All these words apply to what is obscure and confused or shows a lack of clarity. Even when used in an abstract way, the closely related foggy and cloudy can sometimes allude to distinctions between a fog and a cloud. The former is low-lying, enveloping, diffuse, and indefinite in configuration; the latter, partly because it is usually seen from a distance, is typically thought of as being a denser, more remote, and more clearly defined mass that may affect but does not impinge upon the viewer. Similarly, foggy can refer to an enveloping obscurity or confusion in a thinker’s mind: foggy generalizations without any substantiation in fact. By contrast, cloudy can suggest remoteness, lack of relevance, or uncertainty that is external to the thinker or speaker: The prospect for passage of the bill was somewhat cloudy , in view of its evident unconstitutionality. As in this example, the word often refers to a future outcome; foggy is more apt for present confusion: cloudy eventualities; I haven’t the foggiest notion what you’re talking about. Both, of course, can serve as literal description: a foggy day; a cloudy day. Cloudy can also be used to describe milky or unclear liquids: cloudy tap water that clears after a few minutes.

Literally, turbid means muddy or unsettled, and applies particularly to suspensions of foreign particles within a liquid, such as particles of dirt in water. Whereas cloudy in this sense emphasizes a misty appearance, turbid stresses the presence of foreign material that pollutes, muddies or unsettles the basic medium. Turbid , however, is probably more commonly used in figurative than literal senses. It may mean thick and dense, like heavy smoke or fog: a turbid smoke screen of deceitful hints and outright lies. It may mean confused or impure, suggesting a chaotic intermixture of incompatible elements: The stream-of-consciousness technique in fiction has been likened to a turbid confluence of fact, passion and ratiocination.

Murky and opaque suggest lack of light. Murky can mean oppressively dark or, by extension, unclear and confused, as if seen through a mist. Opaque , which in its basic sense means impervious to light, suggests denseness and relative impenetrability: opaque writing, filled with erudite but inept allusions; a murky exegesis of an arcane work, full of dark, involuted clues to which only the author has the key.