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Written by Amelia Bjornsdottir (she, they) beginning Sat Jan 13 21:00:45 UTC 2024.

So I’ve been pondering over the last few weeks the Köppen B-group (desert and steppe), and how I think it’s unrepresentative of the diversity of climates that are deserts and steppes.

For your perusal, Wikipedia’s rendering of the definition of the Köppen B-group.

The isotherm used to separate a h (hot) from a k (cold) climate is an annual average of 18°C. This makes some sense - this is also the isotherm separating tropical from temperate/subtropical climates. This, we will discard (similar to the more radical Trewartha system, which uses a “universal thermal scale”, and separates subtropical and oceanic climates), because we are incorporating the A, C, D and E climate diagnoses into our B-group climate diagnosis.

First, though…

Why is the desert climate called BW?

Ze Jarmans! No, but seriously, you can blame Deutschland for this one. Vladimir Köppen was a Russian-German man and his climate classification was originally promulgated in German.

The German Wikipedia page Effektive Klimaklassifikation shows us the term “Wüstenklimate” as the common name for a BW-Klimate, or BW climate. It is reasonable to guess that Vladimir Köppen was using this word as the basis. This also shows us why K for cold - kaltes. The term “wüst” also means wasteland, which is a whole misrepresentation of desert biomes.

The Köppen-Umbrellix Arid Climate Classification System

This was workshopped with someone who lives in a monsoon-affected mountainous steppe climate in the USA, specifically Fort Collins, Colorado.

It was decided that rather than stopping at using the h/k and 18°C isotherm, we would take the fact that we have a BS climate defined by under 100% of the potential evapotranspiration as precipitation (in the case of Fort Collins) or BW climate defined by under 50% of potential evapotranspiration as precipitation (in the case of Lima, Peru, near which, plants derive moisture from fog, possibly justifying a classification that isn’t BS or BW, but, perhaps, BN) and leave that aside for now. Then, instead of stopping because the precipitation is not high enough to escape B-group, we run the temperature and dry-season calculations (with absolute precipitation thresholds ignored if they render calculation impossible) for the A, C, or D climates (E climates need not apply; all of these have precipitation exceeding evapotranspiration), then lowercase the resulting climate classification and append it to our BW or BS classification.

Running that for Fort Collins gives us a Dwa subclimate. We then lowercase that, and append that to the BS primary classification, getting BSdwa (semi-arid, continental monsoon-influenced (dry-winters) with hot summers) climate.

Running the numbers for Lima gives us an As subclimate (tropical dry-summer). We then lowercase that and append to the BWn (or BN) primary classification, giving BWnas (or BNas).

Baghdad, Iraq: the hypothetical humid version of this city has a Csa (hot-summer Mediterranean-influenced subtropical) climate, owing to its punishingly hot dry summers, which becomes the subclimate, giving us a BWcsa climate.

This method can also be used to subclass ET climates. However, in this case, only C and D would be applicable, the a, b, c and d subclasses of those climates are all inapplicable, and the only place where you’d realistically get a useful subclass is Ushuaia, Argentina, which would have an ETcf climate (oceanic, without dry season, all months averaging below 10°C), which is (unlike the ET climate) hospitable to trees if there’s no permafrost. (This could probably be better written Cfd…)