We present an overview of the spectral variability of the peculiar F-type hypergiant \rho Cas , obtained from our long-term monitoring campaigns over the past 8.5 years with four spectrographs in the northern hemisphere .
Between 2000 June and September an exceptional variability phase occurred when the V -brightness dimmed by about a full magnitude .
The star recovered from this deep minimum by 2001 April .
It is the third outburst of \rho Cas on record in the last century .
We observe TiO absorption bands in high-resolution near-IR spectra obtained with the Utrecht Echelle Spectrograph during the summer of 2000 .
TiO formation in the outer atmosphere occurred before the deep brightness minimum .
Atmospheric models reveal that the effective temperature decreases by at least 3000 K , and the TiO shell is driven supersonically with \dot { M } \simeq 5.4 \times 10 ^ { -2 } M _ { \odot } yr ^ { -1 } .
Strong episodic mass loss and TiO have also been observed during the outbursts of 1945-47 and 1985-86 .

A detailed analysis of the exceptional outburst spectra is provided , by comparing with high-resolution optical spectra of the early M-type supergiants \mu Cep ( Ia ) and Betelgeuse ( Iab ) .
During the outburst , central emission appears above the local continuum level in the split Na D lines .
A prominent optical emission line spectrum appears in variability phases of fast wind expansion .
The radial velocity curves of H \alpha , and of photospheric metal absorption lines signal a very extended , and velocity stratified dynamic atmosphere .
The outburst spectra indicate the formation of a low-temperature , optically thick circumstellar gas shell of 3 \times 10 ^ { -2 } M _ { \odot } during 200 d , caused by dynamic instability of the upper atmosphere of this pulsating massive supergiant near the Eddington luminosity limit .
We observe that the mass-loss rate during the outburst is of the same order of magnitude as has been proposed for the outbursts of \eta Carinae .
We present calculations that correctly predict the outburst time-scale , whereby the shell ejection is driven by the release of hydrogen ionization-recombination energy .