FIGURE 2 Pu-239/Sr-90 Ratio Just Above the Tropopause SON 2 PEP PAY 2 AEC wee 7ogs et Ju | Jy rea TT t-te Au S O N D J F Mr Ap My Ju Jy rere 60 lt *t Au S {ft OF N D J {of F Mr Ap My nia sae meSntees te + Vek Mi ate ee ee if are temples + CONTOURS ARE IN PERCENT. ARROWS INDICATE THE DATE AND RANGE (SMALL ARROWS - KILOTONS; LARGE ARROWS MEGATONS) OF ATMOSPHERIC NUCLEAR WEAPONS TESTS IN EACH HEMISPHERE.