But that carried a risk, because it is cooler at high than at low elevations, and in an especially cool year, it might be too cold to grow crops at all.

在这句中than at low elevations可以省略吗？恢复成完整的句子应该是because it is cooler at high elevations than it is cooler at low elevations，既然成分一致，为什么不能改成because it is cooler at high than low elevations?

【答】在含有比较从句的复合句中，为了简练，从句中和主句重复的成分原则上省略掉，只剩下被比较的部分。被比较的部分应为“并列的成分或结构”，必须是对等关系。例如：

Last year there were many more births than deaths. 去年的出生人数大于死亡人数。

A man's voice is usually lower than a woman's. 男子的嗓音通常比女子的低。

Mary would rather travel by train than by plane. 玛丽宁可乘火车而不愿乘飞机旅行。

The climate is hoter here than in the north. 这儿的气候比北方热。

It is better to laugh than to weep.  笑比哭好。

你问道：为什么不能改成because it is cooler at high than low elevations? 

如果改为你说的这个句子，那么就不符合对等关系。句子变成了：because it is cooler at high elevations than low elevations

被比较的成分是：前者是地点状语at high elevations，后者是名词词组：low elevations，形成不对等关系。

就像这个句子一样：A man's voice is usually lower than a woman. ——a man's voice和a woman不是对等关系。所以判为错句。

当然，在than 从句中如果没有可省略的成分或对等的成分，则要把全句写上。例如：

His illness was more serious than the doctor first thought. 他的病较医生原来以为的更严重。

He escaped more lightly than he had expected. 他比他所预料的更轻易地逃脱了。