Local Winds and Weather — Page 412, Lesson 375

I want to walk you through two more significant local winds that you’ll encounter in professional aviation: the Sumatra and the Harmattan. These are both regionally important, and each has distinct characteristics that affect flight operations. Let’s start with the Sumatra. This wind occurs in the Straits of Malacca — that’s the narrow stretch of water between the Malay Peninsula and the Indonesian island of Sumatra. The Sumatra wind blows from directions between southwest and northwest, and it’s most frequent between April and November, during the time of the southwest monsoon. Here’s how it forms. During the day, thunderstorms build up over the high ground of Sumatra. Those storms are assisted by the sea breeze — the daytime onshore wind that feeds moisture and instability into the developing cumulonimbus clouds. But at night, the situation changes. The subsiding cumulonimbus clouds — that means the thunderstorm clouds that are collapsing or sinking — drift eastward. They move under the influence of two forces: the land breeze, which is the nighttime offshore wind, and the katabatic effect, which is the downslope flow of cold, dense air off the high terrain. As these decaying storm clouds drift out over the warm sea, they are rejuvenated — they get new energy from the warm water. The result is violent storms that occur late at night and in the early morning. When the squall — the line of intense wind and weather — passes through, there is a sudden temperature drop. That’s a key operational cue: a rapid fall in temperature as the squall arrives. Visually, Sumatras take on a pronounced arched shape because the cumulonimbus anvils — the flat, spreading tops of the thunderstorm clouds — spread out at the tops of the clouds. So from a distance or on radar, you’d see that characteristic arched appearance. Now let’s move to the Harmattan. This is the last of the major local winds covered here. The Harmattan blows mostly during winter, from the high pressure desert areas of North Africa. It’s a northeasterly wind that blows towards the ITCZ — the Intertropical Convergence Zone. In fact, the book notes that these are essentially the northeast trade winds — the same large-scale wind belt, but in this region they’re called the Harmattan. The Harmattan is a cool, dusty wind. That dust is critical for aviation. It can reduce visibility to below 1000 metres, especially in areas bordering desert regions — the example given is Kano, Nigeria. The dust layer itself may extend to 7000 or 10,000 feet or more. Visibility tends to improve as you move towards the coast, because the dust concentration decreases. The Harmattan blows from November through to April. By the end of that period, the winds will be light, especially in the south. So if you’re flying in West Africa during those months, you need to plan for reduced visibility, dust aloft, and the potential for low ceilings near the desert fringe. To summarise: the Sumatra is a nocturnal, rejuvenated thunderstorm squall over the Straits of Malacca, with a sudden temperature drop and arched cloud tops. The Harmattan is a cool, dusty northeasterly from the Sahara, reducing visibility below 1000 metres and extending dust up to 10,000 feet, from November to April. Both are important local weather phenomena that you must account for in flight planning and operations.