From now until mid August, you may be able to spot a very rare type of cloud called a Noctilucent Cloud. This is the only time of year when the cloud is visible in the Northern Hemisphere.
The word noctilucent means "night glowing" and atmospheric scientists know very little about this kind of cloud. They form very high in the atmosphere and because they're so high they can still catch a bit of sunlight that reflects into your eyes. When it appears, they are usually very bright and have a blue or silvery white type of colour.
The clouds are typically formed between 76 and 85 km above the Earth's surface and that's why the low, ordinary type of clouds are no longer lit up by the sun when the sun is well below the horizon.
In order to catch these type of clouds you must look towards the direction of sunset. From mid-twilight to the end of twilight, about 30 to 60 minutes before sunrise or after sunset, the clouds will appear criss-crossing low above the horizon. And for areas that do not get darkness at all, these clouds could appear all night long.