Afaik it is a strong greenhouse gas, but under normal circumstances the amount of water vapor in the global air is more or less the same so it won't matter that much.

Now we don't live in normal circumstances, but in a society that hasn't given economic value to an environment that is essential to it's survival. And so it happens like in France where nuclear energy couldn't be used anymore because there wasn't enough water in the rivers for the process anymore, during a particularly hot summer.

Essentially water vapor is one of the runaway feedback loops for cilmate change, more vapor means more greenhouse effect, means warmer, means more vapor, etc. This works until the air is saturated. Climate is driven by a bunch of those complex feedback loops and includes tipping points (e.g. particular big ice structures melting) so predicting how exactly it will pan out isn't trivial.

A side effect of more water vapor is also that storms get stronger and rain falls worse.

Water vapor is an extremely powerful greenhouse gas - however, it's presence in the atmosphere is extremely variable and so it's very difficult to measure long-term trends versus just what the weather is doing today. That's why we use CO2 as our climate change marker. CO2 is nowhere near as powerful of a greenhouse gas, but it's stable in the atmosphere for centuries. That makes it easy to track trends, versus today's weather, and you're also tracking a variable that has long-term consequences. For the record, methane is another strong greenhouse gas but like CO2, it's concentration in the atmosphere is highly variable, so we don't tend to track it.

Water vapour is a very strong infrared absorber, so much so that it is more important for the greenhouse effect than CO2. However, it is usually very difficult to change the amount of water vapour in the atmosphere as a whole (it just rains out).

The stratosphere cycles through much slower than the troposphere (there are very few clouds, for example). That means that if you put water there, it can stay there for a lot longer, increasing the total amount of water in the atmosphere and warming the climate.

I'll also say that we expect water vapour to increase as temperature increases, which is a positive feedback in the climate system,increasing the warming beyond that of CO2.this is known as the water vapour feedback.

Yes. But there's ten trillion tons of water vapor in the troposphere, so, normally this wouldn't be a relevant amount. It's key that the volcano eruption lifted the water vapor completely outside the weather layer, into the stratosphere. A marginal unit of water in the stratosphere is a lot more potent as a greenhouse gas, than the same unit in the troposphere. (I don't know the quantified values).

It absorbs a lot of the spectrum, both UV and IR.

https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_...

However clouds reflect. So high humidity will absorb ir, but thick cloud will reflect.

So its a bit more complex.