  We present the results of fast spectrophotometry of flares on the RS CVn-type
star AR Lac with a time resolution of 34 s and a spectroscopic resolution R
$\sim $ 1300. The observations were performed on July 21-22, 2021 with the 2.0
m Karl Zeiss telescope at the Terskol Observatory. During the flares, an
additional emission appeared in the spectrum of AR Lac at wavelengths nearby Ca
II H\&K ($\lambda $ = 3933, 3968 \AA), nearby Si IV lines ($\lambda $ = 4089,
4116 \AA). Variations in these lines range from 2 to 5\%. We have estimated the
UBV magnitudes from spectrograms through a mathematical convolution of the
spectra with the filter transmission curves. The flare amplitudes in the UBV
band up to 0.8 magnitudes were discovered. A detailed colorimetric analysis has
allowed important parameters of the flares on AR Lac to be estimated: the
temperatures at maximum light and their sizes. The color-color $(U-B)$ -
$(B-V)$ diagrams confirm that all the flares at maximum light radiate as a
blackbody. The temperatures at maximum light were up to 12000 $\pm $ 300 K.
Based on our colorimetric analysis, we have estimated the linear sizes of the
flares at maximum light. The linear size of the flares at the maximum
luminosity is approximately 2\% of the radius of the star.
