Mammatus clouds
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Mammatus clouds(4500-7500m, middle clouds) are
round pouched of moisture. Mammatus clouds are formed under the unusual
conditions of warm, moist air traveling downward, a sort of reverse convection.
How to form?
When a thunderstorm cloud reaches the top of the troposphere, it quits growing vertically and spreads horizontally, creating an anvil, or flattened formation at the top of a mature cumulonimbus. Pockets of warm air continue to rise to the top of the cloud and travel horizontally along the top of the anvil. Due to the large concentration of ice crystals and water droplets suspended in these pockets of air, they are heavier than the surrounding air and begin to fall.
Effect
Contrary to popular belief, the presence of mammatus rarely signals a tornado. The formation of mammatus beneath clouds types other than cumulonimbus is a sign that thicker clouds and rain showers are either approaching or retreating.
Mammatus clouds by Jeff Harrison Ominous Mammatus Clouds at Sunset by Ronnie Wiggin