Dutch Fog
The Netherlands is a small country characterized by variable land use and a strong influence of the North Sea on national climate. Devoid of significant topography, it is an excellent location for assessing the relative influence of various factors on fog occurrence in the absence of complex terrain effects. Using 45 years of observations from weather stations throughout the country, we assessed the regional and temporal variability of fog occurrence. For some of the highlights of this research, please see below. A manuscript describing this work has been submitted for consideration to the Quarterly Journal of the Royal Meteorological Society. |
Location of the weather stations used in the analysis.
Legend can be displayed using the top left icon. |
Dutch Fog in the Past 50 Years
The overall occurrence of Dutch fog is decreasing with time. However, the inter-annual variability of fog occurrence is over twice as large as the long-term trend. For example, in 1958, almost 10% of observations were foggy throughout the country. One year later, just 4% were. We relate the temporal variability in fog occurrence to the wintertime large-scale pressure forcing over Northwestern Europe. In years with more fog, the sea level pressure is higher, resulting in weaker winds, and more clear-sky nights. In other words, more favourable conditions for (radiation) fog formation. In years with less fog, the wintertime atmospheric low over Northwestern Europe is enhanced, resulting in stronger winds and overall less favourable conditions for fog formation.
The overall occurrence of Dutch fog is decreasing with time. However, the inter-annual variability of fog occurrence is over twice as large as the long-term trend. For example, in 1958, almost 10% of observations were foggy throughout the country. One year later, just 4% were. We relate the temporal variability in fog occurrence to the wintertime large-scale pressure forcing over Northwestern Europe. In years with more fog, the sea level pressure is higher, resulting in weaker winds, and more clear-sky nights. In other words, more favourable conditions for (radiation) fog formation. In years with less fog, the wintertime atmospheric low over Northwestern Europe is enhanced, resulting in stronger winds and overall less favourable conditions for fog formation.
Synoptic forcing in years with more/less fog than average. a) and b) the wintertime sea level pressure (SLP) anomaly in foggy/clear years. c) Relationship between fog anomaly and the anomaly in pressure gradient forcing. d) The result of weakened pressure forcing in foggy years: weaker near-surface winds.
Fog in the Absence of Topography
Given that the Netherlands is a relatively flat country, it was possible to investigate different regional influences on fog occurrence, without them being overshadowed by otherwise complex terrain effects. At the same time, the Dutch landscape is highly variable, with large agricultural regions interspersed with dense cities. This landscape variability leads to significant meteorological variability, including influencing nocturnal temperatures.
Given that the Netherlands is a relatively flat country, it was possible to investigate different regional influences on fog occurrence, without them being overshadowed by otherwise complex terrain effects. At the same time, the Dutch landscape is highly variable, with large agricultural regions interspersed with dense cities. This landscape variability leads to significant meteorological variability, including influencing nocturnal temperatures.
Two primary factors were identified as contributing to a location's favourability for fog occurrence: the ocean, and urbanization. The presence of the North Sea means that winters in the Netherlands are relatively mild and moist. However, it also means that the day/night temperature difference is not as high near the coast (water has a higher heat capacity than land, so it does not cool as quickly). As a result, even though there is abundant moisture near the coast, the air does not cool enough to reach saturation. Conversely, inland locations have much larger diurnal variability in temperature, meaning that saturation is more easily reached.
Urbanization influences the downwind temperature and turbulence characteristics, through, for example, the urban heat island effect (warmth from the day is stored in the buildings and concrete, and slowly released at night). We show occurrence of fog in the Netherlands to be related strongly to the overall urban fraction of a given station (i.e., how much of the surroundings are urban vs grass).
Predicting Relative Fogginess Using the Regionally Weighted Index (RWI)
Combining the ocean and urban influences, we distilled an index to indicate the relative fogginess that can be expected between two locations. We call the index the Regionally Weighted Index, or RWI.
Urbanization influences the downwind temperature and turbulence characteristics, through, for example, the urban heat island effect (warmth from the day is stored in the buildings and concrete, and slowly released at night). We show occurrence of fog in the Netherlands to be related strongly to the overall urban fraction of a given station (i.e., how much of the surroundings are urban vs grass).
Predicting Relative Fogginess Using the Regionally Weighted Index (RWI)
Combining the ocean and urban influences, we distilled an index to indicate the relative fogginess that can be expected between two locations. We call the index the Regionally Weighted Index, or RWI.
Essentially, it takes calculates the weighted mean of the area surrounding a given location that is classified as either water or urban. A station with a higher urban/ocean fraction will have proportionally less fog than a station that is surrounded by predominantly grass fields. There are two exceptions to this: Beek and Schiphol, however them being outliers can be easily understood. Beek is located in the southeast of the Netherlands in the most complex terrain of all stations, while Schiphol is the main Dutch airport, and one of the busiest in Europe, with local urbanization and the even airplanes impacting the climatology.
The figure below shows what the Netherlands looks like when viewed according to RWI, then converting RWI to a relative fogginess. Scatter points on the right-hand map show how well the relationship agrees with the observed relative fogginess. The urban centres are apparent, as are the coastal regions, for being less favourable for fog formation.
The figure below shows what the Netherlands looks like when viewed according to RWI, then converting RWI to a relative fogginess. Scatter points on the right-hand map show how well the relationship agrees with the observed relative fogginess. The urban centres are apparent, as are the coastal regions, for being less favourable for fog formation.
Further Reading
- Izett, J. G., B. J. H. van de Wiel, P. Baas, R. B. Schulte, and J. A. van Hooft (2019). Dutch Fog: On the Observed Spatio‐Temporal Variability of Fog in the Netherlands. Q J Roy Meteor Soc. DOI: 10.1002/qj.3597
- Boers, R., van Weele, M., Meijgaard, E., Savenije, M., Siebesma, A. P., Bosveld, F. and Stammes, P. (2015) Observations and projections of visibility and aerosol optical thickness (1956-2100) in the Netherlands: impacts of time-varying aerosol composition and hygroscopicity. Environ Res Lett, 10.
- Duynkerke, P. (1999). Turbulence, radiation, and fog in dutch stable boundary layers. Boundary-Layer Meteorol, 90, 447–477.