Route Climatology — Page 454, Lesson 421

I want to walk you through a new topic: Route Climatology. This is the study of the typical weather patterns you can expect along specific flight routes at different times of year. We're going to look at a particular route — Singapore to Auckland via Darwin and Sydney — and I'll teach you the climatology for the winter season in the Southern Hemisphere, which is July. Let's start with the geographical considerations for this route. The route begins at Singapore, which is at 1° North latitude. It crosses the Equator just south of Singapore, then overflies the Java Sea and many of the Indonesian islands. Next comes the Timor Sea, followed by the central north Australian coast at Darwin, which is at latitude 12° South. From Darwin, the route crosses the dusty, largely flat Australian interior to the mountainous southeast coast at Sydney, at latitude 34° South. The last leg then heads across the Tasman Sea to New Zealand's low-lying Auckland airport at 37° South. Now, for the winter season — July — we look at the pressure systems that dominate this route. First, the ITCZ, or Intertropical Convergence Zone. In July, the ITCZ is in the Northern Hemisphere, well clear of the route. That means it doesn't directly affect this path during winter. Next, thermal lows. Convective thermal lows will occur over the Indonesian islands. Let me define what that means. A thermal low is an area of low pressure that forms due to heating from the ground. Here, these lows are formed by a combination of two factors: island insolation — that's the solar radiation absorbed by the land surfaces of the islands — and high humidity from the surrounding sea. The heating warms the air, it rises, and the moisture from the sea adds energy through latent heat release, which strengthens the convection. So you get low-pressure areas developing over the Indonesian islands. Then we have the subtropical high. The Australian interior in winter lies in the Southern Hemisphere subtropical high belt. This is a belt of high pressure that circles the globe at about 30° to 35° latitude. Because it's winter in the Southern Hemisphere, the seasonal temperature overland is relatively cool. That coolness will reinforce high pressure within the continent. So the Australian interior is dominated by high pressure in July. Let me also bring in the visibility conditions for this region in early summer — and remember, in the Northern Hemisphere context, early summer is around June, but for this route climatology we're looking at the broader picture. In early summer, warm, moist southwest monsoon winds advance north. These winds can bring sea fog to cooler Chinese coastal waters. They can also bring extensive blanket advection sea fog over the cold Oyasiwo sea current. The Oyasiwo current is a cold ocean current that flows between eastern Japan and the mainland Kamchatka peninsula further to the northeast. Advection fog forms when warm, moist air moves over a colder surface — in this case, the cold sea current — and the air cools to its dew point, creating fog. Additionally, some industrial smoke can occur near cities in Japan. Now, upper winds. Over the summer North Pacific — and again, summer in the Northern Hemisphere is July — the equatorial light easterlies of 10 to 30 knots extend to 25° North latitude. Easterlies mean the wind comes from the east. Beyond 25° North, the winds become light westerly — coming from the west. The subtropical and polar front jets are little in evidence over the summer North Pacific. That means the strong jet streams you might expect at higher altitudes are weak or not present in this region during summer. Let's look at tropopause heights. The tropopause is the boundary between the troposphere, where weather occurs, and the stratosphere above. For Singapore, the tropopause height is 56,000 feet. For Japan, it's 38,000 feet. So the tropopause is much higher near the Equator and lower at higher latitudes. Freezing levels also vary. For Singapore, the freezing level is at 16,000 feet. For Japan, it ranges from 3,000 feet to 15,000 feet. That's a wide range, reflecting the seasonal and latitudinal variation. Finally, I want to point you to the figures that illustrate these conditions. For the upper winds in July, you can see the pattern on the chart labelled "Upper winds - July (Winter) Upper Winds" . And for the surface pressure and weather in July, the chart "Surface pressure and weather - July (Winter)" is available . These will help you visualize the pressure systems and wind patterns I've described. So to summarize for this route in winter: the ITCZ is north of the route, thermal lows form over the Indonesian islands due to insolation and humidity, and the Australian interior is under a reinforced subtropical high. Upper winds are light easterly near the Equator, becoming light westerly further north, with weak jet streams. Tropopause heights and freezing levels differ significantly between Singapore and Japan. That's the climatology for this route in July.