Route Climatology — Page 447, Lesson 418

I want to walk you through the route climatology for the summer season, specifically July, across the Asia-Pacific region. This is the kind of knowledge you'll use to anticipate weather patterns on long-haul routes, not just for a written exam but for real operational decision-making. Let's start with the pressure systems. In winter, you have the Siberian High, a massive area of high pressure. But in summer, that is replaced by the Summer Asian Low, centred over a region called Baluchistan — that's in western Pakistan. This low-pressure system has an anticlockwise inflow, which in the northern hemisphere means air spirals inward and upward. That inflow produces the southwest monsoon, and it extends over the route as far north as Central Japan. That northern limit is also the northern limit of the ITCZ — the Intertropical Convergence Zone. Now, the ITCZ itself is critical. The southwest monsoon follows the ITCZ northward as the season progresses. The ITCZ will be over Singapore in March, over China in May, and over Japan in July. Then, as summer ends, the northern extent of the southwest monsoon recedes southwards, driven before the retreating ITCZ. That means the ITCZ passes Hong Kong in September and returns to Singapore again in November or December. So you can see the ITCZ is like a seasonal pendulum, and the monsoon follows it. Next, let's talk about typhoons. In the North Pacific, tropical revolving storms are called typhoons — that's the regional name. Evidence suggests that for a typhoon to form, you need several conditions. First, a sea temperature greater than +27°C. Second, proximity to ITCZ instability — that's where the convergence and rising air can trigger development. Third, the storm must be displaced away from the Equator because Coriolis force is zero at the Equator — you need some Coriolis effect to get the rotation started. And fourth, the storm must be located south of the jet stream belts, because the jet stream's strong winds would destroy the vertical continuity of the storm. Typhoons form in the central Pacific, at around 10° to 15° North. They then drift westward at 10 to 15 knots, driven by the clockwise wind direction around the North Pacific subtropical high — that's the semi-permanent high-pressure system. As they near the Philippines, they generally track near the seasonal ITCZ, but they can also curl northwards, which extends the season in some locations. Operationally, you need to know the numbers. Up to 12 typhoons per year can affect southern Japan, principally in July to September. In Hong Kong, occurrence is commonly in September. The overall season may extend from May to November, depending on latitude. The southern limit for typhoon activity is the Gulf of Thailand. Finally, let's look at surface winds in July. The southwest monsoon extends over the whole route as far north as Central Japan, where the ITCZ then lies. But by late summer, the cold northwesterlies will re-establish behind the retreating ITCZ as the Siberian High begins to rebuild. And where typhoons occur, you get winds of varying strength and direction — that's a reminder that typhoons disrupt the otherwise predictable monsoon flow. You can see the surface conditions for July on the chart.