A　new　apparatus　and　an　interpretation　method　simplified　from　existing　solutions　of　spherical　air　flow　in　soil　were　developed　to　measure　in　situ　air　permeability　(k(a))　of　unsaturated　soil.　By　pumping　air　with　a　controlled　volumetric　flux　(q(v))　into　a　spherical　aeration　bulb　and　measuring　the　corresponding　pseudo-steady-state　air　gauge　pressure　(Delta　p(0)),　k(a)　was　determined　based　on　the　slope　of　a　q(v)-Delta　p(0)　relationship.　The　method　was　applied　to　measure　k(a)　in　a　full-scale　landfill　cover.　The　ka　measurements　were　verified　by　finite-element　modelling　of　air　flow.　The　measurements　show　good　agreement　with　the　computed　results.　Sensitivity　analyses　reveal　that　using　a　smaller　bulb　radius　(r(0))　would　increase　the　measurement　accuracy　byminimising　the　boundary　effects　associated　with　atmospheric　conditions　and　contrasting　k(a)　between　neighbouring　soils.　Yet,　the　use　of　a　small　bulb　would　result　in　more　prominent　effects　of　preferential　flow　and　hence　substantial　overestimation　of　k(a).　For　a　small　bulb　size　(r(0)　=　0.01m),　the　measurement　accuracy　of　k(a)　could　be　within　15%　(i.e.,　2.4　x　10(-13)　to　6.3　x　10(-13)　m(2))　of　the　true　values　when　the　ratio　of　verticalto-horizontal　k(a)　is　between　0.4　and　1.