Route Climatology — Page 454, Lesson 428

Let's start with the route climatology for the Australia region in January, which is summer in the Southern Hemisphere. I want to walk you through the upper wind pattern first. Over the equator, from Singapore down to about 20° south, we have light upper equatorial easterlies. That means the wind at high altitude is blowing from the east, but it's light in strength. Once you go south of about 20° south, the wind shifts — it increases from the west. So the further south you go, the stronger the westerly wind becomes. Now, why does that happen? In summer, the whole continent of Australia is hot, and there is cooler sea to the south. That temperature contrast between the hot land and the cooler ocean drives the wind. Specifically, at the 200 hectopascal pressure level — that's roughly 38,000 to 40,000 feet, in the upper troposphere — the subtropical westerly jet stream is positioned along the south coast of Australia. Its speed is around 70 knots there, and it reduces to about 40 knots as you continue east towards Auckland, New Zealand. Let's move to icing. Icing can be severe above 16,000 feet in cumulonimbus clouds — those are the big thunderstorm clouds. So if you're flying through or near a CB above that altitude in this region during summer, you need to be aware of the risk of significant ice accumulation. Now, the tropopause and freezing levels. I'll give you the numbers from north to south. Over the northern part of the route, the tropopause is at 56,000 feet. As you go south, it drops. Over the southern part, if you're in tropical air, the tropopause is at 51,000 feet; if you're in polar air, it's much lower at 36,000 feet. So there's a big step down depending on the air mass. The freezing level also varies with latitude and season. In the north, the freezing level is at 16,000 feet. In the south, it's at 10,000 feet in summer, and it drops to 5,000 feet in winter. So in January — summer in the south — you'd expect the freezing level around 10,000 feet over the southern part of the route. Let me show you the upper wind chart for January so you can see this pattern visually.