Have you ever noticed clouds have flat bottoms?
Not all clouds, but the puffy little cumulus Toy Story-style clouds do often have them, especially when it's not too windy.
During the day the sun warms the ground, which then radiates heat, causing air near the ground to warm and rise.
As the warm air rises, it reaches a level of the atmosphere where it meets its dew point temperature — where the invisible water vapour in that rising air condenses to form water droplets that we can see as clouds.
As the air continues to rise, that first chunk of cloud gets pushed up as more cloud forms beneath, resulting in a cloud that's puffy on top but flat on the bottom.
That level is called the lifting condensation level (LCL) or convective condensation level (CCL), depending on the lifting mechanism.
Meteorologists can predict where that level will be based on how hot and how humid it is, as well as the temperature and pressure.
- Humid = lower lifting condensation level
- Drier = higher lifting condensation level
Clouds can have flat tops too
If the conditions are just right, the little cumulus clouds grow into big cumulonimbus storm clouds, towering up through the troposphere.
But when they hit the tropopause, the clouds are stopped in their tracks.
- Troposphere – lowest layer of the atmosphere, where our day-to-day weather happens
- Stratosphere – second layer of the atmosphere
- Tropopause – marks the boundary between the two layers
Usually, the atmosphere gets cooler and less dense as you get away from the surface, but at the tropopause there is an inversion, where the air gets warmer with height.
This acts as a cap because the rising air is blocked by this layer of even warmer, more dense air it cannot get through — creating a flat top.
Sometimes the cloud gets pushed out to the side, causing an anvil at the top of the cloud, named after the blacksmith tool of trade.
The tropopause is usually around 10 kilometres up, so it is only the big clouds that get the flat tops.
Clouds come in all shapes and sizes, so keep looking up folks — there is a wonderous world to see out there.
