This is not really about weather but linked to it, note that in the upper North of the country, there is every year, usually from late February to mid-April, episodes of ''smog'', a fine particles pollution due to deforestation fires. The air quality regularly hits unhealthy levels and wearing a N95 mask recommended. There are smartphone apps like Air Visual or websites that gives the Air Quality Index (AQI). Personally, on these dates, I use them more than weather forecast apps.
Backpackers may prefer to travel in Thailand at a time of the year when tourists are fewer and where we can negotiate prices more easily despite the inconvenience due to rains. It is a matter of temperament but generally tourists come from November to April. For a first trip to the country, we recommend you to come for dry season. Otherwise choose your destinatiosn depending on the weather (see maps of the month rains by lowest month).