Mammatus cloud only occur when cumulonimbus are present. However, they can drift up to 25 miles away from a thunderstorm. Mammatus clouds usually form during warm months. In the United States, they occur more often over the midwest and eastern portions of the country, though they can and do occur more infrequently over the west and southwest. Mammatus has often been linked with the occurrence of tornadoes but the fact is mammatus clouds are harmless and do not mean that a tornado is about to form.
How mammatus cloud form? As updrafts carry precipitation enriched air to the cloud top, upward momentum is lost and the air begins to spread out horizontally, becoming a part of the anvil cloud. Because of its high concentration of precipitation particles (ice crystals and water droplets), the saturated air is heavier than the surrounding air and sinks back towards the earth.
 The temperature of the subsiding air
increases as it descends. However, since heat energy is required to melt and
evaporate the precipitation particles contained within the sinking air, the
warming produced by the sinking motion is quickly used up in the evaporation of
precipitation particles. If more energy is required for evaporation than is
generated by the subsidence, the sinking air will be cooler than its
surroundings and will continue to sink downward.
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