Sentence:  The constant temperature contours are much smoother than those observed at 4.3 mm by Coates ( 1959 a ) , and apparently the emission at 8 mm <WSD>is</WSD> not nearly so sensitive to differences in surface features . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)