Cumuliform clouds are defined by the air in the cloud being warmer than the surrounding air. This makes them easy to identify as the cloud will have extensive vertical development – in other words, it will spread upwards more than outwards!
We know that air continues to rise in an unstable atmosphere so cumuliform cloud exists where the atmosphere is unstable. They will typically have a lumpy appearance with a flat base, so they often get called ‘cotton wool clouds’.
Cumuliform cloud is associated with heavy, short-lived rain showers that fall out of individual clouds. The rain is localised so a pilot can often avoid it by simply flying around the showers.
Strong convection is often found inside cumuliform clouds and you will encounter turbulence if you fly through large ones.
An individual cumulus cloud that has greater vertical extent than the surrounding cumulus is often called a Towering Cumulus. When these develop into large rain-bearing clouds with a dark appearance they are called cumulonimbus clouds – what we commonly call thunderstorms!
Stratiform clouds are defined by the air in the cloud being colder than the surrounding air, and therefore heavier. Typically we would expect this to cause the cloud to sink but there are often other forces at play keeping the cloud aloft and above its dew point. Since the cloud itself is trying to descend, it has little vertical development – looking more like a layer of cloud with a flat top.
Some of the forces keeping stratiform cloud aloft (such as frontal uplift, orographic uplift, or convergence) can also be strong enough to keep larger water droplets aloft. This leads to the formation of nimbostratus clouds. Nimbostratus can form in a layer several thousands of feet thick, with a dark appearance and producing continuous widespread rain or drizzle.
Stratiform cloud is associated with widespread, continuous drizzle or rain.