What are the main differences between low and high elevation tea? Why is high elevation tea generally considered better?

In Taiwan, climate plays a major role in the difference in quality and flavor between low and high elevation tea. 

At lower elevations, tea plants face a hotter climate with less fog than in the mountains. These low elevation conditions commonly produce a lesser quality tea. The tea leaves are prone to be more astringent and bitter, thus making it harder to produce a nicer tasting tea. On the other hand the frequent fog, sufficient amounts of rain, and cooler climate at higher elevations make for great growing conditions--allowing for the right chemical balance to produce exceptional tea.

The different harvesting methods used at low and high elevations also affect the tea. Low elevation tea plantations are typically flat and allow for machine harvesting, making harvesting more flexible and less labor intensive. Most tea plantations at higher elevations are too steep to allow for harvesting machines. Therefore, tea leaves must be hand picked, and in effect, make for a  higher quality tea.