Rainfall Trends 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 trend 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.
Lagos monthly rainfall and rainy days
In the monthly averages, August had the lowest rainfall of the rainy season. Then I checked whether the same pattern appeared across the wider Lagos region, using 45 years of satellite-and-station rainfall estimates. Keeping the data on its original grid makes the seasonal change visible without pretending individual neighbourhoods were measured separately.
July, August, and September rainfall across Lagos
Across 45 years, 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.
Forty-five years of Lagos July, August, and September rainfall
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.
How the West African monsoon creates Lagos's August break
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.
How the rainfall comparisons were madeOfficial Lagos monthly climatology and 135 final CHIRPS rainfall grids spanning 1981-2025.
The city-level monthly averages come from the World Meteorological Organization's World Weather Information Service, using Nigerian Meteorological Agency values. The source page does not state its averaging period or rain-day definition, so those figures should not be treated as an exactly matched baseline for the satellite analysis.
The regional analysis uses CHIRPS version 3 final monthly rainfall for every July, August, and September from 1981 through 2025: 135 source rasters and 468 supported land cells on the native 0.05-degree grid. Another 72 cells represent water or low observation coverage and remain visually distinct.
The map summarizes 1991-2020 grid-cell climatology. Annual comparisons use all 45 years and classify August as a minimum only when it is strictly below both July and September. The regional median valley depth is 62.43 percent. A block-resampling interval around the estimated long-term trend includes zero, so this dataset does not establish a robust directional trend.
Monthly grids are blended satellite-and-station estimates, not daily gauge readings, event timelines, or street-level rainfall measurements. Editorial east/west zones are analytical comparisons, not administrative boundaries. No 2026 forecast is included as an observation.
The monsoon diagram is a qualitative schematic based on peer-reviewed climate research. Its coastline uses Natural Earth's public-domain 1:110m land geometry; the rain belts, airflow arrows, and ocean cooling illustrate the reported mechanism rather than measured weather fields.
Sources and further reading
- World Meteorological Organization / Nigerian Meteorological Agency. Lagos monthly climate averages
Published monthly rainfall totals and mean rain days for Lagos.
- Climate Hazards Center, University of California Santa Barbara. CHIRPS version 3 rainfall data
Final monthly satellite-and-station rainfall grids, 1981-2025; DOI 10.15780/G2JQ0P.
- Climate Hazards Center. CHIRPS version 3 technical documentation
Native grid resolution, temporal coverage, data construction, and attribution.
- Climate Dynamics. The impact of air-sea coupling and ocean biases on southern West African precipitation
How seasonal movement of the tropical rain belt and Gulf of Guinea sea surface temperatures shape the little dry season.
- Atmospheric Chemistry and Physics. Impact of the Guinea coast upwelling on atmospheric dynamics and precipitation
How cooler coastal waters can reduce moisture convergence and suppress rainfall over southern West Africa.
- Natural Earth. 1:110m physical land vectors
Public-domain coastline geometry used in the West African monsoon schematic.