Cauliflowers in the sky and a shelf cloud on the leading edge. Why the Baltic breeds storms

A storm at sea looks different from one over land and obeys different physics. One variable decides it: the contrast between the temperature of the water and that of the air moving across it. This is why Baltic thunderstorms can occur in January, why waterspouts accompany them, and why for a yacht crew a spectacular sight on the horizon can be the last warning they get. Grzegorz Zawiślak, meteorologist at Gazeta Morska and a member of the Polish Storm Chasers, explains the mechanism.

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25 july 2026   |   12:36   |   Source: Gazeta Morska   |   Prepared by: Kamil Kusier   |   Print

fot. Grzegorz Zawiślak / PŁB

The engine is a temperature difference

A thunderstorm is, in simplified terms, a heat engine. It needs moisture, warmth near the surface and cold air above. The greater the difference between bottom and top, the more violently air rises and the more energy it has available. Over land that warmth usually comes from a sun-heated surface, which is why land storms peak in the afternoon and fade at night.

At sea the arrangement is inverted. Water has enormous heat capacity, warming and cooling slowly, so there is no daily rhythm dictating when a storm should occur. In summer and early autumn the Baltic is often warmer than the air moving over it, and it then becomes the source of heat and moisture for developing convection.

From this comes the first characteristic of Baltic storms, which Grzegorz Zawiślak describes as a rule.

- We record most Baltic thunderstorms during inflows of cold air masses over the relatively warm waters of the sea, says the Daily Mare meteorologist. - These are so-called advective storms, driven by a large vertical differentiation of air temperature in an environment of increased moisture content between the lower and middle troposphere.

Advection is simply the horizontal transport of an air mass. It requires neither sunshine nor a particular time of day. It is enough for the pressure pattern to move cold air from Scandinavia or the Atlantic over warmer water. A Baltic thunderstorm can therefore strike at three in the morning as readily as at three in the afternoon.

Storms that do not vanish in winter, only change form

The most important consequence of this mechanism is that the storm season at sea does not end in September.

- They most often take a discrete, that is dispersed character, and occur practically all year round, Zawiślak notes.

In the cold half of the year, he explains, during inflows of Arctic cold from northern Europe, these storms manifest themselves as intense showers of snow and graupel combined with strong gusts of wind.

For the recreational sailor this is a curiosity, since in January they are not there. For commercial shipping, fishing crews and rescue services it is a real and troublesome phenomenon, because a winter snow squall cell can cut visibility to a few dozen metres within minutes and add a wind gust on top. This happens on a sea carrying ferry and container traffic at the same time. Polish winter storm warnings for the Baltic regularly mention the possibility of thunder alongside snowfall and vessel icing.

The dispersed character of these storms means, in turn, that they do not form a coherent line that is easy to track on radar and avoid. Individual cells develop and decay at different points across the sea, which makes localised forecasting harder.

Cold fronts, the most dangerous variety

The second type of storm at sea has an entirely different origin and a different order of hazard.

- Frontal storms also appear, associated with dynamic exchanges of air masses, Zawiślak explains. - Storm formations are most often observed on cold atmospheric fronts, when warm, hot streams of air are displaced by rapidly advancing cooling.

Then comes the assessment that, for a reader planning a passage, is the single most important sentence in the material: "Storms of this type display the greatest intensity, bringing not only torrential rainfall but also localised hail with strong gusts of wind."

The practical difference between the two types is fundamental. An advective storm is usually a single cell that can be worked around and that delivers a quarter of an hour of unpleasantness. A cold front is a structure often hundreds of kilometres long, moving at several tens of kilometres per hour, which on the Baltic simply cannot be avoided. Here the decision is taken before leaving harbour, not during the passage.

The shelf cloud, or the last warning

There is, however, a sign that arrives before the wind, and Zawiślak mentions it directly.

- Many storm formations are preceded by a characteristic shelf cloud at the leading edge, heralding a sudden strike of wind, he says.

A shelf cloud is a low, horizontally arranged cloud with a sharply drawn, often ragged edge, moving ahead of the storm cloud proper. It forms where cold air flowing out of the base of the storm meets the warm air that same storm is drawing in. It is not decoration but the physical marking of the leading edge of the downdraught.

For a crew this means one thing. The wind striking beneath the shelf arrives vertically and spreads horizontally, so it changes direction abruptly rather than gradually. Between the moment the shelf is clearly visible above the horizon and the arrival of the squall there is usually a quarter of an hour, sometimes less. That is time for reefing and securing the deck, not for photography.

Waterspouts, a Baltic speciality

Zawiślak points out that waterspouts can accompany any of the storm types described, particularly once the sea has warmed sufficiently. In recent years the phenomenon has become a recognisable feature of the Polish coast.

A waterspout is a vortex of air connecting the base of a cloud to the sea surface. In Polish conditions these are most often of the non-tornadic variety, considerably weaker and shorter-lived than a classic tornado, forming precisely when cooler air moves over warmer water. The diameter of the funnel rarely exceeds thirty metres, and most Baltic formations fall between F0 and F1 on the Fujita scale, corresponding to winds of roughly 90 to 150 kilometres per hour. On reaching land they usually lose energy quickly and dissipate.

Across Europe some 160 waterspouts are recorded annually, and the southern Baltic ranks among the areas particularly prone to them. Poland sees several a season, and has done for more than a century. What has changed is not the frequency but the number of cameras pointed at the sky. Exceptional episodes do occur, however. Between 21 and 23 August 2025, more than a dozen waterspouts were observed off the Polish and Danish coasts, eight of them forming simultaneously within a single storm zone.

For someone on the beach these are essentially harmless, since they form over water and break up over land. For the crew of a small vessel the calculation is entirely different. Wind of around a hundred kilometres per hour will capsize a boat regardless of how narrow the funnel is or how well it photographs.

Beauty that is simultaneously a warning

Zawiślak's material is unusual in that it makes no attempt to conceal fascination with its subject while ending in an unambiguous warning. Both are true at once and worth setting side by side.

- Coastal storms delight with their imposing contrasts across sky and water, particularly when the dark, navy shades of heavily saturated clouds are lit by the rays of the sun, the meteorologist said.

He also draws attention to acoustics, which behave differently over water than over land:

The sound of resonant thunder on open waters, where there are no obstacles to the free propagation of a powerful acoustic wave, carries for tens of kilometres.

That last observation has a practical dimension despite its lyrical phrasing. Over land, thunder is typically audible within a radius of a dozen or so kilometres, damped by buildings, forest and terrain. Over open water the same thunder carries considerably further, meaning that a storm which can be heard may still be beyond visual range. Unlike on land, hearing at sea warns earlier than sight.

The author's conclusion is free of illusions.

- Like every phenomenon associated with active convective processes they nonetheless constitute a serious hazard, especially for maritime navigation and for everyone in the water and on the beach, which is why it is so important to continuously observe the sky and to follow current forecasts and meteorological warnings.

Where to find the warnings

Forecasts and warnings for Polish sea areas are issued by the Central Office of Maritime Meteorological Forecasts of the Institute of Meteorology and Water Management in Gdynia, with the full set of material, including wave, sea level and water temperature forecasts, available at baltyk.imgw.pl.

The Baltic is divided into forecast zones: Western, Southern, South-Eastern, Central and Northern, of which the first three cover Polish waters. Warnings are issued separately for sea zones and for the coastal strip, which can mislead, since an alert for the coast does not necessarily imply a warning for open water, or the reverse.

The thresholds are worth remembering. In Polish terminology strong wind means force 6 to 7 on the Beaufort scale, while a storm warning covers force 8 to 12. English-language bulletins divide this into a strong wind warning for force 6 to 7, a gale warning for 8 to 9 and a storm warning for 10 to 12. NAVTEX receivers, increasingly common aboard yachts, deliver bulletins and warnings automatically without the need to monitor any service.

None of these tools replaces the observation the author returns to. A meteorological warning covers a zone measured in thousands of square kilometres, while a single storm cell is a few kilometres across. Whether it passes over you or beside you is not something a forecast will tell you. The sky will.

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