As the ITCZ retreats, northwesterlies will re-establish behind it — Page 447, Lesson 420

I want to walk you through the weather patterns that affect this region, starting with what happens as the Intertropical Convergence Zone, or ITCZ, moves away. As the ITCZ retreats, northwesterlies will re-establish behind it. That happens because the Siberian High begins to rebuild — that's a large, semi-permanent area of high pressure over Siberia that strengthens during the colder months, pushing winds back out from the northwest. Now, where typhoons occur — and these are the same as hurricanes, just the name used in the western Pacific — winds of varying direction may exceed 100 knots. That's over 115 miles per hour, so well into extreme wind speeds. In sunny conditions, especially in the south, sea breezes will affect coastal wind direction. A sea breeze is a local wind that blows from the sea toward the land during the day, caused by the land heating up faster than the water. Let's move to weather. To the west of Singapore, in the Malacca Straits, a specific type of thunderstorm forms overnight called a Sumatra. That's a katabatic thunderstorm — katabatic means it's driven by cold, dense air flowing downhill, in this case from the landmass of Sumatra out over the straits at night. The southwest monsoon will bring orographic cumulonimbus clouds to southwest-facing coasts. Orographic means the clouds form when moist air is forced upward by a mountain or hill — so the southwest monsoon winds hit those coasts, rise, and produce towering cumulonimbus clouds, which are thunderstorm clouds. East coasts, by contrast, will be more sheltered because they're on the lee side, away from the oncoming monsoon flow. Nevertheless, in these equatorial regions, including Singapore, purely convective cloud will be heavy. Convective cloud means cloud formed by rising warm air — no mountains needed, just the intense heating of the surface. That convection is often strong enough to give daily thunderstorms. Most route weather — the weather that affects flight paths — arises on the ITCZ as it travels. The ITCZ moves from Singapore in March up to Tokyo in July, then back down to Singapore in November and December. As the northbound ITCZ passes China in May, the associated precipitation is known as the Plum Rains — that's a specific seasonal rainy period in East Asia. High typhoon internal wind speeds, coupled with intense rain and thunderstorms, can locally bring much structural damage and flooding. So the combination of extreme wind and heavy rain is what causes the destruction. Finally, over Japan in late summer, some frontal rain occurs as cPc air spreading south from Siberia meets retreating tropical air. cPc stands for continental polar cold air — that's cold, dry air originating over the Siberian landmass. When that cold air pushes south and meets the warm, moist tropical air that's retreating, you get a front, and that frontal boundary produces rain. Let me bring in the figures that show these patterns. shows the weather and winds in July — that's when the ITCZ is at its northernmost near Tokyo, and you can see the southwest monsoon and typhoon tracks. shows surface conditions in January, when the Siberian High is strong and northwesterlies dominate, and the ITCZ is down near Singapore. So to tie it all together: the key drivers here are the seasonal movement of the ITCZ, the rebuilding of the Siberian High, the monsoon winds, local effects like sea breezes and katabatic Sumatras, and the occasional typhoon. All of these shape the weather that a pilot flying through this region has to anticipate.