云朵是可数的。这是因为每朵云都有其自然形成的边缘，而且我们常态沟通中提到cloud，脑海中也浮现它的这种自然边缘的形态。
品读例句：
【例句1】The sun went behind a cloud. (太阳落在云朵后面。)[牛津高级学者辞典]
【例句2】It was scorching and there wasn't a cloud in the sky. （天气酷热难耐，天空中一片云彩也没有。）[牛津高级学者辞典]
【例句3】The sky was a perfect blue - not a cloud in sight.（天空一片湛蓝，看不见一朵云彩。）[剑桥高级学者辞典]
【例句4】She could see the sun through the clouds.（她能透过云层看到太阳。）[牛津高级学者辞典]
【例句5】Dark clouds were gathering in the west. （西边乌云密布。）[牛津高级学者辞典]
【例句6】Dark clouds massed on the horizon.（地平线上乌云密布。）[剑桥高级学者辞典]
【例句7】Dark clouds floated across the moon. (乌云飘过月亮。)[朗文当代英语词典]
【例句8】Do you think those are rain clouds on the horizon? （你认为那些是地平线上的雨云吗？）[剑桥高级学者辞典]
【例句9】Heavy clouds had gathered over the summit of Mont Blanc. (勃朗峰顶上乌云密布。)[朗文当代英语词典]
若表达目的与云的自然边缘无关，而且感觉其无边无垠，那么cloud就不可数，如
【例句10】The plane was flying in cloud most of the way. (飞机大部分时间都在云层中飞行。)[牛津高级学者辞典]
【例句11】The sky was almost entirely obscured by cloud. (天空几乎完全被云遮住了。) [柯林斯高级学者辞典]
【例句12】There was so much cloud, we couldn't see anything.(云层太厚了，我们什么也看不见。)[剑桥高级学者辞典]
【例句13】Visibility was bad due to low cloud. (由于云层较低，能见度很差。)[朗文当代英语词典]
比如对【例句10】，飞机四周的云肯定有边界，但此时对话不关注云的边界。若果真把【例句11】的cloud改成clouds, 则容易让人想到“飞机飞过一朵朵云彩”。若是将cloud改成a cloud, 说话人则有“飞机会飞出这片云”的暗示。
对常态沟通，云朵的边界感很强，故而若没有特殊理由，那么cloud都按可数处理。
对纸张，常态关心的是它作为书写材料，这个材料可以人工切成A4大小，或撕成巴掌大小。A4纸和巴掌大的纸都有边界，但它们以及不是材料了。作为材料的paper, 它是不可数的。