Learn Turkish: Bu mevsimde hava çok değişken, her an yağmur yağabilir. - In this season the weather is very changeable, it can rain any moment.

It is built from the verb root değiş (to change) plus the tendency suffix -gen. Because ş is a voiceless consonant, the suffix undergoes consonant mutation and becomes -ken. This suffix turns the verb into an adjective meaning 'changeable' or 'variable,' describing a strong habit or tendency.

Could I just say hava yağabilir instead of yağmur yağabilir?

Technically yes, but yağmur yağabilir is much more natural. In Turkish, precipitation is almost always expressed by combining the specific weather noun with the verb yağmak (to fall from the sky). You literally say 'rain might rain' (yağmur yağabilir) or 'snow might snow' (kar yağabilir).

Why is the aorist tense used at the end of yağabilir?

The word consists of the verb root yağ, the possibility suffix -abil, and the aorist tense suffix -ir. Combining the possibility suffix with the aorist is the standard way to express that something 'might' or 'could' happen. The aorist handles general possibilities better than the future tense, which would imply a scheduled event.

Does her an literally mean 'every moment'?

Yes, her means 'every' and an means 'moment.' However, when used in the context of anticipating an event like rain, it translates perfectly to the English phrase 'at any moment' or 'any minute now.'