What I'm saying is that it should only be used as a starting point. Sticking to the classification and trying to change the model to get an outcome you want is unnecessary when you can just directly change the climate to whatever you want it to be.
I think I have to agree with zerodv here, I'm not sure if this completely makes sense. Data-based climate classification systems are valuable because they are objective. You can look at how they map out onto areas you are familiar with, and are therefore able to make predictions about any area in the world. A "make it up as you go along" system loses that kind of applicability.
Here's a specific example. In the Köppen system, southern Tunisia is hot desert, yet in real life, this region was inhabited and had cities in it. There are two ways to reconcile this, we could either conclude that the population here was entirely sustained by imports or oases, or we could adjust the definition of desert we're using to be more strict. It pretty much has to be one of those things, either there's a factor we aren't thinking about, or the categories are flawed.
In the WBP system, a new semidesert category is added for regions such as this which are dry but not so dry that they are barren. Adding in this zone allows us to make predictions about human settlement patterns in the rest of the world too, and indeed I would argue that WBP's system also predicts human habitation in Central Asia better as well.
Another example is the North China Plain, there is a region is western Hebei that is marked by many classification systems as steppe, but in real life it seems not to have been naturally vegetated as steppe, nor is it less populated than nearby regions. We could either conclude that it was sustained by irrigation, or something about our semi-arid classification is wrong, or we are using data that's inaccurate for the history we are concerned with. A "just paint that area specifically as continental" approach is tempting, but inelegant.
The length of the growing season is just a count of the number of days warmer than 5 C while GDD takes into account how warm those days get - a day with a mean temperature of 15 C adds 10 C to GDD and one with only 10 C adds 5 C while days with mean temperatures of 10 C and 15 C contribute equally to the length of the growing season.
The -15 C winter would kill off some species of plants that would survive the -5 C winter. Snow can also act to insulate and protect plants from harsh cold.
For an army trying to operate in winter, a colder winter would cause more attrition and drive up demand for food and fuel. Lots of snow would have more of an effect on mobility.
An issue with the Worldbuilding Pasta model is that is trying to predict the vegetation that would naturally grow in a climate, which is usually dominated by perennials. An important consideration for climate in PC is how well can you grow crops for food. Most important food crops are annuals which need to have a certain length of the growing season to complete their life cycles. Whether conifers can grow is not as important a question as can a crop of hardy grains grow.
Yeah, this is an excellent observation. For instance, WBP's distinction between subtropical and temperate forests, which is meant to replicate the broadleaf evergreen to deciduous transition, is based on conditions outside the growing season, and is therefore of little relevance to human agriculture.
Personally, it seems to me that we should use a classification system based mostly on conditions during the growing season, its length and temperature, which WBP did invent his own system for. However, there are still other factors we should take into account. For example, as humans are concerned, tropical diseases (which are to some extent determined by minimum temperatures) are extremely important. The same goes for winters, as I've argued.
