  A photometric calibration of Kurucz static model atmospheres is used to
obtain the following parameters of RR Lyrae stars: variation of stellar angular
radius $\vartheta$, effective temperature $T_{\rm e}$, gravity $g_{\rm e}$ as a
function of phase, interstellar reddening $E(B-V)$ towards the star and
atmospheric metallicity $M$. Photometric and hydrodynamic conditions are given
to find the phases of pulsation when the quasi-static atmosphere approximation
(QSAA) can be applied. The QSAA is generalized to a non-uniformly moving
spherical atmosphere, and the distance $d$, mass ${\cal M}$ and atmospheric
motion are derived from the laws of mass and momentum conservation. To
demonstrate the efficiency of the method, the $UBV(RI)_C$ photometry of SU Dra
was used to derive the following parameters: $[M]=-1.60\pm .10$~dex,
$E(B-V)=0.015\pm .010$, $d=663\pm 67$~pc, ${\cal M}=(0.68\pm .03){\cal
M}_\odot$, equilibrium luminosity $L_{\rm eq}=45.9\pm 9.3L_\odot$ and $T_{\rm
eq}=6813\pm 20$~K.
