Almost all clouds – and all of the rain, snow and storms – occur in the troposphere, the lowest level of the atmosphere, which extends about six to eight miles above the Earth's surface.
Above that is the stratosphere, and even higher up is the mesosphere, which extends from about 30 to 50 miles above the Earth's surface.
For the past 10 years, NASA's Aeronomy of Ice in the Mesosphere (AIM) mission has been studying the mesosphere. It is an extraordinarily clean and dry region. But meteors streaking into the upper atmosphere burn up and can leave meteor dust. What little moisture there is in the mesosphere can sometimes crystallize onto that meteor dust. Those ice crystals can then show up as noctilucent clouds.
The photo above, taken in Alberta, Canada, shows normal water clouds in the troposphere, as a dark layer near the horizon. But above that, glowing in the mesosphere, are brilliant noctilucent clouds.
I once saw a luminous display of these clouds on an overnight trip to Europe (that's why I always want a window seat when flying). Since they are so high in the atmosphere, they still glow even several hours after sunset.