But because of the fact that the missions to Tang China were commanded by the emperor, they had to leave Japan in June or July, the months during which weather conditions were bad (and had to arrive at the capital of Tang within December in order to be present in the ceremony in which the Emperor received New Years congratulations) and had to return to Japan in a season in which weather conditions were severe.
しかし、遣唐使は朝貢の使いであるという性格上、気象条件の悪い6月から7月ごろに日本を出航（元日朝賀に出席するには12月までに唐の都へ入京する必要がある）し、気象条件のよくない季節に帰国せざるを得なかった。