The atmosphere is the layer of thin gases that surrounds the earth’s surface held by gravity forces. Depending upon the height of the earth’s surface, atmosphere layers are classified into different regions
- Troposphere (0-10km)
- Stratosphere (10-30km)
- Mesosphere (30-50km)
- Thermosphere (50-400km)
- Exosphere (>400km)
The temperature and pressure of the atmosphere vary with respect to the height as we go up from the earth’s surface.
The concentration of gases decreases with increasing altitude which means that the mass of gases is lower in the upper atmosphere which can exert pressure. So the pressure of the atmosphere decreases with increasing height.
whereas the temperature shows a different pattern in the atmosphere layers
In some regions temperature increases with the change in height. This is due to different heat sources in these height regions.
In the thermosphere region, the temperature increases, as this is the top of the atmospheric region where gases directly absorb solar and cosmic radiation.
in the stratosphere, the temperature rises as the ozone layer absorb ultraviolet radiation from the sun, and finally, from the troposphere, the solar radiations are absorbed by the earth’s surface. this heat then conducts up in the atmosphere.
Environmental Lapse rate
The rate at which temperature changes over height is known as the environmental lapse rate. It is considered to be positive if temperature decreases over height.
- The lowest layer of the atmosphere (Lowest 10-12 km or so)
- Everything that you and I call “weather” happens in the troposphere
- ELR > O because temperature decreases w.r.t. height
- The top of the troposphere is a LEVEL called the tropopause.
- Just above the tropopause (10-50 km)
- Temperature increases w.r.t. height, so ELR < 0.
- We call layers in which ELR < 0 “temperature inversions” or just “inversions”.
- Contains the ozone layer.
- Very smooth and has very little turbulence.
- The top of the stratosphere is the stratopause.
- Just above the stratopause (50-90 km)
- Temperature decreases w.r.t., so ELR > 0.
- Not much interesting happens up here.
- Topped by mesopause.
- Temperature increases w.r.t., so ELR < 0 (it is an inversion).
- Temperatures are VERY hot because of the absorption of radiation from the sun, but there are very few molecules up here- if you were up here, you would FEEL cold!
Region of interest for Wind Energy
The structures discussed above are interesting but even tropopause is quite high for wind energy harvesting. For wind energy power production, turbines are installed close to the ground which lies in the lowest region of the troposphere called as planetary boundary layer or atmospheric boundary layer.
The boundary layer is the part of the atmosphere that interacts with the ground. it has a diurnal cycle of properties like temperature, wind, humidity, air quality, etc
The top of the boundary layer is between 1km to 4km.
The region above the boundary layer that lies in the troposphere is called a free atmosphere which does not directly experience the properties of the earth’s surface like the boundary layer.
Boundary Layer Parts
The boundary layer is divided into two parts
- Surface layer
- Ekman layer
- This is where the log wind profile is valid
- The height varies over the course of the day, but the top is typically between 50 and 200 m a.g.l.
- Often important for wind power.
- Between the top of the Surface Layer and the bottom of the Free Atmosphere.
- Meteorologists would call this the Mixed Layer (during the day) or the Residual Layer (during the night).
- Wind turbines may extend into this part of the Boundary Layer.