Learn Russian: Машина мчалась, а снег крутился вихрем. - The car was rushing, and the snow spun like a whirlwind.

This is the Metaphorical Instrumental of comparison. In Russian, you can compare two things by putting the noun you're comparing it to into the Instrumental case, skipping the word как (like/as). Снег крутился вихрем means the exact same thing as снег крутился как вихрь, but the Instrumental version sounds more vivid, poetic, and literary.

Why does the Instrumental form of "вихрь" end in "-ем" instead of "-ом"?

The nominative form is вихрь (whirlwind), which is a masculine noun ending in a soft sign. Masculine nouns with a soft stem take -ем (or -ём if the stress falls on the ending) in the Instrumental case. Since the stress stays on the first syllable, it becomes вихрем.

Why use the conjunction "а" instead of "и" here, since the English translation just says "and"?

The conjunction а is often translated as "but," but it also acts as "and" when shifting focus between two parallel subjects. Here, we have two different subjects doing two different things simultaneously (the car rushing, the snow spinning). Using а instead of и highlights this dual focus and creates a slight contrast, which is a very natural way to set a scene in Russian.

Both verbs here ("мчалась" and "крутился") end with the reflexive particle "-сь/-ся". Are they doing the action to themselves?