Cloud watching in paradise
Setting the scene for spectacular sunsets
Cloud watching in paradise
Setting the scene for spectacular sunsets
By Ralph Deans
Of all the fine things that bring visitors to The Bahamas, warm, sunny weather must rank at the top. When chilly winds blow elsewhere, it’s natural to pine for summertime activities like swimming, boating and idly gazing up at the clouds, picking out the outlines of animals, ships and vistas.
If you have just arrived in paradise by air you will have “looked at clouds from both sides now, from up and down,” as songwriter Joni Mitchell wrote in her famous single from 1969.
It would be most unlikely not to see some clouds from a commercial aircraft at 39,000 ft because, according to scientists at NASA, clouds cover 70 per cent of the Earth’s surface at any given time.
That may be hard to believe when you are visiting The Bahamas during our dry season, November to April, when the warm winter sun often beams down from a cloudless sky.
The Bahamas enjoys more than 315 bright sunny days every year. There’s more than 10 hours of sunlight a day in late December and nearly 14 in June. That’s not to say it never rains. It surely does, especially from May to October–our so-called rainy season.
But even then, rainfall averages only six inches a month, mainly in the form of heavy showers or thundershowers that clear quickly. For the rest of the year the average is closer to two inches a month.
Two different weather systems give rise to two different types of clouds in the islands, according to Basil Dean, deputy director of The Bahamas Meteorology Department.
“We really enjoy only two seasons in The Bahamas,” says Dean. During the warmer months, clouds are formed by thermal currents arising from hot spots on land during the day and–since water retains heat longer than the earth–over the ocean at night.
In winter, Canadian cold fronts may move in, displacing our warm subtropical air, lifting it and its moisture into the sky. This produces low-lying stratus clouds, one of the commonest type seen around the world, looking like sheets or layers.
“You will see these clouds only in the winter months after a frontal passage,” says Dean. “The whole sky may be covered with these smooth clouds, flat on the bottom and almost flat on top.”
Even these nondescript clouds become more conspicuous at dawn and dusk when the sun casts colour on them. Anyone with a view of the western horizon is treated almost every evening to a display of colours–from pinks and corals to orange and fiery reds, often streaked with strands of charcoal gray.
In fact, some homes in The Bahamas have a “sundowner”–a deck on the roof where owners can watch nature’s end-of-the-day show with a beverage of the same name in hand.
On rare occasions, sunset watchers in The Bahamas may see a ghostly green flash a moment after the sun sinks below the horizon.
It was once thought that this burst of colour was caused by yellow rays mixing with the blue waters of the ocean to produce green. Not so. The flash, which can also be seen in a desert, is caused by the brief refraction of the green portion of the spectrum through the atmosphere after the camouflaging reds have disappeared.
Castles and anvils
In summer, clouds have a flat bottom but are puffy at the top, “because of the vertical currents that are moving up and down within the cloud,” says Dean.
“Clouds are very dynamic. Water particles are always moving through the clouds and as they move about, they [coalesce] and when these droplets become too heavy … they fall out of the clouds as rain.”
These clouds may look different from those seen in more northern climes, especially the stand-alone pillars that float over the sea like airborne castles.
Dean recalls that his father used these clouds to give him his first lesson in meteorology when he was still a young boy. Father and son had gone fishing.
“Our compass wasn’t working and when we started to lose land … I started to become concerned and my dad said ‘don’t worry about it.’
“We fished, we had a good day and then it was time to head back and I said, ‘Daddy, what you ga do now?’ And he said, ‘You see that patch of tall clouds over there? You just head there and we’ll find land somewhere over there.”’
When he began studying cloud physics, Dean learned what his father and all Bahamian fishermen know from experience–that these clouds form “during the day over land, during night, over the ocean.”
They appear late in the morning or early in the afternoon–after the sun has warmed the earth enough to generate convection. Most of them dissipate, but if they grow, they become “towering cumulus” and eventually the king of clouds, cumulonimbus incus–a mountain of moisture that brings torrential downpours, lightning and bombs of thunder.
Most cumulonimbus stop growing at about 20,000 ft and have a puffy top. But they can boil up to an altitude of more than 60,000 ft–double the height of Mt Everest– where the top of the cloud flattens out into an anvil shape. [Cumulonimbus incus is derived from the Latin words for “heap” “dark cloud” and “anvil.”]
Scientists say the anvil begins to take shape at the tropopause–the boundary between the troposphere, the bottom layer of earth’s atmosphere and the stratosphere, where the temperature may be -40ºC. Cloud physicists call the tropopause a “cold trap” because it stops the ascent of moist air into the relatively warmer air of the stratosphere. Sometimes, however, a parcel of moisture, driven by a strong updraft, may penetrate the lower stratosphere before falling back.
“These are dangerous clouds for aviation,” says Dean. “Pilots don’t fly into them because there are very strong up drafts and down drafts. There is always air rising and when it reaches the top of the cloud its moisture becomes ice crystals.”
When that happens, the cooled air descends quickly and exits the base of the cloud. “You hear about gusty winds. That’s where gusts come from.
“When you see a well-developed anvil shape you know the cell is in a decaying stage and it’s soon to dissipate. The life span is about an hour. When lightning and thundering go on for hours it’s not coming from one cell. You have cells developing and decaying all the time.”
As well as these infrequent thunderstorms in summer, The Bahamas is subject to tropical waves, areas of low pressure that come out of Africa and usually move west and then north across the Atlantic.
“If conditions are favourable– whether at the surface or at the upper levels of the atmosphere –these waves can develop into tropical depressions,” says Dean.
The final stage is a hurricane which can form anywhere in the Atlantic or the Gulf of Mexico.
Dean is well known throughout The Bahamas for tirelessly tracking hurricanes and broadcasting alerts and advice when they threaten.
Scientists, like poets and songwriters, have always been fascinated by clouds, but they would be the first to admit that there is still much to learn. Along with Joni Mitchell, they might say: “It’s cloud’s illusions I recall. I really don’t know clouds at all.”
Except for one memorable occasion, The Bahamas has never experienced frost, snow or sleet.
The exception occurred in the super-cold winter of 1977 when a cold front from Alaska swept across the United States, dumping snow as far south as Miami. On January 19th, it also brought flurries to the northern Bahamas. That was so unusual that Bahamians are still talking about it 35 years later.
Generally, in both Nassau and Freeport, winter temperatures hardly ever fall below 15ºC (60°F) and usually reach a comfortable 24ºC (75°F) in the afternoon. Surface waters around the islands vary between a swimming temp of 23ºC (74°F) in February to 28ºC (83°F) in August.
I am the daughter of Earth and Water
And the nursling of the Sky;
I pass through the pores of the ocean and shores
I change, but I cannot die.
For after the rain with never a strain
The pavilion of Heaven is Gale,
And the winds and sunbeams with their convex gleams
Build up the blue dome of air,
While I gently laugh at my own cenotaph,
And out of the caverns of rain,
Like a child from the womb, like a ghost from the tomb
I arise and unbuild again.
From The Cloud, a poem by Percy Bysshe Shelley
Clouds are the arbiters of the Earth’s energy budget. They are so plentiful and widespread … [T]hey play a major role in governing how much sunlight reaches the surface, how much sunlight is reflected back up to space, how and where warmth is spread around the globe, and how much heat escapes from the surface and atmosphere back into space. In short, clouds are a key component of Earth’s climate system.
David Herring, Climate Program Office, National Oceanic and Atmospheric Administration.
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