bad weather前是否用定冠词，取决于说话人是把这个 bad weather 作为一个特定的因素来看待，还是只把它当作一个抽象的现象来看待。再举几例供你参考（例句均来自词典）：

▲bad weather

Bad weather set back the construction schedule. 恶劣的天气延误了施工进度。

The plane is overdue and has been delayed by bad weather. 飞机晚点了，被坏天气耽搁了。

The game had to be abandoned due to bad weather. 由于天气不好，比赛不得不中止。

Bad weather forced us to postpone Friday’s game. 坏天气迫使我们推迟了星期五的比赛。

The growth of trees in this area is slowed by bad weather. 由于天气不好，这个地区的树木生长缓慢。

▲the bad weather

Owing to the bad weather, classes were canceled. 由于天气不好，不上课了。

The bad weather was doubtless a reason for the delay. 天气不好无疑是造成延误的一个原因。

With the bad weather, we scrapped our plans for a vacation. 由于天气变坏，我们放弃度假的计划。

They’ve had to cancel tomorrow’s football match because of the bad weather.  因为天气太糟，他们不得不取消了明天的足球比赛。

▲注意，有些语境由于明显是指抽象现象（即其泛指意味很浓），此时不用定冠词。如：

Don’t walk over the moors in bad weather. 天气不好的时候不要徒步过沼泽。

In case of bad weather, the wedding will be held indoors. 万一天气不好，婚礼将在室内举行。

They had made all kinds of provisions against bad weather. 他们做好了各种准备应对坏天气。

A red sky in the morning is said to be a sign of bad weather. 早晨天上呈红色据说是坏天气的预兆。

Bad weather was the main drawback to camping in the far north. 恶劣天气是在遥远的北方宿营的主要不利条件。

▲另外，有些语境由于明显是指特定因素（即其特指意味很浓），此时最好用定冠词。如：

We hope the bad weather won’t discourage people from coming along.  我们希望这糟糕的天气不会阻止人们前来。