"لا أُحِبّ اَلْطَّقْس اَلْمُشْمِس."
Translation:I do not like sunny weather.
Not true. I live in a tropical place and I'm tired of sunny weather. We always have sunny days here (it rains sometimes, but even when it's raining, the weather is hot, it never gets cold) and the temperature is always between 25°C and 33°C the whole year. Lots of tourists come here for our "perfect" weather, but I'm tired of it, I really enjoy going to cloudy and cold places with rain and snow to spend a long time there.
It is in Arabic, but English idiom can say it without the definite articles to refer to sunny weather in general. I'm very much a beginner, but to attempt to assist, here's a discussion regarding Labanese Arabic: https://www.lebanesearabicinstitute.com/arabic-definite-article/ Also, here's a discussion about a similar situation in DL: https://forum.duolingo.com/comment/34807995/I-like-hot-weather
Because this is an attributive adjective. An adjective that describes the noun, and does not tell some information or completes the sentence. Attributive adjectives in Arabic follow the noun in its status: definition, gender, number. Thus, since الطقس (the weather) is defined, then the adjective afterward which describes it must be defined (and masculine since طقس is a masculine noun): المشمس.