Today is blue Monday. Days are short this time of year, the sun is low, and often hidden behind a gray cloud cover. The lack of light makes it feel gloomy and many people experience a winter slump. Last December, there were only 29 hours of sunshine, half the average for that month. The last few days have also been very gray. The amount of sunlight varies greatly from winter to winter, but there is no clear trend. We expect little change in the future.
Gloomiest Day of the Year
Since 1958, the amount of solar radiation reaching the ground in De Bilt has been measured. This is expressed in energy in Joules per square meter. Dividing this by the time the sun is up gives us the average strength of solar radiation in Watts per square meter.
In Figure 1, you can see a graph for the gloomiest day of the year with the least sunlight. There are large variations from year to year. The gloomiest day so far was November 25, 2005, with an average of only 2 Watts per square meter of sunlight. The gloomiest day of 1969 still had 19 Watts per square meter. There is no clear trend. The gloomiest day of 2025 so far is January 1 (red circle).
Gloomiest 10-Day Period of the Year
Just like in recent days, gloomy days often follow one another. This is related to the positioning of high and low-pressure areas. A high-pressure area brings several days of weather in Europe with sunny winter sports weather in the Alps, but here we have fog and low-hanging clouds. There is little wind, and the descending air movement in the high-pressure area prevents the cold, moist air layer near the ground from mixing with the warmer air above. Meteorologists call this a cold stagnant layer. The fog and low clouds can persist for days.