The　presence　of　cracks　significantly　influences　the　engineering　properties　of　unsaturated　residual　soil,　particularly　under　external　air　pressure　fluctuation　(i.e.　Δua).　A　formula　is　developed　that　integrates　existing　crack　depth　theories　with　factors　such　as　soil　shear　strengths,　effective　stress　parameters,　effective　tensile　strength　reduction　coefficients,　and　environmental　changes　like　infiltration　and　evaporation.　In　addition,　a　further　equation　is　deduced　to　determine　the　surface　flow　rate　cracking　value　Q0,　which　can　reflect　the　influence　of　seasonal　climate　on　the　matric　suction　of　the　surface　soil.　Our　findings　reveal　that　crack　depth　is　quite　reactive　to　short-term　fluctuations　in　external　air　pressure,　effective　stress　parameters　and　evaporation　intensity,　with　remarkable　increases　in　crack　depth　linked　to　these　variations.　The　theoretical　depth　of　soil　crack　increases　by　0.12　∼　0.27m　when　the　external　air　pressure　fluctuation　(i.e.　Δua)　is　3　kPa.　Soils　subjected　to　climatic　conditions　crack　only　when　Q＞Q0,　and　early　cracking　of　soils　under　Δua＞0　is　conditioned　by　increasing　infiltration　and　effective　cohesion　or　reducing　evaporation.　The　variation　rule　of　the　theoretical　crack　depth　calculated　in　this　research　shows　consistency　with　the　previous　equations,　and　the　model　predictions　at　Δua　=　3kPa　are　proved　to　match　well　with　the　measured　results　of　the　practical　engineering　subjected　to　various　climate　environment　factors.　©　2025　The　Author(s).　Published　by　Informa　UK　Limited,　trading　as　Taylor　&　Francis　Group.