fog, valley fog and radiation fog are probably the easiest. Clear overnight skies and low winds are the name of the game and keeping an eye on the weather forecasts for the saturation levels. Some forecasts will just give the humidity levels as a percentage (over 94% is a pretty good place to start).
There are plenty of mobile apps with various models and forecasts. Clear Outside (www.clearoutside.com) for example will also predict the wind speeds, direction, humidity, and dew points for a given area. Once the dew point meets the air temperature and the humidity is high then this usually results in the formation of mist ad fog in low laying areas. Where it forms however is down to experience (some areas are hot spots) and some of it is a case of traveling around to find it.
You can also use satellite imagery to see if fog is present. As it is low to the ground Infra-red imagery is next to useless as it has a similar signal to the ground. However, if you look at daytime imagery you can often see sea fog and hill fog as it’s pretty static. Whilst the upper-level clouds move through the time sequence the sea fog and hill fog stays in the same place and is fairly pale and homogenous with a clear boundary.
Why does fog evaporate?
Essentially most fog and mist disappear due to a change in the temperature, humidity or wind speeds. Upslope fog with vanish as soon as the wind drops, the air mass becomes dryer, or the sun heats up the land beneath. Radiation fog generally occurs in the blue hour and as the sun warms up the land beneath the moist air warms and rises and mixes with the warmer air above and evaporates. If the cloud cover increases with radiation fog it traps heat in and keeps the air nearer to the ground warmer.
The best time to capture fog and mist
There is no perfect time, but when the dynamic range of the scene is low, is it much easier than in high contrast situations. The blue hours are often perfect as the contrast, which is usually low, is enhanced by the white mists especially in radiation fog. At sunrise for instance the low light is also perfect as the dynamic range is still low, but mist and fog can take on the colours of its surrounding like red and golden light. Occasionally you get fog forming after rainfall, esp. here in Scotland, or also at sunset when it’s clear and the temperature drops quickly.
As soon as the sun gains a lot of height in the sky the intensity becomes so great that the contrasts become difficult to manage in-camera without bracketing or using filters. However, this often looks bleached and washed out or have non-complementary hues. Up in the mountains though these low clouds formation can still look stunning in broad daylight so it’s more of a trial and error as to which kind of scene you want to capture.
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