Relative Humidity in Bangkok, Thailand. 1  The average annual relative humidity is 79.9% and average monthly relative humidity ranges from 74% in January to 85% in September. Nakhon Sawan, Thailand - 221.1 kms (137.4 miles) NNW. 2  Suphan Buri, Thailand - 94.8 kms (58.9 miles) NNW.