First, let's define the key term: tropical revolving storms — Page 447, Lesson 414

I want to walk you through the route climatology for the Singapore to Tokyo via Hong Kong route, starting with the tropical revolving storms that affect the Bay of Bengal and the Gulf of Thailand. First, let's define the key term: tropical revolving storms. In the Bay of Bengal, these are specifically called cyclones. For them to form, they need two essential conditions: a summer warm sea — and the evidence suggests the sea surface temperature must be in excess of +27°C — and the close instability of the ITCZ, which stands for the Intertropical Convergence Zone. Now, here's how the timing works. In July, the ITCZ is positioned north, over the land. Because of that, tropical revolving storms in the Bay of Bengal can only form in early and late summer, when the ITCZ is actually over the sea. That means the formation months are June or October. These storms also occur in the Gulf of Thailand, and they occasionally move west to affect the route. Let's move to visibility. Visibility along this route is generally impaired by two factors: much cloud and frequent rainfall. The reduction in visibility during tropical rainstorms can be considerable — so you can expect very poor visibility in those conditions. Now, winds. At the surface, the winds are southwesterly over the whole route. But let's look aloft. The 200 hPa subtropical jet is now located north of the Himalayas. That means above 20,000 feet, the equatorial region experiences mostly light upper easterlies over the entire route. There's also an important effect from the summer high temperatures over the land mass of Asia. Those high temperatures cause a reversal of the south-north temperature gradient. Aloft, with warm air to the north, the upper pressure gradient movement is southward. The Coriolis effect — or geostrophic force, which we often abbreviate as GF — will turn that southward flow to the right. This gives us a pronounced easterly jet over Rangoon of 70 knots, centred near the 150 hPa level, which is approximately 45,000 feet. Let's talk about icing. Icing can be a problem on this route during the summer, specifically when you are descending through cumuloform clouds — those are the towering, convective cloud types like cumulonimbus. Now, two important altitude references. The tropopause height averages 56,000 feet all year round. The freezing level height averages 16,000 feet all year round. Finally, let's set the geographical stage for the route from Singapore to Tokyo via Hong Kong. This route traverses what's called the Western Pacific Rim, from latitude 01°N to 35°N. It passes close to the east coasts of West Malaysia, Vietnam, and China. The en-route weather is dominated by the changing seasonal pressure over Asia and the temperature differences between the continent and the ocean, and between sea currents. East of Japan, the cold Oyasiwo sea current sweeps down from the Russian Kamchatka peninsula, and it is countered by the warm Kurosiwo sea current flowing northeastward from the Northern Philippines. Much of the area, including Japan, has a mountainous interior. For the winter season, specifically January, we have a figure showing the surface conditions. Let me bring that up for you.