The August Break in Lagos

Doing some open data exploration I began looking at monthly averages for rainfall in Lagos, Nigeria. Slightly after the midpoint of the year I spotted a pattern which I've come to learn is known as the "August Break". July brings about 257mm of rain; August brings about 112mm. That is roughly 56 percent less rain. But July averages 13.2 rainy days, while August still averages 11.6.

Dividing the monthly rainfall totals by the average number of rainy days makes the difference easier to see: about 19.5mm per average rainy day in July, compared with 9.7mm in August. Those are ratios of published averages, not measurements of individual storms, but they show why August can still bring rain almost as often while receiving much less of it overall.

In those city averages, August had the lowest rainfall of the rainy season. The source, published by the World Meteorological Organization using Nigerian Meteorological Agency data, does not say which years its averages cover. To check whether the same pattern appears across the wider Lagos region, I used a separate set of satellite-and-station rainfall estimates from 1981 through 2025. The map below uses 1991-2020 as its baseline, keeping the data on its original grid instead of pretending individual neighbourhoods were measured separately.

Across the full 45-year regional record, August was the lowest-rainfall month of the July-to-September stretch 38 times. That is about 84 percent of the record: common enough to deserve a name, but not dependable enough to treat as a guarantee.

The exception that stood out was 2025. Across the regional grid, July averaged about 83 millimetres, while August reached 134. September rose further to 221. In 2025, the broader region did not have an August minimum, even though the eastern part of the region still did.

The reason lies in the West African monsoon. Early in the rainy season, its main rain belt sits closer to the Gulf of Guinea coast. As the season advances, that belt shifts north toward the Sahel, carrying the heaviest rainfall away from cities like Lagos.

Something else changes at the same time: cooler water rises toward the surface of the Gulf of Guinea. Researchers found parts of the coast cooling from around 30 degrees Celsius in May to 24-25 degrees by August. That cooler ocean weakens the convergence of warm, moist air near the coast, making the storms that deliver most of the rain less likely to form.

As the rain belt moves south again in September and October, the coast enters its second rainy season. The August break is less a break in the weather than a shift in where West Africa's heaviest rain is happening.