Engineering task and calculation objective
The TAL module determines the atmospheric stability class (dispersion category) according to TA Luft 2005 — the German Technical Instructions on Air Quality Control — for dispersion modelling of gases and dusts in the atmosphere. The dispersion class (stability class after Klug/Manier) describes the turbulent state of the atmosphere near the ground and is the central meteorological input of every air quality impact assessment: it governs how strongly a flue gas or dust plume spreads laterally and vertically relative to the wind direction.
Anyone who wants to run a dispersion calculation to TA Luft, determine a stack height, or prepare meteorological time series for a permitting procedure must derive the dispersion class from simple observed quantities. The module implements the TA Luft classification scheme: from the wind speed at 10 m height, the total cloud cover in eighths (oktas), and the date and time of the observation (with sunrise and sunset times to distinguish day from night), it first determines the dispersion class from the basic scheme and then corrects it using the modification rules — for example when the cloud cover consists exclusively of high clouds.
The calculation is used throughout plant engineering and environmental technology wherever emissions from stacks, exhaust chimneys, or diffuse sources are assessed — from site planning through permitting under the German BImSchG (Federal Immission Control Act) to the re-evaluation of existing sources.
Standard and calculation basis: TA Luft: 2005
Calculation workflow
- Record the basic meteorological data: The inputs are the wind speed at 10 m above ground and the total cloud cover of the sky in eighths — the two quantities that span the basic classification scheme of TA Luft.
- Assign day or night conditions: From the date, the time (CET), and the sunrise and sunset times, the module assigns the observation to a daytime or nighttime situation; atmospheric stability differs fundamentally between solar heating during the day and radiative cooling at night.
- Dispersion class from the basic scheme: Following the TA Luft classification scheme (Klug/Manier), the dispersion class is determined from wind speed, cloud cover, and time of day — ranging from very stable (class I) to very unstable (class V).
- Modification of the class: The special rules of the scheme are checked, including the case where the total cloud cover consists exclusively of high clouds. The applied modification code is reported and the modified dispersion class is output as the final result.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Höhe | Höhe | kn |
| Achteln | Achteln | – |
| Anemometerhöhe | Anemometerhöhe | kn |
| Anemometerhöhe | Anemometerhöhe | m/s |
| Anemometerhöhe | Anemometerhöhe | m |
| Windgeschw. | Windgeschw. | m/s |
| Schornsteinbauhöhe | Schornsteinbauhöhe | m |
| Exponent | Exponent | – |
| uH | uH | m/s |
| Wärmestrom | Wärmestrom | MW |
| Abgases(f) | Abgases(f) | m³/s |
| Abgases | Abgases | °C |
| Abgasfahnenüberhöhung | Abgasfahnenüberhöhung | m |
| maximal | maximal | m |
| x | x | m |
| x-maximal | x-maximal | m |
| Quellhöhe | Quellhöhe | m |
| uh | uh | m/s |
| sigmay | sigmay | – |
| sigmaz | sigmaz | – |
| F | F | – |
| G | G | – |
| f | f | – |
| g | g | – |
Frequently asked questions
What do the dispersion classes I to V after Klug/Manier mean?
The classes describe the thermal stratification of the atmosphere: classes I and II stand for stable stratification (typically clear, low-wind nights with little vertical mixing), classes III/1 and III/2 for neutral conditions (overcast or windy), and classes IV and V for unstable stratification under strong solar radiation with intense turbulence. Stable classes lead to narrow, far-reaching plumes with high ground-level concentrations at large distances, while unstable classes produce strong dilution close to the source.
Why are sunrise and sunset times required?
The classification scheme distinguishes between daytime and nighttime hours, because the same combination of wind speed and cloud cover yields a different stability during the day (incoming radiation, unstable tendency) than at night (outgoing radiation, stable tendency). The transition periods around sunrise and sunset are treated separately, so both times must be related to the observation time.
Is the procedure still valid under TA Luft 2021?
TA Luft 2021 has superseded TA Luft 2005 and refers dispersion calculations to Annex 2 in conjunction with the VDI 3783 series of guidelines; the Klug/Manier classification scheme, however, is retained there as the meteorological basis. For re-calculations and the assessment of existing expert reports prepared under TA Luft 2005, the basic scheme implemented here remains authoritative; for new assessments, the currently required calculation method (AUSTAL) should be observed.
What are the typical sources of error when determining the class?
The most frequent errors are a wind speed not referred to the 10 m measurement height (values from other heights must be converted), confusion between CET and daylight saving time, and ignoring the special rule for high clouds: cirrus clouds attenuate incoming and outgoing radiation much less than low cloud cover, which is why the cloud cover is rated differently in that case.