The increased formation of dew on boats in Croatia this summer is an interesting but easily explainable meteorological phenomenon – not unusual, but rather the result of a particular weather situation. SeaHelp has looked into the most important factors and explanations.
Many boaters who have been out and about in Croatia this summer have reported unusually heavy dew formation on their boats. What is behind this phenomenon? Dew always forms when the surface temperature falls below the dew point of the air, there is sufficient humidity, and it is clear and windless – this allows the surface to cool down significantly (known as nighttime radiation).
This summer, however, the air and water are cooler (around 24°C), especially in the evening hours, than in previous years, and the nights are clearer and calmer, causing the boats to cool down more. The temperature difference to the humid air is then often sufficient to reach the dew point, and dew forms.
The approximate equality of air and water temperatures has more of an indirect influence on increased dew formation
Many water sports enthusiasts wonder whether the approximate equality of air and water temperatures can have an influence on increased dew formation. The answer is yes, but only indirectly. This is because when water and air have a similar temperature (around 24°C), the typical “warming effect” of the sea at night is missing.
Consequence: The air cools down more quickly, not least because less heat is stored in the water. When the surfaces of the boats then cool down further due to nighttime radiation, the temperature drops slightly below the dew point and dew forms.
There is also a conclusive answer to the question of why there has been less dew in recent years: the summers have simply been warmer overall, even at night. And fewer clear nights mean less radiation and therefore less surface cooling. In addition, the humidity was lower, at least in some areas, or it was simply too hot to reach the dew point. This year, the conditions are more ideal for dew: cool, humid, clear skies.
Are the dewdrops really bigger this year than in previous seasons?
Some water sports enthusiasts also feel that the dewdrops are bigger this year than in previous seasons. This is entirely possible – but it is not due to increased surface tension of the water. Explanation: the surface tension of water is a constant material property (~72 mN/m at 20 °C) – it does not change significantly due to weather.
Larger drops, on the other hand, form during slow but steady condensation, on smoother or hydrophobic surfaces (e.g., fresh paint), in light winds (the drops do not “run”), and during periods of high relative humidity over a longer period of time.
Conclusion: The increased dew formation this summer can be explained by physical phenomena. Dew forms more frequently because this summer has cooler nights, high humidity, and clear conditions. Larger drops are not a sign of a change in water chemistry, but simply of favorable condensation conditions. Finally, the similarity between air and water temperatures contributes to the air not cooling down significantly, but the surfaces do—thus, the dew point is reached more quickly.
