There is a lot of confused thought, urban myths and bovine excreta concerning weather effects and fishing. Mr Don't-Fish is forever cheerily advising anglers that it's "a grand day for fishing!" when the proverbial cats and dogs are bouncing off the pavement.

Rain, or any other form of precipitation, be it snow or hail, falls from clouds. Cloudy conditions generally create good fishing conditions, but there are other factors concerned with precipitation which can compromise the good effects. Rain is generally formed where masses of cold and warm air meet. Such areas are referred to as 'fronts', and fronts also tend to be areas where atmospheric pressure changes, often dramatically. We have discussed atmospheric pressure change and its negative effect on trout behaviour, so we are really not fond of the arrival of frontal weather.

Dramatic
Rain is a product of frontal weather, so not directly responsible for poor fishing, but the arrival of steady rain indicates that there will also be an associated change (usually a drop) in atmospheric pressure. The same can be said of snow and the drizzly sort of wetness that is often referred to as ‘Scotch Mist’ south of the border.

Here in the UK, the bulk of our rain comes from the Atlantic, therefore the south or the west, and is brought by low pressure systems. What generally happens is that the leading edge of an approaching low pressure system carries the bulk of the rain. The winds along the leading edge are normally from the south-east, and the descent of atmospheric pressure in this area of the weather system is dramatic. The rain ‘batters’ down, wind speed increases, the cloud lowers, temperatures fall, atmospheric pressure drops like a brick, and the trout go off the feed. As the weather system makes its journey east across the country, the winds tend to veer to the south-west and drop in speed, the pressure and temperatures start to rise again, the rain decreases, the cloud ceiling lifts and the trout come on the feed. With the continual easterly movement of the weather system, the wind continues to veer, now into the west, and the atmospheric pressure still rises. The rain often dies away at this point; the sun appearing as the clouds disintegrates. The above is a generalisation of course, but it serves to explain the link between precipitation, atmospheric pressure, and fish behaviour.

Turbulent air mass
So, in general, south-east wind tends to bring falling atmospheric pressure and air temperature, which is bad; south-west sees a rise in atmospheric pressure and temperature, which is good; west winds bring continued atmospheric pressure rise, less rain, but breaking cloud cover may bring out the sun which can negate all other good effects. Settled weather is caused by stable atmospheric pressure, and in such conditions precipitation is unusual; unsettled weather is almost always associated with varying amounts of rainfall and erratic atmospheric pressure. Rain and other forms of precipitation are only a reflection of atmospheric pressure, and it is atmospheric pressure which is the true predictor of fish behaviour.

To underline the point, sudden heavy downpours or hail are almost always associated with ‘thundery’ weather. Thunder storms are small, localised weather systems which contain very turbulent weather effects caused by sudden and extreme falls and rises in atmospheric pressure. It helps to understand a thunder storm if one considers it to be a minute low pressure system, with rapidly falling atmospheric pressure on its approach and dramatically rising pressure in its wake. The rain or hail is produced by the turbulent air mass trapped within this micro-system and is, again, simply a reflection of the rise and fall of atmospheric pressure. Such violent changes in atmospheric pressure can cause wild and unpredictable fish behaviour. Most of us will have seen occasions when fish have gone crazy in such circumstances, savaging any fly which hits the water, but more often than not the first hint of a rumble sees all fish activity cease, normal service being resumed only when the storm or storms are disappearing over the horizon.

Trout fishermen rarely experience snow, but salmon anglers in the spring months get very excited by the onset of snow as it generally heralds rising air temperatures which may be all that is required to make sulky salmon start to take confidently in the winter/spring months. During my Orkney years I saw the arrival of snow turning a hopeless March day into one to remember.

To sum up, precipitation has no direct affect on fish behaviour but the underlying cause (atmospheric pressure change) and associated effects (cloud cover/air temperature change) will. South-east winds with rain are virtually hopeless fishing conditions, but then so are south-east winds in general. South-west winds with or without rain are almost always good. Slight showers indicate relatively stable atmospheric pressure and should make no difference; heavy downpours are a sign of erratic and unstable atmospheric pressure and a termination of trout activity.

One last point: rainfall will oxygenate the water surface layers. All other considerations aside, the after-effects of rainfall should be to attract fish into the oxygen-enriched surface layers. But, as we have learnt, it is impossible to ignore the causes that create precipitation.


Rain stopped play: an awful day on Lough Sheelin meant the anglers fled to the trees for shelter.