Humidity.

What is the effect of Humidity on everything e.g.  what tires to choose / how long they last / weather changes? I've almost done about 3 seasons now but haven't figured out what Humidity affects.

Good question, I'm wondering this as well. I must admit I kinda ignored it every race :P

i think it affects the tire temp which means tire ware

its the amount of water vapour in the air, therefore im guessing more chance of rain, maybe more driver strain or  harder on electronics.

You could basically make the humidity as your percentage of rain... Humidity is just the amount of water droplets in the air which is why when you have 0% humidity, It's considered as dry air while 50% is damp air... Humidity can affect a lot of things on the racetrack like temperature and weather changes...