We r going to taipei & alishan in mid May 2013. How will the weather be at that time?

Taipei will be hot and a little sweltering then, with the average temperature of 26-27C. As it would be during the plum rainy season, it may rain frequently in Taipei, smetimes maybe thunderstorm. In general, summer clothes and raincoat are needed.

As for Ali Mountain, the temperature range would be about 20-30C. While if you are in the mountain, you will feel a little cold. Therefore, besides summer clothes, you also need to take a relatively thick shirt or a coat to keep warm in case.