  Surges are ubiquitous cool ejections in the solar atmosphere that often
appear associated with transient phenomena like UV bursts or coronal jets.
Recent observations from the Interface Region Imaging Spectrograph (IRIS) show
that surges, although traditionally related to chromospheric lines, can exhibit
enhanced emission in Si IV with brighter spectral profiles than for the average
transition region (TR). In this paper, we explain why surges are natural sites
to show enhanced emissivity in TR lines. We performed 2.5D radiative-MHD
numerical experiments using the Bifrost code including the nonequilibrium
ionization of silicon and oxygen. A surge is obtained as a by-product of
magnetic flux emergence; the TR enveloping the emerged domain is strongly
affected by nonequilibrium effects: assuming statistical equilibrium would
produce an absence of Si IV and O IV ions in most of the region. Studying the
properties of the surge plasma emitting in the Si IV 1402.77 {\AA} and O IV
1401.16 {\AA} lines, we find that a) the timescales for the optically-thin
losses and heat conduction are very short, leading to departures from
statistical equilibrium, and b) the surge emits in Si IV more and has an
emissivity ratio of Si IV to O IV larger than a standard TR. Using synthetic
spectra, we conclude the importance of line-of-sight effects: given the
involved geometry of the surge, the line-of-sight can cut the emitting layer at
small angles and/or cross it multiple times, causing prominent, spatially
intermittent brightenings both in Si IV and O IV.
