云是行星表面大气层中由水或多种化学物质构成的可见的液滴或冰晶的集合体。这些悬浮的颗粒物也被称作气溶胶，被气象学的分支云物理学所研究。

地球上的云的形成是地球大气中的空气因两种过程而饱和的结果：空气的冷却和水汽的增加。当饱和度足够，降水将形成并下落到地表；幡状云是个例外，降水在到达地表前就被蒸发了。

云是地球上庞大的水循环的有形的结果。太阳照在地球的表面，水蒸发形成水蒸气，一旦水汽过饱和，水分子就会聚集在空气中的微尘（凝结核）周围，由此产生的水滴或冰晶将阳光散射到各个方向，这就产生了云的外观。因为云反射和散射所有波段的电磁波，所以云的颜色成灰度色，云层比较薄时成白色，但是当它们变得太厚或浓密而使得阳光不能通过的话，它们可以看起来是灰色或黑色的。

In meteorology, a cloud is an aerosol comprising a visible mass of minute liquid droplets or frozen crystals, both of which are made of water or various chemicals. The droplets or particles are suspended in the atmosphere above the surface of a planetary body. On Earth clouds are formed by the saturation of air in the homosphere (which includes the troposphere, stratosphere, and mesosphere) when air cools or gains water vapor. The science of clouds is nephology which is undertaken in the cloud physics branch of meteorology.