OPOD - Nacreous Clouds

Nacreous or Tropospheric?

A collection of rare stratospheric nacreous clouds and not so rare tropospheric iridescent clouds from Norway, Poland, Hungary, Scotland and Russia.

How are nacreous clouds (NCs) distinguished from tropospherics (TCs)?

Time: Nacreous clouds brighten after sunset and can remain visible for an hour. They glow in the dark as high altitude sunlight lights up the 9-16 mile high NCs. TCs are seen only in daytime. Very bright NCs can look like TCs in the day.

Season: NCs form in winter. TCs all year round. The ice crystals of NCs only form when the lower stratosphere cools below -85 Celsius.

Weather: Stormy tropospheric weather can favour NCs although it is not essential. The winds probably help loft necessary moisture across the tropopause into the otherwise very dry stratosphere. TCs any weather.

World Location: NCs are only seen where the stratosphere is cold � high latitudes, Northern Europe, Scandinavia, Iceland, Greenland, Canada and sometimes northernmost US. TCs anywhere.

Local geography: NCs are wave clouds. Downwind of mountains are favourable spots. TCs anywhere but iridescent lenticular clouds also occur in the lee of mountains.

Movement: High NCs stay almost in the same position while dark lower clouds scud beneath them.

Brightness: NCs are very bright indeed - shining in the twilight sky.

Shape & structure: NCs wavy, filmy, curling and uncurling over minutes with slowly changing electric colours. TCs can be wavelike but more often are irregular or have a finer grained structure.

Surrounding clouds: NCs are a type of Polar Stratospheric Cloud (PSC) and are often embedded in other less colourful PSCs of similar shape.

Both NCs and TCs are iridescent. In NCs small similar sized ice crystals diffract sunlight. In TCs the diffractors are small water droplets or, less often, ice crystals.