Wind energy in the atmosphere – altitude, humidity and water vapour transport
Wind turbines extract kinetic energy from the moving atmosphere. To assess the potential impacts of this intervention, it is first necessary to determine the altitudes at which wind turbines operate and how atmospheric properties change with altitude.
Water vapour, which is usually invisible, plays a particularly important role in this context. It is by no means evenly distributed throughout the atmosphere but is heavily concentrated in the lower layers of the troposphere. This is precisely where most wind turbines operate.
The atmosphere changes with altitude
As altitude increases, air pressure, air density and, on average, temperature within the troposphere all decrease. The average temperature lapse rate is approximately 6.5 °C per kilometre. However, the actual temperature profile can deviate significantly from this average.
Warm air at saturation can contain considerably more water vapour than cold air; this is reflected in its higher absolute humidity. When moist air cools as it rises, its relative humidity therefore generally increases. Once the dew point is reached, some of the water vapour condenses, potentially leading to cloud formation.
A clear distinction must therefore be made between absolute and relative humidity:
While the absolute water vapour content generally decreases sharply with increasing altitude, relative humidity may either increase or decrease.
Most of the water vapour is found in the lower troposphere
Water vapour plays a central role in weather and the water cycle. Its vertical distribution is particularly uneven. A large proportion of atmospheric water vapour is concentrated within the first few kilometres above the Earth’s surface; above the middle troposphere, its concentration decreases sharply.
Consequently, a significant proportion of horizontal water vapour transport also occurs within the lower troposphere. Along with this water vapour, energy is transported in the form of latent heat.
When water evaporates from the Earth’s surface, energy is absorbed during the phase transition. The water vapour can then be transported over long distances by air currents. If it later condenses to form cloud droplets or precipitation, the previously absorbed latent heat is released into the surrounding atmosphere. This energy may ultimately be radiated into space.
Water transport and energy transport are therefore directly linked.
At what height do wind turbines operate?
When considering wind turbines, a distinction must be made between two different heights:
At what altitudes do wind turbines operate?
When considering wind turbines, a distinction must be made between two different measurements:
Site altitude refers to the elevation of the terrain above sea level.
Turbine height, by contrast, refers to the height of the wind turbine above the local ground level.
Modern onshore wind turbines often reach hub heights of up to 180 metres. With rotor radii sometimes exceeding 80 metres, their rotors sweep through an atmospheric layer extending from a few dozen metres to more than 200 or even 250 metres above ground level.
At high-altitude sites, the elevation of the terrain must also be taken into account. Alpine wind turbines can therefore operate in atmospheric layers several thousand metres above sea level. Extreme sites on the Tibetan Plateau are situated at elevations exceeding 5,000 metres.
However, a wind turbine’s height above the local terrain is particularly relevant when considering its atmospheric impact, as it determines which layers of the near-surface atmosphere are directly affected by the rotor.
Wind turbines interact with a moist air current
The power of the wind is derived from the kinetic energy of moving air. A wind turbine converts part of this energy into mechanical energy and subsequently into electrical energy.
Behind the rotor, the airflow therefore has a lower average speed than it does in front of the rotor. At the same time, wake flows and additional turbulence are generated. Depending on atmospheric stratification, wind speed and turbine size, this so-called wake effect can extend for dozens of kilometres.
In simple terms, horizontal water vapour transport can be regarded as the product of water vapour content and wind speed. If the wind speed of an air mass changes, its instantaneous water vapour flux will therefore also change, at least initially.
The spatial scale is crucial
However, this physical relationship does not automatically imply that wind turbines reduce the large-scale water cycle or precipitation by the same proportion.
The atmosphere responds dynamically. Air can be accelerated, mixed vertically or channelled laterally into a wind farm. Pressure gradients, atmospheric stability, terrain and large-scale weather patterns also influence how the airflow subsequently develops.
The crucial scientific question is therefore not whether wind turbines influence airflow—this is physically inevitable and well documented through wake effects—but rather:
To what extent does the large-scale extraction of kinetic energy affect the transport of moisture, heat and momentum within the lower troposphere, both spatially and over time?
This question is becoming increasingly important as wind farms grow in size and become more geographically concentrated.
The lower atmosphere deserves special attention
It is precisely the first few hundred to several thousand metres of the atmosphere that are of particular interest in this context. Several factors converge here:
The water vapour content is comparatively high, the horizontal transport of moisture and heat is significant, and this is also the region in which wind energy is harnessed for technical purposes.
Wind turbines therefore interact with the very part of the atmosphere in which a substantial proportion of atmospheric water and energy transport takes place.
This alone does not constitute evidence of a large-scale climatic effect. However, it provides a sound physical basis for quantitatively investigating not only the electrical energy generated but also the resulting changes in atmospheric airflow and in moisture and heat fluxes.
Conclusion
Wind is not only a source of energy but also a transport medium within the atmosphere. Moving air transports water vapour and, with it, considerable amounts of latent energy.
Because atmospheric water vapour is heavily concentrated within the lower few kilometres of the troposphere, and because wind turbines extract kinetic energy from the airflow within this region, there is a direct physical relationship between wind speed, water vapour transport and the use of wind energy.
The actual extent of any resulting regional or supra-regional changes to the water cycle is a separate quantitative question.
This is precisely where a comprehensive assessment of wind energy use should begin: not only the electrical energy generated must be taken into account, but also the effects on the energy and material flows within the atmosphere from which this energy is extracted.
