Sea Science: Don’t let a dewy morning catch you with your chamois down

Aug 15, 2019 by Jordanna Sheermohamed

Sea Science: by Jordanna Sheermohamed

Dewy mornings may conjure up memories of early adventures for some, but many a yacht crew member sees the immediate need to remove moisture. 

It all starts with understanding the different phases of matter, and how it  changes from solid to liquid to gas and back. Condensation occurs when matter changes from the gas phase to the liquid phase – think water droplets forming on a cold beverage glass while outdoors. Similarly, when humid air comes in contact with cooler temperature, condensation occurs. This temperature is also known as the dew point temperature. 

The dew point can be calculated by knowing the relative humidity and temperature of the air. Relative humidity of 100% means the air temperature and dew point temperature are equal. A 10% drop in relative humidity, down to 90%, means the dew point temperature drops 3 degrees Fahrenheit lower. So the air temp would have to continue to drop to match that level in order for condensation to occur. The general rule of thumb is that for every 10 percent lower humidity, the dew point drops 3 degrees Fahrenheit.

Now, this cooler temperature can be an air mass ahead of the humid air or even the surface of an object – your frosty lemonade, the sunglasses that fog up when you walk out of a cold building, or a yacht. 

Dew can also form when the surface temperature cools even lower than the existing dew point temperature. This can occur overnight as the day’s shortwave radiation is radiated back into the atmosphere as longwave radiation, essentially forcing condensation to occur and become morning dew.

Some of the more favorable weather elements to look for include: 

Clear skies, which maximize that longwave radiation being emitted. Cloudy skies would help to reflect some of that longwave radiation right back to Earth’s surface. That makes it hard for the surface temperature to drop below that necessary dew point temperature.

Light winds help to keep the moisture at the Earth’s surface from interacting with drier air higher in the atmosphere. Heavy winds also help in the evaporation process (when matter goes from liquid to gas – think of how a cool breeze helps keep a body dry on a sweaty day).

Moist air at the surface, which is 100% guaranteed in the ocean. As that moist air surface air cools, condensation is likely, provided that temperature drops to the dew point.

In addition, a high level of humidity in the atmosphere can combine with residual salts on the surface of a yacht, allowing the salt to absorb that moisture from the atmosphere and re-create saltwater. This can increase corrosion on metals and also has the ability to permanently stain windows. Salt allowed to dry into crystals is also the equivalent of  fine sandpaper and is quite abrasive. 

Just as dew forms when surface temperatures cool, the air hovering just above the ground can also cool at or below the dew point temperature. When that occurs, that air undergoes condensation and creates fog, and can merge with salt crystals to re-create saltwater.

A basic understanding of the science coupled with knowledge of local weather data can aid in better preparing for the potential of a dewy day – and even more importantly, the need for clean chamois and enthusiastic crew.

Jordanna Sheermohamed is president and lead meteorologist of Weather Forecast Solutions, a weather-forecasting firm ( Comments are welcome below.


About Jordanna Sheermohamed

Jordanna Sheermohamed is president and lead meteorologist of Weather Forecast Solutions, a private weather-forecasting company ( Comments are welcome at

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