▁a ▁tur bine , ▁then , ▁increases ▁the ▁expected ▁wind ▁spe eds ▁by ▁ 1 0 % ▁and ▁the ▁expected ▁power ▁by ▁ 3 4 %. ▁To ▁avoid ▁bu ck ling , ▁dou bling ▁the ▁tower ▁height ▁generally ▁requires ▁dou bling ▁the ▁diameter ▁of ▁the ▁tower ▁as ▁well , ▁increasing ▁the ▁amount ▁of ▁material ▁by ▁a ▁factor ▁of ▁at ▁least ▁four . ▁ ▁At ▁night ▁time , ▁or ▁when ▁the ▁atmosphere ▁becomes ▁stable , ▁wind ▁speed ▁close ▁to ▁the ▁ground ▁usually ▁subs ides ▁whereas ▁at ▁tur bine ▁hub ▁alt itude ▁it ▁does ▁not ▁decrease ▁that ▁much ▁or ▁may ▁even ▁increase . ▁As ▁a ▁result , ▁the ▁wind ▁speed ▁is ▁higher ▁and ▁a ▁tur bine ▁will ▁produce ▁more ▁power ▁than ▁expected ▁from ▁the ▁ 1 / 7 ▁power ▁law : ▁dou bling ▁the ▁alt itude ▁may ▁increase ▁wind ▁speed ▁by ▁ 2 0 % ▁to ▁ 6 0 %. ▁A ▁stable ▁atmosphere ▁is ▁caused ▁by ▁radi ative ▁cool ing ▁of ▁the ▁surface ▁and ▁is ▁common ▁in ▁a ▁temper ate ▁climate : ▁it ▁usually ▁occurs ▁when ▁there ▁is ▁a ▁( part ly ) ▁clear ▁sky ▁at ▁night . ▁When ▁the ▁( high ▁alt itude ) ▁wind ▁is ▁strong ▁( a ▁ 1 0 - meter ▁wind ▁speed ▁higher ▁than ▁approximately ▁ 6 ▁to ▁ 7   m / s ) ▁the ▁stable ▁atmosphere ▁is ▁dis rupted ▁because ▁of ▁fr iction ▁tur bul ence ▁and ▁the ▁atmosphere ▁will ▁turn ▁neutral . ▁A ▁day time ▁atmosphere ▁is ▁either ▁neutral ▁( no ▁net ▁radiation ; ▁usually ▁with ▁strong