when a hurricane is approaching its peak intensity.

[keannu]

This sentence sounds vague. Is it a present tense or a future one? I think the latter.

gz12)The presence of lightning may be a clue to help forecasters understand when a hurricane is approaching its peak intensity. Although satellite and radar technology are generally able to predict the path of a storm with some accuracy, how much and when a storm will intensify is harder to forecast...

[emsr2d2]

Neither. It describes a habitual action. Every time there is a hurricane, the presence of lightning helps the forecasters understand when it is reaching its maximum intensity.

[keannu]

My grammar book says the difference between the present tense and the present progressive tense is like the following.

1.present tense - timeless things(past+present+future, habitual or repetitive things), future, etc
2.present progressive tense - things happening at the moment, recent things, future, etc

Based on this, "when a hurricane approaches its peak intensity" sounds more proper, but there seems to be no difference between the two as a theory is only a theory. What do you think?

[5jj]

What do you think?

I think that your grammar book is rather over-simplifying the situation.