The Q-factor-the ratio between upwelling irradiance and upwelling radiance-
describes the bidirectional structure of seawater apparent optical properti
es as a function of geometry and of marine and atmospheric optical characte
ristics. A 3-yr time series of Q(n)(lambda) measurements-the Q-factor deter
mined by nadir radiance-from the North Adriatic Sea coastal waters, has bee
n analyzed. Scatter plots of Q(n)(lambda) versus sun zenith, theta (0) for
different intervals of the diffuse attenuation coefficient K-d(lambda) have
shown a consistent exponential trend. In addition, least-squares regressio
ns of Q(n)(lambda) versus Q(n)(490) have exhibited determination coefficien
ts R-2 that vary from 0.77 to 0.94 in the spectral range between 412 and 55
5 nm and R-2 = 0.50 at 665 nm. To account for these findings, an empirical
model for Q(n)(lambda) as a function of theta (0) and K-d(lambda (0)) at th
e reference wavelength lambda (0) = 490 nm is proposed for North Adriatic c
oastal waters.