I am going to start a series of the Cloud of the Week. This series contains some information from my book The Message of the Clouds and it will cover the most useful clouds for weather forecasting.
Pannus clouds are probably my favourite cloud, not because they ar e particularly attractive but because they are extremely accurate weather forecasters. Pannus clouds are grey fragmented or wispy clouds which form below the main cloud base of precipitating clouds (clouds producing rain or snow). They form as precipitation from above moistens the air until it is so damp that these dark clouds form. They can be known as virga or fallstreaks when they are in a wispy form like the photo below.
Pannus clouds are often fragmented due to turbulent winds and they appear to rapidly move and change shape. They indicate that the clouds above are already raining and that there will be rain on the ground in the next ten minutes. These clouds increase in numbers as the rain falls until they cover the whole sky. Pannus clouds form below cumulonimbus clouds and nimbostratus clouds, although they mainly form below the latter.
Pannus clouds are useful because they almost always indicate rain in the next few minutes so if you see these clouds remember to wear the waterproofs and pack up the picnic!
I hope you enjoyed the first cloud of the week and come back next week for more! If you would like more information on weather forecasting using this cloud and with many other clouds you might find my book The Message of the Clouds helpful. It contains all the information you need to forecast the weather yourself just by glancing up at the clouds as well as detail on how the local terrain can affect the weather. Find out more about The Message of the Clouds here.