  The observed abundance peculiarities of many chemical species relative to the
expected cluster metallicity in blue horizontal-branch (BHB) stars presumably
appear as a result of atomic diffusion in the photosphere. The slow rotation
(typically $v\sin{i}<$ 10 km s$^{-1}$) of BHB stars with effective temperatures
$T_{\rm eff}>$ 11,500 K supports this idea since the diffusion mechanism is
only effective in a stable stellar atmosphere. In this work we search for
observational evidence of vertical chemical stratification in the atmospheres
of six hot BHB stars: B84, B267 and B279 in M15 and WF2-2541, WF4-3085 and
WF4-3485 in M13. We undertake an abundance stratification analysis of the
stellar atmospheres of the aforementioned stars, based on acquired Keck HIRES
spectra. We have found from our numerical simulations that three stars (B267,
B279 and WF2-2541) show clear signatures of the vertical stratification of iron
whose abundance increases toward the lower atmosphere, while the other two
stars (B84 and WF4-3485) do not. For WF4-3085 the iron stratification results
are inconclusive. B267 also shows a signature of titanium stratification. Our
estimates for radial velocity, $v\sin{i}$ and overall iron, titanium and
phosphorus abundances agree with previously published data for these stars
after taking the measurement errors into account. The results support the
hypothesis regarding the efficiency of atomic diffusion in the stellar
atmospheres of BHB stars with $T_{\rm eff}>$ 11,500 K.
