create atmospheric étages

[raissameyer]

crosslink to #998

Most of Earth's clouds are located in the troposphere. The troposphere expands from Earth's surface to the tropopause and its height is about 8 km at the poles and 18 km at the equator (numbers vary depending on the source).

According to WMO, the troposphere itself can be divided into three atmospheric levels or étages. Occurrence in a certain atmospheric level or étage is a useful quality to define cloud genera.

I would like to add the three atmospheric levels (étages) to ENVO. However, there are some inconveniences to defining the atmospheric levels:

1. Depending on where you are on Earth, the extend of the middle and high atmospheric levels may differ (the extend of the low atmospheric level stays the same)

2. The middle and high atmospheric level seem to overlap

   Each level is defined by the range of heights at which clouds of certain genera occur most frequently. The levels overlap and their limits vary with latitude

   Low atmospheric level

   • polar region: From the Earth’s surface to 2 km (0 – 6 500ft)
   • temperate region: From the Earth’s surface to 2 km (0 – 6 500ft)
   • tropical region: From the Earth’s surface to 2 km (0 – 6 500ft)

   Middle atmospheric level

   • polar region: 2 – 4 km (6 500 – 13 000 ft)
   • temperate region: 2 – 7 km (6 500 – 23 000 ft)
   • tropical region: 2 – 8 km (6 500 – 25 000 ft)

   High atmospheric level

   • polar region: 3 – 8 km (10 000 – 25 000 ft)
   • temperate region: 5 – 13 km (16 500 – 45 000 ft)
   • tropical region: 6 –18 km (20 000 – 60 000 ft)

[raissameyer]

Following the definition on WMO, I would like to propose the following definitions for the low, middle and high atmospheric levels for further refinement:

NTR: low atmospheric level Parent: troposphere Definition: A part of the troposphere which extends from the Earth's surface to 2 km (0 - 6500ft) height. Source: https://cloudatlas.wmo.int/en/useful-concepts.html#levels Synonym: low étage

NTR: middle atmospheric level Parent: troposphere Definition: A part of the troposphere which extends from 2 km to 4 km (6 500 – 13 000 ft) height in the polar region, from 2 km to 7 km (6 500 – 23 000 ft) height in the temperate region, and from 2 km to 8 km (6 500 – 25 000 ft) height in the tropical region. Source: https://cloudatlas.wmo.int/en/useful-concepts.html#levels Synonym: middle étage

NTR: high atmospheric level Parent: troposphere Definition: A part of the troposphere which extends from 3 km to 8 km (10 000 – 25 000 ft) height in the polar region, from 5 km to 13 km (16 500 – 45 000 ft) height in the temperate region, and from 6 km to 18 km (20 000 – 60 000 ft) height in the tropical region. Source: https://cloudatlas.wmo.int/en/useful-concepts.html#levels Synonym: high étage

I would add these definitions as a comment, as these are not the defining features of the levels but the result of whatever it is that defines the levels (I assume temperature, density, etc. but have not yet found clear information about this). To explain this I would add an editors note and would like to eventually add a proper definition. This approach of adding the terms with imperfect definitions will allow me to already use the atmospheric levels in defining the cloud genera.

[raissameyer]

The upper limit of the low atmospheric étage coincides with the upper limit of the planetary boundary layer (synonym atmospheric boundary layer already in ENVO).

Some sources state that the planetary boundary layer extends down to the Earth's surface [ref, ], while others state that there are three other layers below it: the laminare Unterschicht, bodennahe Grenzschicht (Geiger-Schicht), and bodennahe Luftschicht (Prandtl-Schicht) [ref].

I'll go ahead and include the atmospheric boundary layer in my definition for the lower atmospheric level.

Beside that, my research so far has shown that the altitude ranges of the atmospheric levels are a matter of convention (the space above the atmospheric boundary layer is usually called "free troposphere", ref).