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) em>.