It is the temperature of air that surrounds our parcel that determineswhether it continues to rise or stabilises.
疑惑为何前面有It is？
the temperature of air that surrounds our parcel是定语从句修饰后的主语，determines是谓语，whether itcontinues to rise or stabilises是宾语从句，从结构上看无需It is啊。
另外，determines是谓语，在宾语从句里，that是在谓语后面起引导用途或者省略，而它前面有个that是为何呢？
也考虑过它是否会是关系代词，后面也是一个定语从句，使determines whether itcontinues to rise or stabilises修饰our parcel，若是如此的话，前面有It is从语法上应该就没问题了，但句意就不对了。
麻烦老师帮忙解惑，谢谢！
由于这里是强调句结构it is...that... it is和that之间是被强调的内容，去掉it is that句子仍然完整。
【It is】 the temperature of air that surrounds our parcel 【that】 determines whether it continues to rise or stabilises.
强调的部分是the temperature of air that surrounds our parcel
正是包裹周围的空气温度决定了它是继续上升还是稳定下来。
去掉it is...that...后句子就变成
The temperature of air //determines// whether it continues to rise or stabilises.
这种强调句结构是非常容易见到的，举几个例子：
It was the beautiful birds that stunned me.
It was yesterday that I saw him.
