The hottest day will be caused by the sun staying at its highest point for the

longest period over Bangkok, and would also increase temperatures nationwide, especially in Chiang Mai and Kanchanaburi, said the department's Weather Watch Office director Dulyapol Pisalayabutr.

Meteorological Department director-general Suparerk Tansriratanawong said Thailand, except the southern region, had

been under the influence of cold winds from China and predicted the rainy season would arrive sooner than usual - and stay longer this year.

Anon Sanitwong na Ayutthaya, an expert on weather changes at Chulalongkorn University, said the world now faced the "La Nina" syndrome - unusually cold ocean temperatures in the Equatorial Pacific - which will bring more rain and a longer rainy season. La Nina is forecast to last for between 18 months and two years in Thailand and other Southeast Asian countries.