Translation:I noticed that the weather suddenly became cold.
As I understand it, the English sentence means that I noticed (in the past) that the weather was becoming cold (at that very same moment in the past). I would interpret the Esperanto sentence as meaning that I noticed (in the past) that the weather had become cold (at an earlier moment). But I'm not 100% sure about my interpretation of the English sentence, as I'm not a native speaker.
Native English Speaker here, New England version.
I'm with GastonDorron. [This is an edit -- coming back to this, I realized that GastonDorron is right.]
The English sentence happens in an indefinite time in the past, the narrative past. The action in that sentence -- noticing the cold -- happens in the narrative past's immediate present.
It's easier to understand with a little more context: "I was walking down the road. I noticed that the weather suddenly became cold." So as the narrator walks down the road, she notices the weather becoming cold; it's in her present.