Route Climatology — Page 462, Lesson 429

I want to walk you through the route climatology for the Cairo-to-Johannesburg route via Nairobi. This is a real-world planning tool we use as professional pilots to anticipate weather, winds, and hazards along a specific flight path. Let's start with the geographical context. The route runs from 30° north latitude down to 27° south latitude, between 28° and 37° east longitude. Over Egypt and Sudan, the terrain is almost entirely low-lying Sahara desert. When we reach the Kenya border at about 6° north latitude, the land begins to rise — this is the start of the equatorial vegetation belt. By the time we get to 2° south latitude at Nairobi, the elevation is over 5,000 feet. From there, the route crosses the eastern edge of the Kalahari plateau and finally reaches the high veld of Johannesburg. Now let's look at January conditions — that's northern winter and southern summer. Pressure Systems In the northern winter, pressure is high over the comparatively cool Sahara desert. The Intertropical Convergence Zone, or ITCZ, is at its southern extreme over Zimbabwe. This arrangement leads to overland low pressure extending south from Nairobi all the way to Johannesburg. Weather The northern section, from Cairo down to 6° north, is dry and dusty. Near Nairobi, we start to see convective cumulus and cumulonimbus — that's CU and CB — along with some nimbostratus, or NS. Further south, instability is enhanced by the presence of the ITCZ. At Johannesburg, orographic low cloud and fog can occur in the early morning — "orographic" means caused by the terrain forcing air upward. That fog clears quickly, giving way to convective CU and showers in the afternoon. This is the wet season. Additionally, cyclones originating in the Mozambique Channel can sometimes move west to affect Zimbabwe and northern South Africa. Surface Winds The southerly Khamsin wind blows from Egypt toward the Mediterranean between December and April. Further south, over Sudan and Kenya, the clockwise outflow from the Sahara High becomes first northerly, then northeasterly, turning into the trade winds blowing from dry Saudi Arabia. South of the equator, those winds back again to northerly — that's a Coriolis effect change — and they blow clockwise around southern Africa's summer low pressure, which is about 1005 hectopascals. For this reason, they become easterly again near Johannesburg. Visibility Visibility is poor over the dusty Sahara, but good toward Nairobi except in showery precipitation. At Johannesburg, early morning fog can be caused by easterly surface winds from the Indian Ocean rising orographically onto the Kalahari plateau. Icing Icing can be severe above 16,000 feet in cumulonimbus clouds near the ITCZ. Now let's move to July — northern summer and southern winter. The ITCZ equatorial trough has moved north with the sun to approximately 18° north latitude, pushing the winter Sahara anticyclone northward toward the Mediterranean. Behind and south of the ITCZ, cooler winter temperatures over the southern African landmass cause pressure to build. That covers the key route climatology for this sector. Let me know if you'd like to go deeper into any of these elements, or if you're ready to move on.