The top graph in the image below shows the minimum and maximum temperature during the year in the Netherlands. The bottom graph shows the corresponding duration of the day and night time.
The top graph shows the expire of the difference between the minimum and maximum temperature between winter (~ 5 degrees with a daytime of ~ 8 hours) and summer (~ 10 degrees with a daytime of ~ 16 hours). However, both graphs are not 100% in correspondence because the highest average temperature is about 1 august and the longest daytime is about 21 of June.
We can put both graphs together and the image below shows the result. It is visible that in summer the surface area of the Netherlands is accumulating so much heat that – despite of the decrease of daytime and the maximum height of the sun – the average day temperature rises during 2 month before it becomes nearly equal again to the temperature of 21 June (solstice). In other words: there is a long period of heat accumulation during summertime.
The measured accumulation of heat in summertime is the average temperature all over the country. The local measurements differ a lot, like the image below shows. The dotted lines (blue and red) show the temperature of the air, the continuous red and blue lines show the surface temperature. In other words: the emitted radiation that can be detected for example with the help of a satellite.
The accumulation of the solar radiation by buildings, etc. can be tested in a very easy way. The image below shows the top view of some houses in a row, 2 very big trees and separations between the gardens of the houses. These separations are made of concrete and the dimensions are 2,0 x 3,0 meter. The thickness of the concrete wall is 5 cm.
In August there was a sunny day – blue sky – and the air temperature in the shadow was 23 degrees Celsius (centre of a city). After nearly 2 hours sunshine at the south side of the grey concrete wall (A), the surface temperature of the 5 cm thick concrete at the other side of the wall (north side) was 34 degrees.
At the other side of the garden is another concrete separation and besides this concrete wall is a dense hedge (B). The air temperature close to the hedge (south side) was about 28-29 degrees but when the sun no longer shined upon the hedge the temperature decreased rapidly to the air temperature in the shadow (23 degrees Celsius).
However, the temperature at the surface at the north side of the concrete wall A remains higher – in relation to the air temperature – for a couple of hours, after the sun didn’t shine upon the wall any more.
This simple measurement is not an accurate measurement because of the influence of the blowing of the wind, reflecting surfaces, etc. However, it still shows the big difference between the heat accumulation of plants and the heat accumulation of concrete. And it isn’t difficult to conclude that in man made cities the local climate isn’t equal to the average climate of the whole countrry.
The image above shows the climate of a region that is about 1000 km to the south (Bordeaux, France). It is remarkable that the micro climate in the centre of a city in the Netherlands showed to be quite equal to the climate of a region in the south of France during spring, summer and autumn (click the image to enlarge).
Moreover, we cannot blame this difference in average temperature solely on the increase of greenhouse gasses in the atmosphere and the lengthen of the day time at higher altitudes. It is mainly because the original covering of the land by woods and fields is replaced by a concentration of buildings, roads, etc.
Next chapter: "The water cycle"