An　experimental　system　of　frost　deposition　on　cryogenic-temperature　surfaces　under　natural　convection　conditions　was　set　up　and　a　series　of　frost　formation　experiments　were　conducted　under　various　conditions,　focusing　on　the　frosting　phenomena　on　horizontally-　and　vertically-placed　cryogenic-temperature　surfaces　under　natural　convection　conditions　especially　the　early　stage　frost　formation　phenomena.　The　influences　of　surface　temperature,　air　temperature　and　air　relative　humidity　were　also　carefully　studied　of　frost　formation　on　vertical　cryogenic　surfaces.　A　very　peculiar　and　important　phenomenon　for　frost　formation　on　cryogenic-temperature　surfaces　observed　is　that　liquid　air　droplets　were　first　formed　on　cryogenic-temperature　surface　if　surface　temperature　is　lower　than　-165　degrees　C　before　frost　crystals　appeared.　The　liquid　droplets　on　horizontally-placed　surfaces　may　exist　for　a　quite　long　time　and　even　form　a　continuous　film,　and　have　significant　influences　on　frost　deposition.　It　was　found　that　frost　crystal　clusters　deposited　on　liquid　air　droplets　are　not　static,　they　moves　both　as　and　relatively　to　the　droplets.　The　frost　layer　growth　on　the　vertical　surfaces　of　cryogenic-temperature　is　highly　non-uniform　and　exists　a　downward　growth　period　during　which　the　frost　layer　grows　mainly　along　the　downward　direction　of　the　vertical　surfaces.　It　was　also　found　that　under　very　low　surface　temperature　and　natural　convection　conditions,　as　the　surface　temperature　drops,　the　frost　layer　thickness　decreases.　This　is　completely　different　to　the　phenomenon　observed　on　ordinary　low-temperature　cold　surfaces.　(C)　2016　Elsevier　Ltd.　All　rights　reserved.