Tourists tend to avoid Southeast Asia in the monsoon months, for many expect it being rainy every day and all-day. They fear mosquito-born diseases and floods.
Well, in fact it's not all that bad. Rainy season can be a very rich and beautiful time in the tropes. There are many more animals active, it's all green around and it's considerably cooler than in most of the dry season.
Another positive aspect of the monsoon season is the lack of tourism; prices drop, places are not crowded, life is more relaxed.
Besides it's not raining all-day long. It's mostly rather raining for an hour or two, or there is a shower coming down, while it afterwards might be sunny again or cloudy only. In north Sumatra, let's say Bukit Lawang or at Lake Toba, one can almost set the clock for the afternoon rain, what starts usually around four o'clock. There it comes mostly as heavy pouring and continues over many hours, sometimes over the whole night. Next morning it's clear and sunny again. One can be very active over most of the day outdoors.
As a rule of thumb one can say as closer one approaches the equator, as more rain is to expect.
Southeast Asian Monsoon
Monsoon, actually the name of a wind, is altogether a complicated climate activity around the equatorial zones of the earth. It's triggered by the changing position of the sun around the equator along the annual seasons. High- and low pressure areas and temperature differences on the surface of the earth and water temperatures in the oceans cause strong winds and overregional air streams who last for months. The monsoon affects different parts of Southeast Asia in a different way. It depends on their proximity to the equator, to the open sea and to the existence or absense of mountains and mountain chains.
Considered the annual variations of the monsoon, generally spoken in Indochina the southwest monsoon starts approximately in May and lasts until September / October / November. It brings heavy rain. At the leeside of the mountains rainfall is significant less than at the luvsides. A good example for that is the Tenasserim Mountain Range in the south of Thailand. Ranong is the most rainy province in Thailand, for it get's much of the southwest monsoon which is coming from India and is continuing northeast deeply into China. As further distant an area is from the coast, as less rain it receives. In Surin, northeast Thailand, there happens a third of the rainfall only as comes down at the coasts of the Andaman Sea.
The climax of the southwest monsoon, in all-day-language called the 'rainy season', is then usually in September, might stretch into October, then ceasing. From early November on it's usually over. The four months from November to February mark the best times concerning climate/weather conditions in Southeast Asia, for it's dry, still green and less hot than in the other nine months of the year.
After a certain, shorter time of transfer it's changing into the northeast monsoon. In this time, between December and February, the wind is coming from inner Asia, namely China and brings cool and mostly dry air. In some regions as the north of Laos or Thailand (e.g. in the Golden Triangle) it can be quite cool in the evenings and particularly in the mornings and forenoons. Sometimes the wind is coming there down from the altitudes of the Tibetian Plateau in the Himalaya.
There is also a southeast monsoon streaming out of the Australian mainland. It starts dry and get's wet over the sea in the Indonesian Archipelago. The southeast monsoon is reaching in it's last outstretches maximal until Borneo, the southern tip of the Malay Peninsula including Singapore.
In the shorter transitional times between the two monsoons the speed of the wind is lower than normally. Sometimes there are heavy thunderstorms and blizzards, when a low-pressure area is hanging over a region, particularly queuing at mountain ranges.
Monsoon and Nature
Monsoon winds exist on earth since the formation of oceans and landmasses, means since the early stage of the formation of the planet earth after it's cooldown. Wildlife and natural vegetation evolved over billions of years under the influence of the local climates, here the monsoon. They are highly adapted to the rythm of rain and draught. The tropical rainforests are sometimes also called 'monsoon forests', because they are what they are due to the climatic pattern what shaped them.
Since the monsoon is a very sensitive climatic system, global warming and climate change has a clear influence on it.
Monsoon and Culture
The seasonal repeating monsoon winds who bring rain are crucial for the people's agriculture since the time of the first human settlements. Rice and vegetable cultivation is following a clear seasonal pattern aligned to rainy season and dry season. All changes in these rhythms are influencing the crop and food supplies of the people.
For the seafarers of former centuries the monsoon winds made it possible to overcome the distances between the Arabic countries and also Europe to mainly India and later to Southeast Asia (called by the Europeans in former times the 'East Indies').
Since the last few decades, increasing areas of Southeast Asia are getting sealed with concrete and asphalt. That comes together with a ruthless deforestation. Because of the destruction of the natural environment the annual rainy season causes floods particularly in wider parts of Thailand. If the disasters are very big and damaging the globalized industries, as happened in 2011, they make it into the international headlines.
Typhoons and Cyclones - Tropical Storms
Under a combination of certain preconditions heavy tropical storms can arise on the oceans. In Southeast Asia, in the Pacific Ocean, they are called typhoons, while they are called cyclones in the Gulf of Bengal / Indian Ocean. They start as a huge amount of wet, hot air over the sea, arising upwards and forming huge cloud towers. Next the air pressure under the rising cloud is collapsing, the low pressure is sucking air from the sides, triggering a storm. The rotation of the globe spins the thing around, a whirl is appearing. This effect is self-accellerating and getting very strong.
In the case that such a tropical typhoon or cyclone is clashing to a coastline it's getting slowed down rapidly, and it's energy is unloaded on the coastal region, tearing trees and buildings down and flooding the area. Particularly endangered are coastal regions of The Philippines, Vietnam and Burma/Myanmar.