For land surfaces, it has been shown that the albedo at a particular solar zenith angle θi can be approximated by the proportionate sum of two terms:
fɔːɹ lˈænd sˈɜːfɪsᵻz, ɪt hˈæzbiːn ʃˈoʊn ðætðɪ ælbˈiːdoʊ æɾə pɚtˈɪkjʊlɚ sˈoʊlɚ zˈiːnɪθ ˈæŋɡəl θˈiːɾəi kæn biː ɐpɹˈɑːksɪmˌeɪɾᵻd baɪ ðə pɹəpˈoːɹʃənət sˈʌm ʌv tˈuː tˈɜːmz: