It's kind of a zig zig. Look them up and you'll see.

Noctilucent clouds can be seen on clear midsummer evenings that form on the border between earth and space. So they are the highest.

Noctilucent clouds have been documented at heights of 76 to 85 kilometers (47-53 miles). http://science.nasa.gov/headlines/y2008/25aug_nlc.htm

Noctilucent clouds are the highest and least understood.

Cirrus and cirrostratus are the high-type clouds. Any high cloud will have the prefix "cirr".

With the exception of rare stratospheric noctilucent clouds, cirrus forms the highest cloud layer (also cirrostratus or cirrocumulus). In the tropics (where the troposphere is thicker, that is tropopause is higher) cirrus can generally occur up to 50-60,000 feet (15-18000 metres), sometimes higher when associated with tropical cumulonimbus anvils.

Note that the possible height of cloud varies with location and airmass, so the highest cloud can occur in the tropics, the lower heights (with cirrus possible at 10,000 ft or 3500 m) in polar regions.