UNDERSTANDING FOG AND HOW IT AFFECTS FISHING

Text: Alan Bulmer                              Images: Mark Hoffman and Alan Ang

Several times recently I’ve ventured out onto the flats at dawn and encountered foggy conditions. On the first occasion it was a thick “pea souper” with low visibility and two of us only hooked one fish in the entire session. Interestingly the fish took as the fog was finally starting to lift but it was an isolated incident as the fog soon returned with a vengeance and after that nothing else took our offerings.

Fog - swirling

On the second occasion there was no fog present when we ventured out and fish were feeding actively all along the shore line. We caught a couple of fish almost immediately and thought that it was going to be one of those days where the fish took almost anything cast in front of them.  However, suddenly a blanket of fog appeared and engulfed us within minutes. As soon as this happened the fish stopped feeding and did not re-appear. A promising start spoiled completely by the arrival of fog.

This has happened to me many times before and in my experience if fog is either present at the start of a session then fishing is poor or it arrives during a session then the fish will stop feeding, especially in autumn and winter.

This is also the case in Scotland, where salmon fishing does not start if a thick sea fog called a “haar” creeps menacingly inland. Experienced anglers and ghillies will not venture out until the fog lifts as they know that there is minimal fish activity during a haar.

However, this pattern does not seem to be the case elsewhere. For example in the USA fog is often linked to excellent fishing. Ed Mitchell in his book Fly Rodding Estuaries (ISBN 0-817-2807-2) states “foggy days are usually windless, and as on calm nights, fish commonly come to the surface. Granted, the fog makes it much harder to navigate a boat or even navigate on the beach, but if you can do it safely, it is well worth it. Look for swirls, wakes, fins slicing the surface and busting fish. As with calm nights, try working flies right on the top. They should be deadly. Oddly enough, foggy days can also hold opportunities for sight fishing. Yes, fog reduces the light levels considerably, but it also produces a very uniform, diffuse type of soft illumination, which results in little or no glare off the water. Coupled with the calm conditions, it may permit you to see cruising fish in shallow water and this really cranks up the fishing”.

Fog - background

Why is fog fishing excellent in some places and poor in others? The answer, as always, is in the detail and is probably quite simple. Fog is not the same the world over and there are several different types of fog. Fish behaviour is likely linked to the fog type so it is important for anglers to understand the conditions which cause each type.

WHAT IS FOG AND WHAT CAUSES THE DIFFERENT TYPES OF FOG?

Fog is caused by tiny water droplets suspended in the air. The thickest fogs tend to occur in industrial areas where there are many pollution particles in the air on which water droplets can grow.

Fog is classified according to visibility. The term “Aviation fog” means the visibility is officially less than 1,000 metres. “Thick fog” is a term used for the general public and motorists and it means the upper limit of visibility is only 180 metres. “Dense fog” is a term used to describe conditions where visibility falls below 50 metres.

Fogs which are composed mainly or entirely of water droplets are generally classified according to the physical process which produces saturation or near saturation of the air. The main types of fog are:-

RADIATION FOG

Radiation fog usually occurs in the winter, aided by clear skies and calm conditions. The cooling of land overnight by thermal radiation cools the air close to the surface. This reduces the ability of the air to hold moisture, allowing condensation and fog to occur. Radiation fogs usually dissipate soon after sunrise as the ground warms. An exception to this can be in high elevation areas where the sun has little influence in heating the surface.

VALLEY FOG

Valley fog forms where cold dense air settles into the lower parts of a valley condensing and forming fog. It is often the result of temperature inversion with warmer air passing over the valley. Valley fog is confined by local topography and can last for several days in calm conditions during the winter.

ADVECTION FOG

Advection fog occurs when moist air passes over a cool surface and is cooled. A common example of this is when a warm front passes over an area with snow cover. It is also common at sea when moist tropical air moves over cooler waters. If the wind blows in the right direction then sea fog can be transported over coastal land areas.

UPSLOPE FOG

Upslope fog or hill fog forms when winds blow air up a slope (called orographic uplift). The air cools as it rises, allowing moisture in it to condense.

EVAPORATION FOG

Evaporation fog is caused by cold air passing over warmer water or moist land. It often causes freezing fog and sometimes frost. When some of the relatively warm water evaporates into low air layers, it warms the air causing it to rise and mix with the cooler air that has passed over the surface. The warm, moist air cools as it mixes with the colder air, allowing condensation and fog to occur.

Evaporation fog can be one of the most localised forms of fog. It can happen when:-

  • Cold air moves over heated outdoor swimming pools or hot tubs, where steam fog easily forms.
  • Cold fronts or cool air masses move over warm seas. This often occurs in autumn when sea temperatures are still relatively warm after the summer, but the air is already starting to cool.

FREEZING FOG

Freezing fog is composed of supercooled water droplets which remain liquid even though the temperature is below freezing point. One of the characteristics of freezing fog is that rime – composed of feathery ice crystals – is deposited on the windward side of vertical surfaces such as lamp posts, fence posts, overhead wires and pylons.

image

To simplify matters, fog is often just classified as land fog or sea fog depending on where it has formed.

Land fog is most common in New Zealand. For land fog to form the air must be moist. The sky by night must be comparatively clear of cloud, for the land cools by radiating heat into space and this radiation and consequent ground cooling is inhibited by skies more than half covered by cloud. Light winds must be present in order to mix the condensation in the surface layer. Too little wind and the condensation lies on the ground as dew. If the wind is too strong the fog is dispersed or lifted from the ground surface. Fairly specific values can be placed on these conditions:-

  • Relative humidity greater than 80%
  • Skies less than half covered by cloud
  • Wind speed between 3 – 9 knots

Look for the bright sunny day, clear of cloud and almost calm, yet clothes on the line do not dry, lawns remain wet. This indicates moist air.  Before dawn, night cooling under clear skies may produce fog. If on such a day fog forms overnight the likelihood is that fog will occur again night after night so long as the conditions by day are unchanged.

Seasonally late Spring to Autumn is he most prevalent time of the year. Geographically river valleys, coasts and marsh lands are the most likely areas. Once fog is formed how soon will it disperse? As a rough rule:-

  • The earlier the fog forms the later it is likely to disperse.
  • If fog has not dispersed by mid-afternoon then it is unlikely to do so that day.
  • Fog is often most dense just before dispersal because maximum mixing of the fog cloud occurs when the air just begins to rise from the surface.
  • An increase in wind strength or the onset of rain will aid dispersal.

Sea fog, or advection fog as it is also known, is caused by warm moist air passing over cold sea. If the sea temperature is low enough to cause condensation of the moisture in the lower layer of the air, fog will result.

As with land fog, wind is also a requirement in order to mix the saturated air in the surface layer. However at sea fog can occur with wind speeds up to 20 knots. Winds stronger than this will lift the fog to 200 – 300 metres from the water surface giving a layer of very low cloud.

Once formed sea fog tends to be very persistent. It remains unchanged by day because the temperature of the sea, unlike that of the land, remains unaltered between day and night. Sea fog normally only disperses when there is a change to drier or colder air – usually signalled by a change in wind direction bringing air from a different source. Wind strength above 20 knots will lift it and rain will thin it.

WHEN IS IT BEST TO FISH IN FOG?

From my experience, if the air temperature is cooler than the water when fishing in fog or the temperature drops when the fog arrives then this seems to discourage fish from feeding. This may be due to the fact that the surface water temperature starts to drop which constrains the feeding activity of bait fish.

Conversely if the air is warmer than the water in foggy conditions and the temperature continues to rise when the fog arrives then this seems to encourage fish to feed. Conditions are generally still and the fog cover reduces the amount of sunlight reaching the water which favours predators.

If the fog is a radiation or evaporation type then it definitely pays to wait around until the fog lifts as this often signals an increase in fish activity as again the air temperature is increasing.

If the fog is an advection fog (sea fog) and it has been hanging around for a while then it is probably not worth going fishing until there is a forecast increase in wind strength sufficiently strong enough to shift it (refer back to the rough rules above).

Fog - Spinning

Fishing is not an exact science so don’t take the statements above as gospel. They are a guide only. I still venture out in sea fog and every now and again I get lucky even though logic says the fish will not be feeding.

REFERENCES

  1. Weather for New Zealand Sailors by Lt. Cdr. K.E. Brierley ISBN 0-86481-140-3.
  2. UK Meteorological  Office (http://www.metoffice.gov.uk/learning/fog)

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