Linear dunes align in roughly the direction of strong prevailing winds. Their length is substantially greater than their width, and they lie parallel to each other, sometimes with a wavy pattern.
我觉得应该是significantly ,就是很显著的意思。Their length is substantially greater than their width,如果翻译成他们的长度事实上大于他们的宽度,就会不太合理,假如前面有提到(举个例子)人们都会以为他们的长等于宽,或者是宽度比长度还要大,那么这样的话翻译成事实上,就会正确了。