In winter, most precipitation falls with southwest winds. This is primarily due to the prevalence of this wind direction and the fact that days are generally wetter during these winds. Compared to the period 1961-1990, the number of days with precipitation has hardly changed, but when it rains, more rain falls on average per day, except with winds from northern directions. The strongest increase occurs with southwest winds.
Precipitation Amount by Wind Direction
In the winter half-year (October to March), an average of 437 millimeters of rain fell at the De Bilt measuring station during the period 1991-2020. This is a 7 percent increase compared to the previous 30 years (409 millimeters in 1961-1990). The amount of precipitation is strongly dependent on wind direction.
To investigate this, we calculate the amount of precipitation per wind direction (figure 1). We do this for sectors of 90 degrees, in increments of 1 degree, so 0-90 degrees, 1-91 degrees, and so on. The precipitation for example with southwest winds is the sum of the precipitation on all days with an average daily wind direction between south and west. Of the winter precipitation in the period 1991-2020, the largest portion, 71 percent, comes from the southwest. Only 13, 4, and 12 percent of winter precipitation comes from the northwest, northeast, and southeast, respectively.
Compared to 1961-1990, precipitation has increased the most for southwestern wind directions. Northern wind directions show hardly any increase.