The trade wind flow is very light and readily deflected by the many mountain chains and valleys which characterize continental Southeast Asia. Precipitation and cloudiness is highest on the exposed windward side of the terrain, particularly on the east slopes of the coastal mountains of Vietnam, that first intercept the ocean wind flow. The mountain chain that extends the length of Myanmar and forms the backbone of the Malay Peninsula is affected in the same way. In the protected interior of Cambodia and Thailand, however, sunshine is a little more abundant.
Air masses across Southeast Asia are very unsettled and showers and thundershowers grow at the least provocation. It may take little more than two wind flows that collide, a dark surface that collects a little more heat from the Sun, or a range of hills, to set off the buildups. Nearly every afternoon is dotted with clouds, and nights are humid. As in India, the westerly jet also carries upper disturbances into Southeast Asia, each bringing its own retinue of cloud and rain. These systems tend to pass to the north of the eclipse track, except over Myanmar, but are variable enough that they contribute substantially to the cloud cover along the shadow's path.
Heading southeastward past Vietnam, the track encounters the Intertropical Convergence Zone (ITCZ), where the winds from the northerly trades converge with those of the retreating southerly monsoon. This boundary is usually found lurking over the Malay Peninsula, past the northern tip of Borneo and into the Pacific along the 8th parallel. It is not a well-defined boundary, but rather a diffuse area of showers and thunderstorms that wonders about its average position. The land masses of Indochina distort the wind flows that create the ITCZ, and it becomes very broad and undefined between the Bay of Bengal and New Guinea.
Many of the thunderstorms that develop along the ITCZ are part of organized clusters (Figure 9) that throw up large and solid cloud umbrellas. They will almost certainly make eclipse viewing impossible should they arrive on the critical day, and because of their size, will be difficult to outrun. Over the Bay of Bengal, about 2 to 3 of these weather systems can be expected during the month. This grows to 4 to 6 per month off the coast of Vietnam and to 8 along the west coast of Borneo.
Past Borneo and the islands of the Celebes Sea, the path moves into the light southerly flows found on the south side of the ITCZ. Cloud cover decreases slightly north and east of the island of Sulawesi (Celebes), in most part because thunderstorm clusters avoid the area. From a climatological point of view, this is probably the best location for observing from a ship, but a timely weather forecast for other areas would probably give more advantage.
October is in the midst of the typhoon season, a particularly vicious type of storm that is essentially a Pacific hurricane. They have every bit of the nasty personality, and then some, of their North American cousins. Typhoons approach the coasts of Indochina from the east, passing over the Philippines on their way toward Vietnam. They weaken rapidly once they cross onto land, but leave large areas of cloud to plague eclipse-viewing. Luckily, they are quite rare at the latitude of the eclipse track, affecting it perhaps one year in five.
Beyond the Celebes Sea, the eclipse track moves into quieter and slightly sunnier weather of the Pacific equator, where the path ends at sunset in Micronesia. It is a region of cumulus cloudiness, with plenty of blue sky between the convective buildups. Occasional thunderstorm clusters may spoil this idyllic pattern, but they are not as common as farther north toward the ITCZ. Unfortunately, because the eclipsed Sun will be low in the sky, even a little cloudiness will go a long way to obscuring the view.
Shipboard observers will find compatible wave heights along much of the eclipse track, unless a recent typhoon or cyclone sends a large swell into the eclipse area. From the coast of Vietnam to New Guinea wave heights average between one-half and one metre. Beyond New Guinea, and on the Bay of Bengal, waves average a little over one meter in height.