Mammatus under a thunderstorm anvil, showing a high degree of ordering.
Mammatus is one of the more bizarre cloud forms. Many people who see mammatus
for the first time are awe-inspired. Indeed, the mammatus with its inverted
bulges of smooth cloud, sometimes showing high ordering, is a remarkable sight.
Mammatus occurs whenever there is a conditionally unstable cloud layer above
clear air: when an air parcel from this cloud layer descends into the clear air
below, the cloud droplets evaporate, cooling down the air parcel and making it
descend even faster. A blob of cloud material is dragged down, creating the
pouch of cloud.
Where to look for mammatus
Mammatus at sunset can be spectacular. The spoked appearance is caused by solar (crepuscular) rays.
Thunderstorms are the ideal mammatus producers. The anvils of these
storms contain ice crystals, and the anvil is usually sharply bounded by clear
air below it (since right next to a thunderstorm, convection is usually
somewhat inhibited). The best place to look for mammatus is at the back
side of a storm (referring to its direction of movement); a trailing
anvil is left over and quite frequenty shows the mammatus.
Photography of mammatus requires either a super-wide angle lens, if you want to
include the landscape (because good mammatus is very high up in the sky, up to
overhead), or a zoom lens and zooming in on the mammatus without a foreground.
Mammatus is actually translucent, as this photo shows (look right from center).
If you are seeing mammatus under a thunderstorm anvil, position yourself so
that you will be on the west side of the storm after sunset, so you might see
spectacular red hues of the sunlight onto the mammatus.
Also, sometimes anvil lightning is visible in between the mammatus pouches.
Wait until twilight to capture these very nicely. You can use exposure
times up to 20 to 30 seconds before the mammatus starts to smear out
noticeably on a wide-angle photograph.
Mammatus is usually a small cloud, and subject to evaporation. This is why it is actually
quite transparent, although it may not appear so as seen from below the cloud. When you are in
an airplane and flying through mammatus, you will notice that it is actually hardly visible at close range.