sonnig
sunny
Referring to the weather: with lots of sunshine, bright and cloudless.
Für morgen ist sonniges Wetter vorhergesagt.
Sunny weather is forecast for tomorrow.

sunny
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sunny
Referring to the weather: with lots of sunshine, bright and cloudless.
Für morgen ist sonniges Wetter vorhergesagt.
Sunny weather is forecast for tomorrow.

The adjective 'sonnig' (sunny) is derived from the noun 'die Sonne' (the sun). The ending '-ig' turns the noun into an adjective and means 'like the sun' or 'with sun'.
Ich liebe diese sonnigen Frühlingstage.
I love these sunny spring days.
Like many adjectives, 'sonnig' (sunny) can stand before a noun or be used with the verb 'sein' (to be). The form before the noun is declined.
Wir hatten einen sonnigen Urlaub am Meer.
We had a sunny vacation by the sea.
You can tell whether the literal or figurative meaning is intended from the noun. If 'sonnig' (sunny) refers to the weather or a place, it is literal. If it refers to a person's character or a mood, it is meant figuratively.
Ein sonniger Tag hebt sofort meine Laune.
A sunny day immediately lifts my mood.
In its figurative, positive meaning, 'sonnig' (sunny) is often combined with words that describe a person's character or charisma. Typical phrases are 'ein sonniges Gemüt' (a sunny disposition), 'ein sonniges Wesen' (a sunny nature), or 'ein sonniges Lächeln' (a sunny smile).
Er ist trotz aller Schwierigkeiten ein Mensch mit einem sonnigen Gemüt geblieben.
Despite all the difficulties, he has remained a person with a sunny disposition.
bright / light
'hell' and 'sonnig' can both describe a pleasant situation with lots of light. 'hell' only says that there is a lot of light or that something is not dark. The light source does not matter. 'sonnig' is used mainly for weather or places with lots of sun. A room can be 'hell' even if no sun is shining into it. A day is 'sonnig' when the sun is shining. 'sonnig' can also describe a cheerful nature or mood. So 'hell' is more general, while 'sonnig' is more closely connected with sun and cheerfulness.

Morgen soll es sonnig werden, deshalb planen wir einen Ausflug an den See.
Tomorrow is supposed to be sunny, so we are planning a trip to the lake.
Nach dem Unterricht war der Klassenraum noch hell, obwohl es draußen bewölkt war.
After class, the classroom was still bright, even though it was cloudy outside.