Mid-infrared 7-20 $mu$m imaging of Jupiter demonstrates that the increased albedo of Jupiters South Equatorial Belt (SEB) during the `fade (whitening) event of 2009-2010 was correlated with changes to atmospheric temperature and aerosol opacity. The opacity of the tropospheric condensation cloud deck at pressures less than 800 mbar increased by 80% between May 2008 and July 2010, making the SEB ($7-17^circ$ S) as opaque in the thermal infrared as the adjacent equatorial zone. After the cessation of discrete convective activity within the SEB in May 2009, a cool quiescent band of high aerosol opacity (the SEB zone at $11-15^circ$ S) was observed separating the cloud-free northern and southern SEB components. The cooling of the SEBZ (with peak-to-peak contrasts of $1.0pm0.5$ K), as well as the increased aerosol opacity at 4.8 and 8.6 $mu$m, preceded the visible whitening of the belt by several months. A chain of five warm, cloud-free `brown barges (subsiding airmasses) were observed regularly in the SEB between June 2009 and June 2010, by which time they too had been obscured by the enhanced aerosol opacity of the SEB, although the underlying warm circulation was still present in July 2010. The cool temperatures and enhanced aerosol opacity of the SEBZ after July 2009 are consistent with an upward flux of volatiles from deeper levels (e.g., ammonia-laden air) and enhanced condensation, obscuring the blue-absorbing chromophore and whitening the SEB by April 2010. Revival of the dark SEB coloration in the coming months will ultimately require sublimation of these ices by subsidence and warming of volatile-depleted air. [Abridged]
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