【Dirty】 , 【filthy】 , 【foul】 , 【nasty】 , 【squalid】 mean conspicuously unclean or impure.

【Dirty】 is the general term for what is sullied or defiled with dirt of any kind.

【Filthy】 is a much stronger term than  【dirty】 in its suggestion of offensiveness; it often suggests gradually accumulated dirt which besmears or begrimes rather than merely soils.

【Foul】 carries a still stronger implication of revolting offensiveness; it often implies an unwholesome or malodorous state resulting from the decay of putrescible matter. It may come near to loathsome or disgusting.

【Nasty】 applies chiefly to what is repugnant to a person who is fastidious about cleanliness.

Sometimes 【nasty】 is softened to a mere synonym for "objectionable, disagreeable".

【Squalid】  adds to the idea of dirtiness or filth that of extreme slovenliness or neglect.

All of these terms may imply moral uncleanness or baseness or obscenity.

【Dirty】 , however, stresses meanness or despicability.

【Filthy】 and 【foul】 imply disgusting obscenity,  【filthy】 stressing the presence of obscenity and 【foul】 , its ugliness.

【Nasty】 implies a peculiarly offensive unpleasantness.

【Squalid】 implies sordidness as well as baseness.

The first four terms are used also of weather, meaning the opposite of clear and thereby implying rainy, snowy, stormy, or foggy weather.