Context :

Aims : We study the transition region ( TR ) properties above sunspots and the surrounding plage regions , by analyzing several sunspot reference spectra obtained by the SUMER ( Solar Ultraviolet Measurements of Emitted Radiation ) instrument in March 1999 and November 2006 .

Methods : We compare the SUMER spectra observed in the umbra , penumbra , plage , and sunspot plume regions .
The hydrogen Lyman line profiles averaged in each of the four regions are presented .
For the sunspot observed in 2006 , the electron densities , differential emission measure ( DEM ) , and filling factors of the TR plasma in the four regions are also investigated .

Results : The self-reversals of the hydrogen Lyman line profiles are almost absent in sunspots at different locations ( at heliocentric angles of up to 49 ^ { \circ } ) on the solar disk .
In the sunspot plume , the Lyman lines are also not reversed , whilst the lower Lyman line profiles observed in the plage region are obviously reversed , a phenomenon found also in the normal quiet Sun .
The TR densities of the umbra and plume are similar and one order of magnitude lower than those of the plage and penumbra .
The DEM curve of the sunspot plume exhibits a peak centered at \log ( T / K ) \sim 5.45 , which exceeds the DEM of other regions by one to two orders of magnitude at these temperatures .
We also find that more than 100 lines , which are very weak or not observed anywhere else on the Sun , are well observed by SUMER in the sunspot , especially in the sunspot plume .

Conclusions : We suggest that the TR above sunspots is higher and probably more extended , and that the opacity of the hydrogen lines is much lower above sunspots , compared to the TR above plage regions .
Our result indicates that the enhanced TR emission of the sunspot plume is probably caused by a large filling factor .
The strongly enhanced emission at TR temperatures and the reduced continuum ensure that many normally weak TR lines are clearly distinctive in the spectra of sunspot plumes .