The massive walls require a large and relatively long input of heat from the sun (radiation) and from the surrounding air (convection) before they warm through to the interior.
ðə mˈæsɪv wˈɔːlz ɹᵻkwˈaɪɚɹ ɐ lˈɑːɹdʒ ænd ɹˈɛlətˌɪvli lˈɔŋ ˈɪnpʊt ʌv hˈiːt fɹʌmðə sˈʌn ɹˌeɪdɪˈeɪʃən ænd fɹʌmðə sɚɹˈaʊndɪŋ ˈɛɹ kənvˈɛkʃən bᵻfˌoːɹ ðeɪ wˈɔːɹm θɹuː tə ðɪ ɪntˈɪɹiɚ.