A Fingerprint Of Human Effects On Weather And Climate: Literally! by Jos de Laat

A fingerprint of human effects on weather and climate: Literally!

This blog frequently reports about human fingerprints on weather and climate. It is a topic that also has my attention as a scientist (see for example here).

A few weeks ago there was a particular weather situation in my home country (the Netherlands) that nicely illustrates some effects of human activity on weather, and noted on the website from MeteoConsult, a Dutch commercial weather company. I noted this in an email to Roger, and he invited me to write a small guest-blog about this particular observations. So here we go.

A few weeks ago there was a particular weather situation in my home country (the Netherlands) that nicely illustrates some effects of human activity on weather, and noted on the website from MeteoConsult, a Dutch commercial weather company. I noted this in an email to Roger, and he invited me to write a small guest-blog about this particular observation. So here we go.

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The weather here in the Netherlands during the last week or so has been stable, with high pressure dominating Western Europe. In this time of year that means quiet weather and cold nights with frequent formation of fog during night. During morning the fog starts to disappear because of solar insolation, and although this may take quite some time, most of the time it results in a sunny afternoon.

This was also the case on the morning of Sunday, 11 March 2007. The satellite image above (about 10:15 local time) shows that the western and central part of the country covered by fog and low-level clouds. Note that the east-west distance of this image is about 400 km. At many locations in the north and southeast it was already sunny with clear skies. What is remarkable about the image is the odd shape of the low-cloud area: it has fingers! The interesting question is: why did these fingers occur? As it turns out, these fingers are caused by the location of urban areas in the Netherlands.

On the second satellite picture below, the location of major urban areas in the western part of the country have been added. Clearly the cities of The Hague, Leiden, Haarlem, Amsterdam an Utrecht (close to where I live) are located at the most southern parts of the cloud-free bands, and there is just the slightest wind from the southwest (indicated by the red arrow). These are also the major urban regions in this area (look here for a geographical map of the Netherlands). A coincidence? Not likely.

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During night, Earth’s surface cools radiatively. If sufficient moisture is present, the air near the surface may become saturated and fog or low clouds may start to appear. This results in a temperature inversion between the lowest atmosphere near the surface and layers above. The inversion layer prevents mixing with dryer air from above, and radiative cooling by the cloud (droplets) maintains cool surface temperatures. During daytime, when solar radiation increases and the atmosphere is heated, or when the air is more turbulent due to increased winds causing mixing with dryer layers from above, the clouds droplets start to evaporate and the clouds can disappear. If the radiative cooling outweighs the absorption of solar radiation, and mixing is insufficient, such cloud and fog areas can be very persistent, sometimes lasting throughout the day. However, in this particular case it appears that the break-up of the clouds was accelerated by the urban areas. But why?

There are two important reasons. First of all, urban areas add additional heat to the atmosphere, the so-called waste-heat effect. During this time of the year, with cold temperatures during night (close to freezing), people tend to warm their houses. In a densely populated area this can add quite some energy (heat) to the air. For example, the current amount of energy consumption averaged over the entire area of the Netherlands is nearly 4 W/m2 (National Institute for Statistics, CBS), by no means an insignificant number. For highly populated areas like cities, these values can even be much higher. And as the temperature of the air rises, clouds start to evaporate, and when the clouds disappear, solar radiation will reach the surface, further heating the air leading to evaporation of surrounding clouds.

A second process that plays a role is the presence of tall buildings in urban areas. These buildings distort the wind flow, causing enhanced turbulence (in meteorological terms: increased surface roughness) and more mixing. If the cloud/fog layer is sufficiently thin, and/or buildings are tall enough, the enhanced turbulence will mix in dryer air from above. This further accelerates the break-up of the fog and low clouds. This process is sometimes referred to as cloud-top entrainment instability.

The weather situation on this Sunday morning apparently was ideal to demonstrate the effect these processes can have on low clouds. Due to the particular location of urban areas, and with a little bit of fantasy, the fog area looks like the palm of a hand with outstretched fingers: an example of a true human fingerprint on weather and climate!

Jos de Laat, PhD, Royal Netherlands Meteorological Institute (KNMI).

With a special thanks for Casper Hootsen, Meteo Consult, the Netherlands, for noting this feature and writing an editorial about it on the Meteo Consult website, and Meteo Consult, for allowing the use of the editorial and satellite images. The satellite image is obtained from Meteosat MSG.

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