Why is April the driest month?

In the Netherlands, we don’t have distinct wet and dry seasons; rain is spread throughout the year. However, one thing that stands out is that in spring—and particularly in April—rainfall is at its lowest. Why is that?

Rain throughout the year

The animation below shows the average amount of precipitation for each month in the period from 1991 to 2020. The images are based on precipitation observations from the KNMI network, which consists of around 300 volunteers spread across the Netherlands who read rain gauges daily. What stands out is that April stands out as a relatively dry month.

Two types of rain

April is a transitional month. In winter, strong westerly winds bring storms toward Europe, accompanied by heavy rain, as was the case this winter in Spain and Portugal. The rain falls within these storms where cold and warm air meet along fronts that stretch thousands of kilometers. Meteorologists refer to this as large-scale precipitation.

As the transition to summer begins, the westerly winds weaken, and the land heats up significantly. Showers form when the land warms the lower atmosphere and moistens it through evaporation from vegetation and the ground. The warm, moist air rises, cools, clouds form, and eventually rain falls. Meteorologists call this convective precipitation. Showers can produce a lot of rain in a short time and in a localized area.

Perhaps counterintuitively, December and August are the wettest months (Figure 2). In August, the amount of precipitation is roughly the same as in December, but it falls in a much more localized and shorter timeframe. This is why you might get the impression that August is drier than December.

Rain in April

With the weakening of the westerly winds and the reduction in storm activity, large-scale precipitation in April is also much less. However, the sun is not yet strong enough to heat the land significantly during the day, and there is still little evaporation from plants. As a result, the contribution of convective showers to April’s precipitation is much lower than in summer.

Additionally, the sea is still cold from winter, which suppresses convection and means the air contains less moisture. All these factors combined make April the driest month of the year. March still experiences more winter storms, while in May, the sun is already stronger, and showers form earlier.

September is also a transitional month, but in September, coastal areas in particular receive a lot of precipitation because the sea is still warm from summer.

All months have become wetter, except April

If we compare precipitation levels between the period 100 years ago (1901–1930) and the recent period (1991–2020), we see that all months have become wetter, except for April (Figure 2). Winter precipitation has increased because sea temperatures are rising. Storms therefore carry more moisture, resulting in heavier rainfall. According to the KNMI’23 climate scenarios, this trend is expected to continue in the future as the sea continues to warm.

Higher moisture levels in the air have also led to an increase in summer precipitation, though less so than in winter. The climate scenarios do not expect this trend to continue into the future. Further warming will cause the land to dry out, reducing summer precipitation. You can see on the climate dashboard that this decrease may have already begun in recent years.

KNMI climate report by Frank Selten

More information

• All KNMI climate reports
• Unpredictable spring rains (climate report May 2023)
• Summer almost as wet as winter (climate report May 2023)

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