Curious about rainbows, haloes around the sun and moon, sun pillars, iridescent clouds etc, then I thoroughly recommend Atmospheric Optics. A fine site, which used to be known as Sun Dog, has not only pictures of strange atmospheric optical phenomena but also good explanations of what causes them.
With the exception of Noctilucent Clouds, everything you see here is visible during the daytime, and requires no optical equipment - just luck to be in the right place at the right time!
As I have them will add photos of things I have seen, in order of increasing height...
The mesosphere sits above the Stratosphere and here temperatures are very low (think -150 degrees Celsius) and the pressure is around 1 millionth of that at sea level. It is therefore surprising that clouds can form at such altitude, but they do, though mainly at high temperate to polar latitudes. It also requires special conditions to see them - the clouds have to be illuminated by the Sun but it has to be dark at ground level so the blue sky does not overwhelm their faint blue appearance. Typically this restricts their appearance at latitudes such as the UK and northern Europe to a month or two around mid summer, and they are best seen around 1.5 hours after sunset or before sunrise.
When they do appear, noctilucent clouds light up the sky with an almost metallic blue colour, against which any normal clouds will be seen as dark silhouettes - or orange if lit up by city lights. They exhibit a fine structure, e.g. wisps or herring-bone patterns, and can be seen to move slowly across the sky if watched or photographed over a period of several minutes.
They are something of an enigma because they were hardly known before the 20th century, so global warming is implicated, along with the increasing amount of water injected into the high atmosphere by rocket and Space Shuttle launches. Peversely, with more heat being trapped at low levels the mesosphere is cooling, and it is also cooler when the Sun is at or near the minimum of the Solar Cycle. In 2009 the Sun was especially quiet so this year was well favoured.
Living in the south of England it is comparatively rare to see them, but here is a selection of pictures that I have taken...
2005, from Surrey, England
2006, from Surrey, England
2008, from Reykjavik, Iceland
2009, from Surrey, England