ing outside the bay, but there was no certainty about what would happen next.
As it turns out, we had plenty of wind our first day, forecasts notwithstanding. Savai'i is about 40 miles long, with a huge, round-shouldered spine of volcanoes—last active in 1911—rising to 6,000 feet and filling the center of the island. This land feature enhanced the sea-breeze effect by creating anabatic winds when the sun on the higher elevations heated the surrounding air, caused it to rise, and air was pulled in from the sea to replace it. These are the reverse of the nighttime katabatic winds, which form as cooling air sinks down a mountain's flanks. The effect was to add to the flow of rising air from the flat land along the coast, thus requiring more air to flow in from farther offshore. The effect of the sea breeze was thus felt even as we drew away from the coast.
By noon, the island had also generated a cloud street, a train of cumulus clouds that looked like the plume flowing downwind of an active volcano. The northeast wind gave us a slight reaching angle, and it was a lovely day for a sail. I'd encountered cloud streets before, and I was curious now to see just how far out this one extended, so I held our course a few degrees from the rhumb line to stay on its north side. Six miles from Savai'i, a break in the line of clouds tempted us to duck through, but I still couldn't discern the end of the street, so we kept on.
As the afternoon waned, the wind went even more northerly, and the seas grew lumpy. Showers appeared along the cloud street. We let our passage drift down toward the rhumb line until we came
Ambient: Of the surrounding area or environment. Anabatic wind: Cool air becomes heated along a slope and rises. This is known as a valley breeze or an anabatic wind. Katabatic wind: A wind that's created by air flowing downhill.
Source: National Weather Service Glossary (www.weather.gov/glossary/)
under the clouds themselves, and suddenly the wind blew out of the south. Thunder began grumbling, a constant sound. Finally, it moved off to the east, leaving haze, long sheets of rain, and virga; a low sun shining through made everything silver. At 2000, the thunderstorms were all around again.
At this point, we were 35 miles beyond the tip of Savai'i, and the satellite photo showed no significant weather in our vicinity. We'd never found the end of the cloud street; instead, it had just dribbled into showers and squalls. On the chance that the line of squally stuff was narrow and that the south side would be more stable, at a lull when we could distinguish the dark edge of a shower ahead and one still behind, we jibed to port and ran southward to cross between the two weather cells. For a quarter of an hour, Oddly Enough did 7 knots on a broad reach; the seas were as smooth as a dinner plate. The wind eased, and the sailboat started rolling again, but suddenly the thunder and now-visible lightning were to the north of
us! A steady downpour lasted 15 minutes, followed by showers. It felt as if the tension had gone out of the air. I made sure we stayed south of the cloud line, and the weather improved as we moved out of the electrical storms and, eventually, out of the showers.
At dawn the air was hazy and muggy, and fluffy, moisture-laden cumulus clouds remained all day, but any sign of the cloud street was gone.
The wind, meanwhile, had died off, its speed coming more into line with the 6 to 8 knots we'd expected from the forecasts. The rest of the passage was anticlimactic.
Looking back at the previous day's conditions, our area of disturbed weather wasn't large enough to show on satellite photos and maps. Although the cloud street itself would no longer have existed 35 miles from Savai'i, it's very possible, and in fact likely, that the island was still affecting our weather, evidence of another mesoscale event.
The cloud street was, in fact, a visual manifestation of a disturbance in the air stream caused by the volcanic ridge atop Savai'i. The wind that hit Savai'i on its eastern side had been forced to rise by the flank of the volcano. When it reached the mountaintop, the air had cooled enough to form a cloud, as the significant amount of tropical moisture in the air condensed.
As the airflow continued west, it reached the western flank of the mountain and sank, adding, incidentally, to the effect of the breeze coming from Savai'i's western end. The airflow that rose up to accommodate the mountain didn't simply resume its previous course; instead, the hump that was created by the mountain propagated itself downstream as a series of bumps, or waves. In the language of meteorology, the wave resulted as the air stream tended to seek equilibrium.
As the air sank down the mountain, it became warmer, and the water evaporated; the cloud disappeared. When the air rose in the next wave, it again condensed, and a cloud formed. The air sank, the water evaporated, and the cloud disappeared again. Neither the wave pattern nor the clouds moved, only the air. In this fashion, the pattern of clouds and the disturbance they represented was carried downstream, dissipating gradually with distance. The convection caused by rising air currents and the gustiness caused by sinking air could be felt many miles away.
When we left for the Wallis islands, we headed downwind of the island, and so experienced the disturbed air as far out as it extended.
Tracking the effects of such local weather as Savai'i's sea breeze and the atmospheric wave generated by the volcano is difficult for the meteorologist; it's going to be next to impossible for cruising amateur weather observers looking for clues on large-scale maps. But an understanding of how the land mass we're leaving may affect local winds and convection currents can predict what we'll encounter on our first day of a passage.
Ann Hoffner and her husband, photographer Tom Bailey, have finished a cruise of the Kimber-ley region of northwestern Australia and are back in Darwin deciding where to go when the monsoon season ends.
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Lets start by identifying what exactly certain boats are. Sometimes the terminology can get lost on beginners, so well look at some of the most common boats and what theyre called. These boats are exactly what the name implies. They are meant to be used for fishing. Most fishing boats are powered by outboard motors, and many also have a trolling motor mounted on the bow. Bass boats can be made of aluminium or fibreglass.