Weather, Temp, Rainfall

Jan-May In Jan and Feb, it is quite cold as it is considered winter. Besides this, there are often rain especially during the 'plum season'. The rain usually lets up around late March - early Apr so this window of time is good to go but from 29/3 to 4/4 is considered the golden week in taiwan so expect to compete with the locals going vacationing. Also, there is the said children's day and tombsweeping. If you book early, you should be ok. Rainy season starts to come in late May so it might not be wise to go then. Jun-Sept Disadvantages to travelling in Taiwan during the summer: (1) More bugs, insects, and mosquitoes. (2) Risk of dengue fever. (3) High UV index. (4) Higher risk of dehydration (5) Typhoon season Taipei is very hot as it is in a valley surrounded by mountains that create quite a heat sink. Unlike Singapore, which is an island, there is no light breeze in Taipei to cool the city despite the heat so the city is dry and hot. Oct-Dec is good time to go. Weather not too hot or cold. A light cardigan will do the trick and little rainfall. Also, because of the cooler weather, there are less bugs and mosquitos if you are travelling into forested areas. advise from taiwanese friend

Rules, Weather and what to bring?

Rules

Today the new Changi Airport Rule is formally in place. Carry-ons should not contain liquids, aerosols & gels in containers more than 100ml and the total of such content shouldn't exceed 1 Litre.

Weather

Looking good. 90% of the areas are sunny and in the region of 22-32deg oC. But still must bring Torch and raincoat/brolly if wish to visit caves.

When To Go to Taiwan?

Spring can be clear and dry, or wet and grey, or even blighted by a sandstorm from China. Sometimes you get all three conditions in one day.
Summer (June to August) is a great time to visit the east coast, especially for outdoor activities, but be aware of typhoons, which can hit the island from June to October. Summer is not a good time for travel to the outer islands as they are crowded with tourists, and flights and hotels are difficult to secure. Try October to November when rates have dropped but the weather is still great. The cities are always hot and sticky in summer.
Autumn (September to November) is best, as conditions tend to be warm and dry, air pollution is at a minimum, and prices lower.
Avoid travelling during the Chinese New Year holiday as the entire country pretty much shuts down. Some good holidays not to avoid, however, include Lantern Festival, Dragon Boat Festival, and the Matsu Pilgrimage.
In general, July and August, Chinese New Year, and Saturday nights are considered high season and accommodation prices are often double the off-season rates. Outside touristy areas, however, there is little change in price from month to month (only perhaps weekday and weekend).

Weather

Taiwan is sub-tropical:
> the mountains can be chilly in summer (June to August)
> snowy in winter (December to February)

The dry season lasts from November to January in the north but extends to April in the south.
Summer is hot and sticky all over the low parts of the island, with drenching rains in the mountains.
Daytime temperatures in Taipei are around 33°C in summer and 19°C in winter, while the southern regions experience warmer winters with daytime highs of around 24°C (75°F).
In winter monsoon winds can sweep across the island from Central Asia and cool the air by 10°C or more in a few hours.

the above section is extracted from: http://www.lonelyplanet.com/worldguide/taiwan/when-to-go