The cloud characteristics included differences in the ease with which moist air in the tropics travels into the upper atmosphere, the speed with which raindrops fatten and the level of humidity required for clouds to form.

• 云层的特征包括热带的潮湿空气缓慢流向高层大气时出现的种种区别，也包括雨滴变大的速度与云层形成所需要的湿度。