Learn Russian: Мы ехали медленно из-за тумана. - We were driving slowly because of the fog.

No. Потому что (because) must be followed by a full clause with a subject and a verb, like "because there was fog." Из-за translates to "because of" or "due to" and is followed directly by a noun. It is the perfect choice for pointing to an external cause like weather.

Why is the word for fog written as тумана?

The preposition из-за always requires the noun that follows it to be in the Genitive case. The dictionary form for fog is туман (a masculine noun ending in a consonant), so we add -а to form the Genitive: тумана.

Does из-за always imply that the cause was a bad thing?

Usually, yes. It is typically used for negative causes, obstacles, or frustrating circumstances (like bad weather or a mistake). If you wanted to express a positive cause (like "thanks to the good weather"), you would use the preposition благодаря plus the Dative case.

Why use ехали instead of the multidirectional verb ездили?

Ехали is the past tense of the unidirectional verb of motion ехать. It shows that the speakers were in the middle of a single, specific journey going in one direction when the fog slowed them down. If you used ездили, it would imply a completed round trip or a general habit of driving slowly.