Route Climatology — Page 447, Lesson 416

I want to walk you through the route climatology for a specific route — this is the kind of analysis you’ll use to plan a flight across East Asia in January. Let’s start with the pressure systems that drive everything. The Intertropical Convergence Zone, or ITCZ, lies well south of the route in January. That means the band of converging trade winds and heavy convection is far away, so it doesn’t directly affect the weather along this path. What does dominate is the Siberian High — a large, cold, high-pressure system that is well established to the west over Asia. This high is the engine for the surface winds you’ll encounter. Now, some polar front lows — the low-pressure systems that form along the boundary between cold polar air and warmer mid-latitude air — do traverse the extreme north of the region. So the far northern part of the route can see passing frontal depressions. Let’s look at the surface winds. Because the Siberian High is a high-pressure system in the northern hemisphere, the wind flows clockwise outward from it. That clockwise outflow establishes the wind direction over the route, and you can see this pattern in Figure 23.3. From Singapore up to Vietnam, the northeast monsoon blows — that’s a steady wind from the northeast. From Vietnam onward into China, the wind remains from the north or northeast. As you get near Japan, the wind is from the north or northwest. Now, the weather. In the south of the route, the northeast monsoon sweeps down from the warm expanse of the South China Sea. That warm, moist air moving over the sea produces intense convective instability — the atmosphere becomes unstable enough to generate strong updrafts. This produces cumulus clouds, cumulonimbus clouds, heavy showers, and thunderstorms along any windward coast in its path. For example, the east and northeast coasts of West Malaysia and Vietnam get hit hard. Inland areas that are sheltered by mountains will remain drier, aside from some convective weather — so the mountains block the moisture. Moving toward Hong Kong: after the ITCZ has passed southbound in September — meaning it moved south of Hong Kong — some shelter is afforded from the north and northeast winds by the Chinese mountainous landmass. So from October to December, the weather in Hong Kong is fine and dry. But a change occurs in January. The wind veers — that means it shifts direction clockwise — and the source area for the wind becomes the warm Kurosiwo sea current. That’s a warm ocean current flowing northward off the east coast of Asia. These new warm, moist winds then flow over seasonally cooled coastal Hong Kong waters. When warm, moist air moves over a colder surface, it forms advection fog — fog created by horizontal movement of air over a cooler surface. You also get low stratus, drizzle, and generally gloomy conditions. This coastal condition is known as the Crachin, and it lasts in Hong Kong from January to April, after which the northward movement of the ITCZ will dispel it. In the north of the route, very cold, dry southeastward outflow from Siberia crosses the comparatively warm Sea of Japan. That temperature difference generates moderate instability, which is then orographically enhanced — meaning the mountains force the air upward, amplifying the instability — over the Japanese northwest coast and central mountains. This causes cumulus clouds and heavy snow showers. The eastern lee areas — the side sheltered from the wind, like Tokyo — will be drier and less cold. That’s due to the Föhn effect, where air descends on the lee side of mountains, warming and drying as it compresses, plus additional warming from the Kurosiwo sea current. Visibility: in the south of the route, visibility is good between showers — so when it’s not raining, you can see well. At Hong Kong, visibility is excellent from October to December, but abysmal from January to April in the Crachin conditions we just discussed. Near Tokyo and other big Japanese cities, visibility can be reduced to near fog limits by industrial smoke — so human pollution adds to the hazard. Finally, upper winds. From Singapore up to about 10° north latitude, you have equatorial easterlies blowing at 10 to 30 knots. Further north, the winds become westerly and increase in speed as you go toward 25° north to 40° north. That’s where the 200 hectopascal subtropical jet stream blows — frequently up to 150 knots, and occasionally up to 300 knots near Japan. That exceptional speed is due to a combination of two factors: the strong low-level geostrophic southeastward outflow from Siberia — that’s the wind driven by the pressure gradient of the Siberian High — and the extreme thermal wind component generated by the marked temperature difference between Siberia and the Pacific Ocean. Further north still, there are some occasional westerly jets in association with polar front lows — the low-pressure systems we mentioned earlier.