Route Climatology — Page 439, Lesson 411

I want to walk you through the winter weather patterns along this route, which is a key part of route climatology for your ATPL studies. We're looking at the northeast monsoon season, and I'll explain how the pressure systems, weather, visibility, winds, and icing conditions all fit together. Let's start with the pressure systems. The dominant feature here is the Siberian High — that's a large, cold, high-pressure system that sits over Siberia during winter. Continental outflow from this Siberian High establishes the NE Monsoon over the whole route. The NE Monsoon is simply the northeast wind that blows from the Asian continent out toward the sea. The ITCZ — the Intertropical Convergence Zone, where the trade winds meet — is south of the route during this season, so it doesn't directly affect the weather we're about to discuss. Now for the weather itself. In winter, the route weather is generally very good. Calcutta and Bangladesh are protected from the north by the Himalayas, so the cold air from Siberia doesn't hit them directly. Abeam Burma — that means off the coast of Burma — the NE monsoon will have had a long land track. Although isolated convective cumulus clouds are possible, the dry cold air will in general ensure clear skies. So you get mostly blue skies along that stretch. Further south, the monsoon crosses the Gulf of Thailand. As it moves over the warm water, it picks up warmth and moisture, which encourages some CU — cumulus — and CB — cumulonimbus — to form over the isthmus of southern Thailand. So you start seeing more convective cloud development there. On the last section of the route, the NE monsoon arrives warm and moist from the expanse of the South China Sea. This gives large-scale orographic cumulus and thunderstorms over the east coasts and central highlands of West Malaysia and Sumatra. "Orographic" means the clouds form because the air is forced to rise over the mountains. The West Coasts are more sheltered and generally less wet, although all equatorial land areas including Singapore have considerable convective heating resulting in almost daily thunderstorms. So even the sheltered side still gets its share of storms due to the intense heating at the equator. Let's talk about visibility. Early morning mist can occur in the moist river delta regions of Bangladesh and Burma. Otherwise, visibility is very good outside showers and thunderstorms. So if you're not in a shower or thunderstorm, you'll have excellent visibility. Now for the winds. Low-level winds are northeasterly over the whole route under the influence of the NE monsoon. That's consistent with what we'd expect. Above 20,000 feet, upper winds overrun the monsoon — meaning they flow above and independently of the surface monsoon flow. The 200 hPa subtropical jet lies just south of the Himalayas and is therefore north of the route. So the jet stream is to your north. You get decreasing westerlies of around 40 knots over Calcutta, which reduce to zero near 10°N, and thereafter become the normal light equatorial easterlies. So as you fly south, the westerly wind weakens, stops, and then reverses to a light easterly wind near the equator. Finally, icing. Icing can occur above 16,000 feet, but it's a lesser problem in January than in July because skies are generally clear on the route. So you're less likely to encounter icing conditions in winter compared to summer. Let me bring up the relevant diagrams so you can see these patterns visually. That covers the winter weather along this route — the NE monsoon, the generally good weather with some convective activity in the south, the visibility conditions, the wind patterns from surface to upper levels, and the icing considerations.