brighten up

No — say "The weather brightened up.". while 'brighten' is a word, 'brighten up' is the standard phrasal verb used for weather and mood improvements.

Definition

phrasal verb. to become happier or to make a place look more cheerful and full of light.

phrasal verb. to become or make more cheerful, lively, or luminous; can be used intransitively for mood/weather or transitively for physical spaces.

simple

“The sun is starting to brighten up the room.”

contextual

“She bought some colorful flowers to brighten up her new apartment after a long winter.”

complex

“The atmosphere in the office began to brighten up significantly once the management announced the unexpected year-end bonuses.”

often used with 'weather', 'mood', or 'rooms' as the subject or object.

use this to teach the suffix '-en' (bright to brighten) and how adding 'up' adds a sense of completion or improvement to the action.

The weather brightened.

The weather brightened up.

while 'brighten' is a word, 'brighten up' is the standard phrasal verb used for weather and mood improvements.

Particles

up

Separability

optional

Pattern

brighten (+ object) + up