2004 37¢ Altocumulus Undulatus
Issue Date: October 4, 2004
City: Milton, MA
Printed By: Avery Dennison
Printing Method: Photogravure
Perforations: Serpentine die cut 11
“The moon is wading deep in snow,” is one of many sayings people used to forecast weather from the clouds.
Clouds form when moist air rises to a cooler altitude and condenses around tiny particles. Strong winds move the clouds around the world until the water falls as rain or snow.
In 1803, Luke Howard (1772-1864), a British amateur meteorologist, categorized clouds using Latin names. Modern cloud classification is based on his system.
The Latin cloud names describe their appearance. Layer-like clouds are called stratus clouds. Cumulus clouds are piled-up masses of white clouds. Cirrus clouds are curly white clouds.
Stratus and stratocumulus are low-level clouds. Mid-altitude clouds (alto-) are generally found between 6,000 and 20,000 feet. High altitude clouds (cirro-) are generally found above 20,000 feet.
Cumulus and cumulonimbus clouds may reach heights as great as 60,000 feet from their base. Beware when the top of such a cloud flattens out to the shape of an anvil. Then, it’s a thunderhead!
Once, people depended entirely on folklore to predict weather. Today, forecasters may use modern scientific instruments, but they still have to look to the clouds.