Since "when rainy season" is a bit ambiguous, I have broken this guide down into three parts. You can jump to the section that best fits what you are looking for:
It is caused by shifts in global wind patterns (atmospheric circulation). when rainy season
If you are traveling during this time, here is a quick survival guide: Since "when rainy season" is a bit ambiguous,
: Declared when there is consistent, widespread rainfall and specific wind shifts (e.g., lower-level southwesterly winds in Thailand). Natural and Ecological Impacts when rainy season
If you tell me the specific place, I can provide a concise summary of its rainy season timing and any recent trends.