Weather

The Thailand climate is controlled by tropical monsoons and the weather in Thailand is generally hot and humid across most of the country throughout most of the year. While Thailand’s seasons are generally divided into the hot season, cool season, and rainy season, in reality it’s relatively hot most of the year. The weather in central, northern, and northeastern Thailand (the landlocked provinces) is determined by three seasons, whereas the southern, coastal regions of Thailand feature only two, making the weather in Thailand quite easy to understand and plan a trip around.

In Thailand’s inland provinces the seasons are clearly defined: Between November and May the weather is mostly dry and the cool season and hot season occur from November to February and March to May respectively.

The other inland season, the rainy season, lasts from May to November and is dominated by the southwest monsoon, during which time rainfall in most of Thailand is at its heaviest.
The southern, coastal region of Thailand really has only two seasons – rainy season and dry season. Fortunately, for those planning a beach holiday, Thailand’s two coasts have slightly different rainy seasons, allowing visitors to find sunny beaches nearly year round.
On the Andaman or west coast, where Phuket, Krabi, and the Phi Phi Islands lie, the southwest monsoon brings heavy storms from April to October, while on the Gulf of Thailand or east coast, where Koh Samui, Koh Phangan, and Koh Tao lie, the most rain falls between September and December.

Cool Season (November - February)

The weather in Thailand around the central, northern, and northeastern regions is mostly cool and dry between November and February, consequently these are the most popular months to visit Thailand.  Considering its location in the tropics however, the Thailand climate is quite warm most of the year and genuinely “cool” weather really only occurs in the northern mountains, while areas like Bangkok and Ayutthaya receive perhaps only two or three weeks of “cool” weather in late December or early January.
The southern region of Thailand really has only two seasons – “rainy” and dry, not technically experiencing “cool” weather, per se, but featuring glorious sunshine without unbearable heat, beginning in late November and continuing onto April or  May.

Hot Season (March - June)

The weather in Thailand classified as the hot season lasts from March to June when higher relative temperatures and occasional rain are the norm.  Around the inland areas, including Bangkok and Ayutthaya, this often means punishing heat and high humidity.  The temperatures in the hot season begin climbing in February and by April the unrelenting heat makes many residents eager for the upcoming rains, which begin sporadically falling around mid-April.  This is traditionally the least popular season for travelers to visit, although the weather in Thailand is still quite nice along Thailand’s coasts.

Rainy Season (July - October)

The rainy season lasts from July to October and is dominated by the southwest monsoon, during which time rainfall in most of Thailand is at its heaviest.  However, like the “cool” season, the name “rainy season” is slightly misleading.  While it certainly does rain during this season it’s more likely to consist of flash-flood afternoon downpours than a continual drizzle for days.  If you can bear the heat and humidity, the weather in Thailand is typically sunny throughout the rainy season, but when the rain comes, it’s fast and it’s furious.

Fortunately for beach lovers, Thailand’s two coasts have slightly different rainy seasons, allowing visitors to find sunny beaches nearly year round. On the Andaman or west coast, where Phuket, Krabi, and the Phi Phi Islands lie, the southwest monsoon brings heavy storms from April to October, while on the Gulf of Thailand or east coast, where Koh Samui, Koh Phangan, and Koh Tao lie, the most rain falls between September and December.  While the monsoon on the west coast brings a fairly steady season of continual rain that forces businesses outside the major tourist destinations to shut their doors for the season, the east coast storms are more similar to the north’s, generally sunny days with occasionally heavy downpours.

Overall, the southern parts of Thailand, particularly the Andaman Coast, get the most rain: around 2,400 millimeters every year, compared with the central and northern regions of Thailand, both of which get around 1,400 millimeters.

Recommended Clothing
 
Selecting appropriate clothing for visiting Thailand depends on the season and your itinerary, including your intended activities.  However, while shorts, sleeveless t-shirts, and sandals would seem most practical, Thailand has somewhat conservative dressing standards and Thais tend to look disapprovingly on those too casually dressed, particularly for those intending to visit temples or establishments catering to well-to-do Thai clientele, both of which are unlikely to allow you entry if you are dressed in such a manner.
That said, streets do flood during the rainy season, and temples and even some Thai businesses expect you to remove your shoes before entering.  So sandals are quite practical.
Loose fitting, lightweight clothing that breathes well and dries quickly is your best bet for “rainy” season garb, and a poncho and/or travel umbrella is also highly recommended gear.
A hat to protect you from the sun is a good bet year round; one that protects against sun and rain is an even better idea.
During the cool season, if you plan on visiting the northern regions, perhaps Mae Hong Son or Chiang Rai, a long sleeved shirt or light jacket is advisable.  In fact, people have died of exposure in Northern Thailand, and at 2,565 meters (8,415 ft), Doi Inthanon may yet see snow as the Thailand climate undergoes change.  Check weather conditions before your trip, and pick up some warmer clothes at the markets in Chiang Mai or Chiang Rai before heading up into the hills.
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                          Information and Picture from : Amazing Thailand