Unless one is indicating a definite time change, one should stick to a single tense in a piece of writing / conversation.
A mistake that is regularly committed is that especially in narrations or long pieces of writing, people shift between the past and the present.
One should rather write in the perfect tense, especially while reporting.
The perfect tense, comprising the verb 'have' accompanied by the main verb in the 'en' form (e.g. have eaten, had shown, would have gone), denotes an action that is complete but without a delimiting time frame.
Thus rather than saying….
'On the outside, everything was fine. But we could see rubble and heavy smog around the building'
'On the outside, everything was fine. But we saw rubble and heavy smog around the building'