CHARACTERISTICS AND FORMS OF THE URBAN CLIMATE
2.4 Humidity / Precipitation / Vegetation

In rural areas a substantial proportion of solar radiation is used to evaporate water stored in the soil and in vegetation. This amount is significantly greater than changes in temperature produced by building shadows.

Areas of vegetation – particularly forests – also carry out a sizeable filtering effect on the air. As such, forest air has between 200 and 1000 times less dust and soot particles than air in cities. Dust levels are also noticeably less in inner-city parks than in built-up areas. This underscores the high importance of inner-city green spaces for the urban climate.

A common property of all forms of vegetation is the prevention of soil capping, to which concrete climatic effects can be assigned:

Only small quantities of water can evaporate from capped, built-up surfaces. This is a very significant contributor to the surplus temperatures ubiquitous in built-up areas. The discharge factors for rain water discharge according to DIN 1986 Part 2 show that 90% of precipitation water flows off when falling on plasters with joint grouting, black covers, or concrete surfaces (compare with Chapter 6.1.4, Tab. 6/2).

The influence of non-evaporating water pockets on air warming is enunciated by the following comparison: Approximately 2250 kJ of energy are required to vaporize 1 liter of water at standard air pressure. However, the same amount of energy can increase the temperature of 100 m3 of air by 18 degrees Celsius.

Owing to the decreased water vapor pressure in warmer built areas, a strong vapor pressure gradient and corresponding potential for evaporation set in to the cooler and more humid surroundings. This leads to the so-called Oasis Effect, which holds down air temperatures at the edge of the built area while increasing the evaporation in the more humid neighboring area. To this extent, the built structures surrounding a green space produce a disadvantageous long-range effect on the green spaces spread like oases throughout a built-up area.

As an aside, it should be noted that the there is dispute as to the quantitative potential of vegetated surfaces for oxygen generation, as well as the importance of this factor for humans (ROBEL, 1975; BERNATZKY, 1985; MUERB, 1992).

One can assume that the proportion of oxygen in the atmosphere has remained constant for roughly 200 million years at about 21%. In that time period, therefore, a remarkably stable equilibrium must have existed between oxygen generation and oxygen-requiring processes. The latter have increased substantially in recent years due to technical and industrial processes involving combustion. The oxygen use of organisms and technical processes is always so quickly balanced by air exchange movements, however, that fluctuations in concentration between only 1/100 and 1/1000 percent by volume can be observed in areas where humans and plants exist. Fluctuations of this size are insignificant for oxygen-breathing animals and humans. In particular, the decisive factor for effective respiration is the partial pressure of oxygen, which changes according to air pressure. Hence it follows that larger drops in air pressure, especially as a result of changes in the weather, have a greater effect on respiration ability than slight fluctuations of the oxygen concentration in the air.

During the process of photosynthesis, chlorophyll-containing plants extract carbon dioxide from the air and release oxygen. In order to supply the annual oxygen requirements of a human being, an area of approximately 130 m2 of vegetation must engage in photosynthesis for the entire summer season, assuming a yearly oxygen production of 2 kg for every square meter of vegetation. According to BERNATZKY (1985), a 100-year-old freestanding beech tree 25 meters high bears a total of 1,600 m2 of exterior leaf surfaces, thus producing enough oxygen for 10 human beings each year.

One must take into account, however, that the oxygen produced by plants during photosynthesis is not a lasting gain for the breathable air. Nearly one-third of that oxygen is consumed during the process of plant respiration, during which plants break down organic substances with the help of oxygen. The remaining two-thirds is used up in lengthy decomposition processes to break down dead vegetation substance. An enduring gain in oxygen can only result from the long-term conservation of organic substances, which was the case for example during the prehistoric formation of coal and oil deposits. The earth"s oceans are the largest locations of oxygen production with a positive balance: Their phytoplankton supplies roughly 70% of the oxygen used on earth. The remaining 30% comes from land-based plants, especially from the great continental forests. Our green areas and forests exert only a spatially limited, stabilizing effect on the oxygen content of the air; this cannot, however, be overlooked in light of the numerous anthropological sources of combustion near the ground. The concept of city parks and green spaces as "Lungs of the City" should be avoided, however, as their function in this regard is minor.

Because of the heat-island effect, the relative air humidity in cities is lesser than in the surrounding areas, although the absolute humidity differs only slightly owing to the intake of water vapor from the countryside due to burning processes. On average, the humidity in a city is about 6% less than in the surrounding areas on a yearly basis. Particularly large differences are visible in the formation of dew (up to 65% less in cities). As an example, Figure 2/8 shows the measured humidity distribution in the area of the city of Karlsruhe on pleasant, windless summer evening (FIEDLER, 1979).

In contrast to the claims of earlier literature, the occurrence of fog in cities – limited by the strengthened heat-island effect and the substantial decrease in airborne dust – has become less than in the surrounding countryside. The average range of visibility in cities has also increased considerably.

The amount of precipitation and the number of rainy days in cities is 5 to 10% greater and the number of summer days with thunderstorms is 15 to 20% greater. During storms, the strongest precipitation occurs above all leeward from the city ((Fig. 2/8a). Intensified hail activity in German cities cannot be proven thus far. Days with snowfall are less frequent in cities than in the countryside, and snow cover disappears quicker in cities on account of the higher temperatures present (KUTTLER, 2010).