Route Climatology — Page 447, Lesson 412

I want to walk you through the summer climatology for this route. We're looking at July conditions, and I'll start with the pressure systems because they drive everything else. In winter, you'd have the Siberian High dominating, but by July that's completely gone. It's been replaced by the Asian Low, centred over Baluchistan. That's a huge thermal low-pressure area sitting over the hot landmass of Asia. Now, the Intertropical Convergence Zone — the ITCZ — is the band where the northern and southern hemisphere trade winds meet, and it migrates with the seasons. In spring, it moves northbound rapidly across the route. It reaches Singapore by March, then Calcutta by May. Then in autumn, it comes back southbound: Calcutta in October, Singapore again in November or December. So in July, the ITCZ is located well to the north of our route. Here's the key mechanism: the inflow of air into the ITCZ flows cyclonically — that means counterclockwise in the northern hemisphere — around the Asian Low. This circulation establishes the southwest monsoon, exactly as shown in Figure 23.2. Let me bring that up for you. The southwest monsoon is supplied from the Southern Hemisphere. The southeast trade winds cross the Equator, and when they do, they veer — that means they turn to the right — becoming southwesterly winds. So the source air is the southeast trades, which then become the southwest monsoon. Now, what's critical for you as a pilot: these winds have a lengthy equatorial sea track. That means they travel over warm ocean along the Equator for a long distance. This ensures two things: high temperature and high humidity. So wherever this air makes landfall — wherever it hits a coast — the orographic uplift, which is the forced rising of air over high ground, will trigger intense instability. That means thunderstorms and severe weather. Let's move to the weather itself. In summer, flying conditions are poor. The whole route lies on the windward side — the side facing the wind — of the Bangladeshi, Burmese, Thai, and West Malaysian coasts. So thunderstorms and severe weather will occur throughout the route, with one exception: the extreme south, in the Straits of Malacca. There, the mountains of Sumatra provide some protection from the southwest monsoon. But don't relax. The high mountains on either side of the straits create a new hazard. At night, katabatic winds — that's cold, dense air flowing downhill from the mountains — flow down from each side. These are aided by the land breeze effect, where the land cools faster than the sea at night, so the wind blows from land to sea. These two flows meet in the middle of the straits, causing a convergence line. Convergence means air piles up and is forced to rise, and that uplift triggers thunderstorms. Along the straits, this double-sided uplift can cause a line of night-time thunderstorms, arched in the middle. These are known as Sumatras. Here's a memory aid: Sumatras occur in Summer. Finally, let's talk about cyclones. In the Bay of Bengal, tropical revolving storms are called cyclones. For a cyclone to form, you need a summer warm sea. Evidence suggests the sea surface temperature needs to be in excess of plus 2 — and the excerpt cuts off there, but you can see the threshold is a warm sea surface temperature, which the Bay of Bengal certainly has in July. So to summarise the summer picture: the Asian Low drives the southwest monsoon, the ITCZ is well north, the whole route is on the windward side with severe thunderstorms, except the Straits of Malacca where you get night-time Sumatras instead, and the Bay of Bengal is primed for cyclone formation.