【Infrequent】 , 【uncommon】 ,  【scarce】 ,  【rare】 ,  【occasional】 ,  【sporadic】  are comparable when they mean appearing, happening, or met with so seldom as to attract attention.

Something is  【infrequent】 which does not occur often, especially within a given period of time, or which does not recur except at very wide intervals of time or of space.

Something is  【uncommon】 which does not occur or is not found ordinarily and which therefore is singular, exceptional, or extraordinary.

Something is  【scarce】 which at the moment in mind is not easily found or which does not exist or is not produced in sufficient quantities.

Something is  【rare】  (see also CHOICE THIN ) of which but few examples, specimens, or instances are found; also, the term often carries such implications of  【uncommon】 as being exceptional or of extraordinary character.

Something is 【occasional】  which happens or is met with merely now and then.  【Occasional】 more than any of the preceding terms implies irregularity or nonconformity to a rule or law that might govern occurrences or appearances.

Something is  【sporadic】 which has no continuous existence or continuity in its manifestations and which comes into existence or occurs only in 【rare】 and, usually, isolated instances.