Clouds of the Day - April 17, 2020
/Solar heating warms the Earth’s surface. Sunny days warm the ground more than cloudy days. As air temperatures warm, currents of rising and sinking air develop. These currents create a natural ventilation system as warm air rises and cooler air aloft sinks. The system works because of gravity - denser air sinks while less dense air rises. If moisture in the air is sufficient clouds will form. Otherwise the motions may still be there but we don’t see them. Turbulence in clear air is an example of vertical motions that are invisible but we can feel when flying.
Cumulus clouds reveal how rising currents can create puffy clouds. When lower level air temperatures are cool the warm sunshine warms the air, making it less dense. It rises while denser cooler air aloft falls to the surface. In the photos below, the lower cumulus clouds are formed by this less dense rising air. The higher altocumulus clouds in the photos form when an entire layer of air aloft is rising due to upper level upward forcing. This forcing is much different than what is happening in the lower levels. Changes in wind direction and speed, in both horizontal and vertical directions, cause the up an down motions at higher levels. The vertical motions at higher altitudes are usually much gentler than the motions that form near the surface. Examples of both types of motions are found in the photos from today.
There is much more going on in these photos. I will explain in later posts.
Altocumulus above Cumulus
Altocumulus - cumulus in the mid-levels
Altocumulus above cumulus.
Altocumulus above cumulus
Altocumulus
Cumulus
Later in the morning and early afternoon the cumulus clouds grew as temperatures warmed.
Cumulus (congestus)
Cumulus
During the early afternoon the cumulus clouds expanded and merged into a solid cloud deck.
Cumulus (Mediocris foreground; and congestus in background)
Cumulus merge to create overcast conditions
Cumulus and Stratocumulus
Cumulus and Stratocumulus
Here is what the cumulus cloud field looked like on National Weather Service radar from Des Moines, Iowa. The sky looked like there was precipitation falling aloft but it was not reaching the ground due to evaporation
Radar image is a screengrab from The Radarscope radar display program.
The solar radiation chart shows how cloud cover changed during the day. The profile for today (17th) shows sunrise and a steady climb in radiation as the Sun moved higher in the sky. Clouds were causing variations in the radiation. As clouds moved over the sun radiation decreased. This creates the jagged line in the graph. More extensive cloudiness moved in after lunch with large variations during the afternoon. See the pictures above to see how the clouds varied during the day.
