Wombat and Nighttime Squalls, Explained

May 14, 2019

Day 221

Noon Position: 24 28N  59 12W

Course(t)/Speed(kts): NNW 6

Wind(t/tws): SSE 11

Sea(t/ft): SW 3

Sky: Squally; big thunderheads moving slowly

10ths Cloud Cover: 8

Bar(mb): 1022+

Cabin Temp(f): 86

Water Temp(f): 79

Relative Humidity(%): 60

Sail: Twin headsails poled out.

Noon-to-Noon Miles Made Good (nm): 60

Miles since departure: 29, 692

Avg. Miles/Day: 134

Leg North Miles: 6,764

Leg North Days: 55

Avg. Miles/Day: 123

Mo made two and three knots most of yesterday and lay becalmed soon after the sun went down. I put away the sails, had a glass of wine in the cockpit, and admired the stars. Especially one bright object on the horizon to the west, which I did not recognize.

It twinkled like a star, but for whole minutes it stayed right there and did not slip into the sea as it should. Then I saw the shimmer of green and then red. “Sailboat WOMBAT” said the chart plotter.

I can count the number of times I’ve seen a sailboat out here on the first two digits of one hand, so this was something to take note of, especially as her heading was a direct intercept.

For the longest time it was unclear if she saw Mo. I finished the wine (things happen slowly at sea), switched on the engine and motored due north for a time to create some room between us. Her course, about 60 degrees true, lay for the Strait of Gibraltar; her departure could only be the Caribbean.

In the dark I never saw more than her lights, white above, green and red below, and later, white aft. She did not call on the radio and neither did I. I still wonder why we both stayed mum.

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Some time back I mused over a strange weather phenomenon, namely squalls that, for a time, were forming in the night and dissipating with the day. For those unfamiliar with weather at sea, squalls are nothing more than what you would call thunder or cumulonimbus clouds. In this cloud, hot air is accumulating and rising as an organized mass; as it reaches altitude, it condenses and, if large enough, later rains some of its moisture back down to sea.

Note the root of accumulate, “to heap together” is the same as cumulus. Nimubus typically refers to the cloud’s rain potential.

During the day, the driver, that thing that heats things up and starts the convection process, is the sun.

But, lacking solar radiation, what on earth could create enough heat at night to form squall clouds, and having formed, how could they possibly dissipate when the sun arrived in the morning. I just seemed backwards.

Recently we received an answer, an email into the Figure 8 site.

It went like this…

“Randall,

“My name is Dennis Decker, and I’m a retired National Weather Service Meteorologist. I found your recent weather quandary interesting, and, as it’s a rainy day here in the mountains of North Carolina, I thought I would tackle your question about the diurnal tropical rainfall.

“A little reading reveals that it’s not an easily described process. One major component of night time tropical convection is the change in the vertical temperature profile. To put it simply, when it’s warmer than normal in the lower levels or cooler than normal in the upper levels, upward motion or convection will begin.

As you well know, the daily air temperature fluctuation over tropical oceans is very small even though there can be a lot of daily sunshine. That’s because the ocean absorbs most of the incoming solar energy.  So, to produce an unstable atmosphere there must be cooling in the mid to upper levels. This takes place when an upper cloud layer radiates energy into space. This causes the atmosphere at that level to cool and the temperature profile becomes favorable for convection. During the day the same upper region can absorb solar energy and warm and stabilize the atmosphere.

“These are very subtle changes in the atmosphere and can only be observed over the tropical oceans. Over continents or islands the solar heating of the surface overpowers these subtle affects and produces a daytime rainfall maximum, as does the presence of fronts or organize tropical systems or surface convergence.

“Hope this helps.

Dennis”

Hey Dennis, many thanks for taking the time to explain the mystery of nighttime squalls. Much appreciated, and yes, that make sense now.

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Today, found stuck to the side of the boat, the world’s smallest flying fish…