|Noctilucent Clouds, Sweden, A display captured by P-M Hedén (photography site, more NLCs) September '08. ©P-M Hedén, shown with permission.
The season for NLCs, Earth's highest clouds, starts mid May for Northern Hemisphere watchers. They can be visible even sooner (Moon permitting!)..
They are high (50-53 miles) in the immensely rarefied upper atmosphere where the pressure is around five millionths of that at sea level. The clouds of tiny ice crystals are a few miles below the mesopause, the atmosphere's coldest point. The observing 'season' comes about because the clouds need exceptionally low temperatures (below -123C) for the crystals to form in the ultra dry conditions. Paradoxically, the mesosphere is at its coldest in summer. Another factor is that the clouds shine by reflected high-altitude sunlight and the sun is at the necessary short distances below the horizon in summer to give us reasonably dark skies while still shining at the clouds' height.
Although best far North they are visible at times in middle latitudes for example Austria and Hungary and Utah and Colorado. Look an hour or so after sunset low in the northwest to where the sun is located below the horizon. Binoculars help to distinguish their skeiny structure from lower tropospheric clouds.