Route Climatology — Page 462, Lesson 432

I want to walk you through the route climatology for a very important region—the area around the Nile Valley, East Africa, and southern Africa, focusing on how the Intertropical Convergence Zone, or ITCZ, drives the weather patterns you'll encounter as a professional pilot. Let's start with the weather itself. In July, the ITCZ reaches its northern extreme just north of Khartoum. For Khartoum, this is normally the city's only wet month of the entire year. The ITCZ brings with it a tropical rain belt, and along with that come line squalls and dust storms known as Haboobs. A Haboob often appears as a wall of dust lifted to 10,000 feet. These can appear in Northern Sudan from May to September as the ITCZ sweeps north and then south again. They form during the day when convection is strong. Now, contrast that with Johannesburg, which is in winter during this same period. There, continental high pressure prevails, and it is the dry season. However, even in the dry season, stratocumulus and cumulus—abbreviated ST and SC—turbulence cloud may form. This happens when air rises orographically, meaning it's forced upward by the terrain, as it moves from the Indian Ocean up to the Johannesburg high veld. Let's move to surface winds. High pressure over the Mediterranean, combined with low pressure over Arabia, gives northerly surface winds over the route from Cairo down to the ITCZ. These northerlies are an extension of the Mediterranean Etesian winds. In July, the ITCZ is near 18° North. South of the ITCZ and north of the Equator, winds will be from the southwest due to the Coriolis effect. Then, south of the Equator, the winds revert to the southeast trade winds. Over southeastern Africa, these southeast trades are known as the Guti. The southeasterly Guti blows anticlockwise around the overland winter high pressure system, often being in place for about five days at a time. It can bring the orographically formed stratocumulus and cumulus to Johannesburg. Now, visibility. Over the Sahara, visibility will be appalling in Haboobs, and poor elsewhere due to Sahara dust. Near Nairobi, visibility will be good except in showers. At Johannesburg, visibility may be reduced below the low stratocumulus and cumulus formed by the Guti southeast wind. Icing: as in winter, icing can be severe above 16,000 feet in cumulonimbus clouds, which we abbreviate CB. Let's turn to the Nairobi region. This region is of special interest because the two ITCZ transits in the year each give their own instability rainfall pattern. I want you to look at Figure 23.13, which shows Nairobi weather from March to May. During Northern Hemisphere spring, the ITCZ passage is northbound, from March to May. It is followed by the moist southeast trade winds from the Indian Ocean. Rainfall will be extensive and is known as the Long Rains. Now look at Figure 23.14, which shows Nairobi weather from November to December. During Northern Hemisphere autumn, the ITCZ passage is southbound, from November to December. It is followed by the dry northeast trade winds from Saudi Arabia. Rainfall will still occur but will be less, and this is known as the Short Rains. Now, orographic uplift. At each ITCZ passage, the surface wind velocity—abbreviated W/V—will be alternating between northeast and southeast. Nairobi has an elevation of over 5,000 feet and is only 200 nautical miles from the east coast. Especially during the long and short rains at ITCZ passage, and between 0200 and 0800 local time, orographic uplift in the generally easterly winds can frequently produce low stratus. This stratus often lowers to the undulating surface as fog. Thunderstorms: convective thunderstorms can occur at any time, but they are very infrequent from June to September, when the ITCZ is well north. Finally, let's look at tropopause and freezing levels. Tropopause heights average 56,000 feet all year. Freezing level heights are 16,000 feet in equatorial regions and average 14,000 feet in the higher latitudes in winter.