Context : RRab stars are large amplitude pulsating stars in which the pulsation wave is a progressive wave .
Consequently , strong shocks , stratification effects , and phase lag may exist between the variations associated with line profiles formed in different parts of the atmosphere , including the shock wake .
The pulsation is associated with a large extension of the expanding atmosphere , and strong infalling motions are expected .

Aims : The objective of this study is to provide a general overview of the dynamical structure of the atmosphere occurring over a typical pulsation cycle .

Methods : We report new high-resolution observations with suitable time resolution of H \alpha and sodium lines in the brightest RR Lyrae star of the sky : RR Lyr ( HD 182989 ) .
A detailed analysis of line profile variations over the whole pulsation cycle is performed to understand the dynamical structure of the atmosphere .

Results : The main shock wave appears when it exits from the photosphere at \varphi \simeq 0.89 , i.e. , when the main H \alpha emission is observed .
Whereas the acceleration phase of the shock is not observed , a significant deceleration of the shock front velocity is clearly present .
The radiative stage of the shock wave is short : 4 \% of the pulsation period ( 0.892 < \varphi < 0.929 ) .
A Mach number M > 10 is required to get such a radiative shock .
The sodium layer reaches its maximum expansion well before that of H \alpha ( \Delta \varphi = 0.135 ) .
Thus , a rarefaction wave is induced between the H \alpha and sodium layers .
A strong atmospheric compression occurring around \varphi = 0.36 , which produces the third H \alpha emission , takes place in the highest part of the atmosphere .
The region located lower in the atmosphere where the sodium line is formed is not involved .
The amplification of gas turbulence seems mainly due to strong shock waves propagating in the atmosphere rather than to the global compression of the atmosphere caused by the pulsation .
It has not yet been clearly established whether the microturbulence velocity increases or decreases with height in the atmosphere .
Furthermore , it seems very probable that an interstellar component is visible within the sodium profile .

Conclusions :