怪不得 (no wonder)

That explains the cold!

• no wonder!怪不得guài bù dé = "no wonder / that explains it" (= (it's) no wonder (that...)难怪nán guài).
• original原来yuán láito rain下雨xià yǔ(completed / change)了le，no wonder!怪不得guài bù déso much这么zhè mecold冷lěng = "oh, it rained — no wonder it's so cold."
• A reason appears, and everything clicks.

Reason，no wonder!怪不得guài bù dé + Observation

• no wonder!怪不得guài bù dé reacts once the explanation surfaces.

so it turns out原来yuán láirain下雨xià yǔ了le，no wonder怪不得guài bù déso这么zhè mecold冷lěng。

• no wonder!怪不得guài bù déso much这么zhè mecold冷lěng = "no wonder it's so cold."

Recap

Read the full reference: HSK 5 grammar — Complex sentence patterns.