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Lindsey: 'Dew on boats in the morning, sailors take warning' actually checks out
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Lindsey: 'Dew on boats in the morning, sailors take warning' actually checks out

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Gene Doughty, who works on the water in Morro Bay, asked an insightful question. “The old-time fishermen tell me that when we see the morning’s dew on our boats, there is a northwesterly gale on the way.” Why is that? 

For those that work on the sea, the threat of gale-force winds, which can generate large seas, is a constant concern. Out of necessity, before modern weather forecasting was available, sailors came up with sayings or proverbs such as, “Distant shores loom up nearer before the rain” or “Rain is most frequent at the turn of the tide.” Many of these sayings have proven mostly untrue over the years. However, some are legitimate such as, “Dew on boats in the morning, sailors take warning” and here is why. 

Like the tides, the ebb and flow of the low marine clouds along the California coastline is reassuringly timeless. At times, the coastal clouds will surge into the inland valleys and relieve the summer’s heat. At other times “May Gray” or “June Gloom” can persist along the coast for days on end.

If the skies are overcast, the clouds act like a blanket keeping the temperatures warmer. On the other hand, clear nights will allow more of the atmosphere’s heat at the surface to radiate out in space, and this is key in answering the question about the morning dew on boats.

You see, on these clear nights, boats, cars and other things near the Earth’s surface cool more rapidly than the air surrounding them by emitting infrared radiation at a faster rate into space.

As the moisture-laden northwesterly winds come off the Pacific Ocean, this relatively warmer air comes in direct contact with these colder surfaces, like boats or blades of grass; the air cools to its dew-point temperature and condenses on these objects and dew forms. Think of a cold glass of iced tea on a warm day. The water vapor in the air condenses on the outside of the cold drink.

The dew-point temperature is the temperature at which air must be cooled for it to become saturated. At that point, the air can no longer hold its water vapor, some of which condenses into water, as dew or clouds. Dew point is simply the temperature when dew forms. Usually, the air temperature can never be colder than its dew-point temperature. When the dew-point temperature and air temperature are the same, the relative humidity is at 100%. 

Along the coastal regions of the Central Coast, dew-point temperatures usually average in the 40s during the winter and the 50s in the summer.

If the northwesterly (onshore) winds are strong enough during the overnight, they can mix out the temperature inversion/marine layer leaving behind clear skies. Consequently, morning dew can develop on the boats.

As the day goes on, the Central Valley warms, producing a deeper thermal trough; this is a predominant weather feature during the spring and summer. This condition generates a deeper trough of low pressure and creates a stronger pressure gradient between the waters off our coast and the interior, producing stronger northwesterly (onshore) winds.

On days like this, the northwesterly winds can increase from fresh to strong (19 to 31 mph) levels in the morning to moderate gale-force to fresh gale-force (32 to 46 mph) levels by the afternoon, putting boaters in harm’s way. So yes, this old fisherman hypothesis often checks out.

John Lindsey is Pacific Gas and Electric Co.’s Diablo Canyon Power Plant marine meteorologist and a media relations representative. Email him at pgeweather@pge.com or follow him on Twitter @PGE_John.

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