Translation:the light humidity
High humidity was accepted without any article. Why light humidity should be with THE?
Adjectives in any language have 2 situations or purposes let's say: a) tell a status (predicative), b) describe a noun (attributive).
The predicative form is what occurs in English with the verb (to be), like saying the humidity IS light - Here, we used "is" to tell the status of the humidity, which is "light". This is predicative status. In Arabic, predicative style does not require "AL" (the) before the adjective.
The other case, attributive, is when the adjective comes attached to the noun, as when you say in English light humidity. In English, "a/an" or "the" just comes before this compound. In Arabic, attributive adjectives mimic the noun in gender, definition and number. So, if the noun is plural the adjective must be plural, if the noun is feminine the adjective must be so, and so on.
Under the light of this, the sentence above is "attributive" in nature, and since الرطوبة (the humidity) appears defined with AL (i.e. the), the attributive adjective attached to it must be defined as well: الخفيفة (the light).
Suppose I want to convert this sentence from attributive to predicative? We remove "AL" from the adjective: الرطوبة خفيفة - and the English translation of this becomes The humidity IS light instead of the light humidity.