Environmental Engineering Reference
In-Depth Information
Annual
Annual
Atmosphere
Atmosphere
precipitation
precipitation
496,000 km
3
/a
496,000 km
3
/a
Evaporation
from the sea
Evaporation
from the sea
Evaporation
from the soil
Evaporation
from the soil
Precipitation
Precipitation
Surface of
the earth
Surface of
the earth
385,000 km
3
/a
385,000 km
3
/a
111,000 km
3
/a
111,000 km
3
/a
425,000 km
3
/a
425,000 km
3
/a
71,000 km
3
/a
71,000 km
3
/a
Earth
Earth
40,000 km
3
/a
40,000 km
3
/a
Fig. 2.36
Water cycle of the earth (see /2-23/)
The potential energy that the water reaches in the cloud and rain-forming layers
of the atmosphere is only utilisable to a very limited extent. For the part raining
down over the oceans, this energy is mainly converted into non-utilisable low
temperature heat. Of the proportion raining down on the continents, around 64 %
drain into the ground; the resulting potential energy can thus not be entirely used
either, as portions of the drained water only surface again at sources located fur-
ther downhill at a lower elevation.
Therefore, ultimately only the water flowing off the surface is available for po-
tential utilisation, corresponding to approximately 36 % of the total rainfall on
continents. Hence, only the potential energy resulting from the respective differ-
ences in altitude between the respective precipitation site and the sea level is theo-
retically useful. Without any technical utilisation measures, the energy stored in
waterways or lakes in the form of potential energy would be converted into ther-
mal energy at a temperature level close to ambient temperature by erosion in the
riverbed and whirling.
Precipitation.
Within the atmosphere of the earth different atmospheric layers
contain different steam volumes. This steam is converted into a visible form, if the
air temperature sinks below the dew point, and the water molecules bind to con-
densation nuclei (i.e. fine, floating aerosol particles), forming small water drop-
lets. If the temperatures are above the freezing point, precipitation occurs, if the
cloud droplets attach among each other (coalescence) and can no longer be carried
by the air current /2-24/.
Liquid precipitation is called rain. It is distinguished between drizzle with a
drop radius between 0.05 and 0.25 mm and rain with a drop radius between 0.25
and 2.5 mm. Liquid precipitation can also fall from clouds containing ice crystals
(e.g. thunderstorm clouds). On its way from the atmosphere to the surface of the
earth, such precipitation is generally transferred from a solid to a liquid phase. If,
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