Multifrequency spatially-resolved radio continuum observations of the M supergiants, α Ori (M2 Iab) and α Sco (M1.5 Iab + B2.5 V), have been obtained using the VLA A array and VLA+Pie Town configurations, to study changes in the extended (i.e. scale 1 − 10 stellar radii) atmospheres of these stars and to model the conditions in their wind acceleration regions. Strong modelling constraints on the atmospheric thermal properties are derived, because the radio emission is resolved at multiple wavelengths. Changes seen in the α Ori source flux density and radio visibility data occurring on several year timescales are described, based on observations obtained in 2002 February and April and in 1996 December. The need for multicomponent models of the plasma conditions in both the warm and cool gas around α Ori is discussed. The radio properties of the α Sco system, both of the M supergiant itself and in the H II region surrounding the B-type companion, provide important tools for estimating conditions within the M supergiant's wind.