Noctilucent clouds: the highest clouds on Earth

Why they need the mesosphere to be coldest in summer

The mesosphere is the coldest part of the atmosphere, reaching −130 °C at about 85 km. Counter-intuitively, it is coldest in summer at high latitudes, and this is a consequence of the global circulation.

In summer, air rises over the summer hemisphere and flows poleward, and as it does so it expands and cools. This means that at 80 km over the Arctic in July, the temperature can be 100 °C colder than it is at the same altitude in January. Noctilucent clouds need that cold: at 80 km the air is so dry that ice can only form below about −120 °C.

When and where to look

Noctilucent clouds are visible only between roughly 50° and 70° latitude in both hemispheres, and only in the summer months of that hemisphere — from late May to early August in the north, and from late November to early February in the south.

They are seen during the deep twilight period after sunset or before sunrise, roughly when the sun is between 6° and 16° below the horizon. At that time the sun still illuminates the mesosphere at 80 km while the troposphere below is in shadow, so the clouds shine pale electric blue against a dark sky.

Noctilucent cloud observing window (northern hemisphere)

Sun elevation	What you see	Best time relative to solar midnight
Above 0°	Twilight too bright; NLC invisible	Too early or too late
0° to −6°	Sky still bright blue; NLC may appear washed out	About 2 to 3 hours before solar midnight
−6° to −16°	Optimum window; NLC show maximum contrast	1 to 2 hours either side of solar midnight
Below −16°	Sky too dark; NLC fade and the troposphere blocks them	After solar midnight, window closes

What they look like, and what they do not

Noctilucent clouds are pale blue or silvery-white, with a herringbone, banded or wavy structure that is often extremely fine. They do not move fast relative to the stars, and they are not sharply defined at the edges the way tropospheric cloud is.

The key identification test is that they must be illuminated while everything else is dark. If a cloud is still lit but there is cirrus nearby at normal altitude that is also lit, you are looking at ordinary twilight cloud. Noctilucent clouds are the only clouds still catching the sun when the troposphere has gone completely dark.

• Colour. Electric blue to silvery white, never orange or pink.
• Structure. Very fine bands, billows or a herringbone pattern.
• Position. Low in the sky, 5° to 25° above the horizon, towards the pole.
• Timing. Only in deep twilight, in the two to three hours around solar midnight.
• Behaviour. Slow drift, gradually fading as the sun gets lower.

Why they are changing

Noctilucent clouds were first reported in 1885, shortly after the eruption of Krakatoa, and for decades they were widely assumed to be a volcanic curiosity. They are now observed far more often than in the twentieth century, and there are two proposed reasons.

More water vapour in the mesosphere. Methane oxidises in the upper atmosphere to produce water vapour, and atmospheric methane has more than doubled since pre-industrial times. More methane means more water at 80 km, where water vapour is normally almost absent.

Colder mesosphere from CO₂. Carbon dioxide cools the stratosphere and mesosphere by radiating infrared energy to space, even as it warms the surface. A colder mesosphere makes ice formation easier.

Both effects push in the same direction: noctilucent clouds should become more frequent and extend to lower latitudes, and the observations so far are consistent with that. They are, in a real sense, an early visible indicator of changes in the upper atmosphere.