<Dd> Altostratus (As) </Dd> <Dd> Altocumulus (Ac) </Dd> <Dd> Altocumulus castellanus (Ac cas) </Dd> <Tr> <Th> Low - level </Th> <Td> <Dl> <Dt> Tropospheric stratiform, stratocumuliform and cumuliform genera </Dt> <Dd> Stratus (St) </Dd> <Dd> Stratocumulus (Sc) </Dd> <Dd> Cumulus (Cu) </Dd> <Dd> Horseshoe </Dd> <Dt> Stratiform and cumuliform species </Dt> <Dd> Fractus </Dd> <Dd> Cumulus humilis (Cu hum) </Dd> <Dt> Stratocumulus variants (non-WMO terminology) </Dt> <Dd> Actinoform cloud </Dd> <Dd> Asperitas </Dd> </Dl> </Td> </Tr>

What layer of the atmosphere do stratus clouds form