Njengokuba umthi ukhula, ungavelisa izangqa ezibonisa ukukhula njengoko ukhuni olutsha lungqengqe phantsi lujikeleze oludala. lungakhula lude lubeneminyaka eliwaka. kwindawo apho kukho imozulu etshintsha-tshintshayo, ukhuni oluveliswayo ngamaxesha onyaka ahlukileyo izangqa zingamane zisiba bukhanya ziohinde zibemdaka. Apho imozulu ishushu, nakwimozulu ekwiingingqi ezikufuphi neEquator enexesha lonyaka elimanzi-nelomileyo ngokutshintshatshintshana kwazo, izangqa zokukhula komthi zibakho ngonyaka, ipere nganye yezazinge ezibukhanya nezimdaka ngombala ibonakalisa okokuba umthi unonyaka omnye ubudala. Kwiindawo apho kukho amaxesha amabini ngonyaka apho kubamanzi okanye kome, kusenokuba kho iipere ezimbini zezazinge ezibukhanya nezimdaka ngebala ngonyaka; kwaye kwezinye (kwiingignqi ezizinkqantosi yimbalela apho imvula ihlala-ihlale ibekho), kungakho isazinge esitsha sokukhula kwimvula nganye.