Question

What is the subordinate clause in the following sentence?: He was late to school because of the snowstorm.

Answer 1

The subordinate clause in this sentence is " because of the snowstorm ".

Explanation:

Here's a definition of clause (slightly redacted) from
https://www.merriam-webster.com/dictionary/clause:

Definition of clause
A group of words containing a subject and predicate and functioning as a member of a complex or compound sentence.

The sentence "When it rained they went inside" consists of two clauses: "When it rained" and "they went inside."
[End of quote]

"When it rained" cannot stand alone: When it rained . . . what? What happened when it rained?

Again, from the Merriam-Webster site:

Definition of subordinate clause
A clause that does not form a simple sentence by itself and that is connected to the main clause of a sentence.

In the sentence "I went home because I felt ill," "because I felt ill" is a subordinate clause.

A subordinate clause has a subject and verb, but cannot stand alone as a sentence, because it begins with a Subordinate Conjunction or a Relative Pronoun, that connects (subordinates) the clause to the main one.

Check out this site for a really good list of subordinating conjunctions, relative pronouns and examples (and it's printable):

http://www.chompchomp.com/terms/subordinateclause.htm

N.B.: There is no verb in " because of the snowstorm", so it can be considered a "verbless clause".
[See: https://www.thoughtco.com/verbless-clause-1692588 ]

If your sentence above were " He was late to school because the snowstorm was heavy", then the subordinate clause would be more obvious.