"High humidity is difficult."
Translation:اَلْرُّطوبة الْعالْية صَعْبة.
there are 2 ways of using an adjective: the first one is when you make a statement about a noun, for example "the house is big" - il-bayt kibiir. The other one is when you join the adjective to the noun, so it becomes one thing "The big house is mine" - il-bayt il-kibiir milky.
In the second case, arabic requires "al/il" in the beginning of both words, so it becomes il-bayt il-kibiir.
EDIT: I'm learning egyptian arabic, so there might be small differences in pronunciation, but the general idea is the same
Egyptian Arabic is very different from MSA in regards to more than just pronunciation, but in this particular case, the rule is the same, yes.
That said, I believe what Ayman meant is why don't we say:
رطوبة عالية صعبة
without a definite article at all, even with the noun? It's that way in English, after all. And the answer is that in Arabic, when we're speaking about something categorically, in general terms, we use the definite article, like you would in French, for example. To say "Running is good," you'd say "The running is good" in Arabic.