Polar mesospheric clouds
Encyclopedia
Polar mesospheric clouds (PMCs) are a diffuse scattering layer of water ice crystals near the summer polar mesopause
.
s. From satellites, PMCs are most frequently observed above 70°-75° in latitude and have a season of 60 to 80 days duration centered about a peak which occurs about 20 days after the summer solstice
. This holds true for both hemispheres. Great variability in scattering is observed from day-to-day and year-to- year, but averaging over large time and space scales reveals a basic underlying symmetry and pattern. The long- term behaviour of polar mesospheric cloud frequency has been found to vary inversely with solar activity.
When mesospheric clouds are viewed above the atmosphere, the geometrical limitations of observing
from the ground are significantly reduced. They may be observed ‘edge-on’ against the comparatively
dark sky background, even in full daylight. The photometer field of view must be well baffled to avoid
interference from the very bright Earth about a degree beneath the cloud layer. It is a much more
difficult task to observe the clouds against the bright background of the illuminated Earth, although
this has been achieved in the ultraviolet in the 200 to 300 nm spectral region, because of the very
small albedo of the earth in this part of spectrum.
American and Soviet astronauts observed the phenomenon from space as early as 1970. Most
observations are reported from the night side of the orbit and the observer is looking towards the
twilight sector. At this time the observer's eye is dark-adapted and polar mesospheric clouds would
appear with maximum contrast against a comparatively dark background. Soviet astronauts have
reported sightings of mesospheric clouds even when the Sun is above the horizon.
Satellite observations allow the very coldest parts of the polar mesosphere to be observed, all the way
to the geographic pole. In the early 1970s, visible airglow photometers first scanned the atmospheric
horizon throughout the summer polar mesospause region (Donahue et al., 1972). This experiment,
which flew on the OGO-6 satellite, was the first to trace noctilucent-like cloud layers across the polar
cap. The very bright scattering layer was seen in full daylight conditions, and was identified as the
poleward extension of noctilucent clouds. In the early 1980s, the layer was observed again from a
satellite, the Solar Mesospheric Explorer (SME) . On board this satellite was an ultraviolet
spectrometer, which mapped the distributions of clouds over the time period 1981 to 1986. The
experiment measured the altitude profile of scattering from clouds at two spectral channels (primarily)
265 nm and 296 nm (Thomas and MacKay, 1985). This phenomenon is now known as Polar
Mesospheric Clouds.
The general seasonal characteristics of polar mesospheric clouds are well established from the five
years of continuous SME data. Over that period, data for four cloud ‘seasons’ in the north, and five
‘seasons’ in the south were recorded. In both hemispheres, the season begins about one month
before summer solstice and ends about two months afterwards. Since there are no biases due to
such factors as changing number of hours of visibility, weather conditions, etc. this is a ‘true’
behaviour. It is believed to be a result of the fact that summertime mesopause region becomes
coldest during this period causing water-ice to form, in contrast to most other regions of the
atmosphere which are warmest in summer. Temperatures at latitudes equatorward of the boundary
of detection never get low enough for water-ice to form.
Polar mesospheric clouds generally increase in brightness and occurrence frequency with increasing
latitude, from about 60 degrees to the highest latitudes observed (85 degrees). So far, no apparent
dependence on longitude has been found, nor is there any evidence of a dependence on auroral
activity (Thomas and Olivero, 1989). This indicates that control of polar mesospheric clouds is
determined by geographical rather than geomagnetic factors. The brightness of polar mesospheric
clouds and noctilucent clouds appears to be consistent at the latitudes where both are observed, but
polar mesospheric clouds near the pole are much brighter than noctilucent clouds, even taking into
account the lower sky background seen from space. Polar mesospheric cloud observations have
revealed that the well-known phenomenon of the northward shifting with latitude of date of peak
noctilucent cloud occurrence is partly due to the increased number of hours of noctilucent cloud
visibility with latitude and partly due to an actual northward retreat of the boundary towards the end of
the season.
Mesopause
The mesopause is the temperature minimum at the boundary between the mesosphere and the thermosphere atmospheric regions. Due to the lack of solar heating and very strong radiative cooling from carbon dioxide, the mesopause is the coldest place on Earth with temperatures as low as -100°C...
.
Description
Observed from the ground, this phenomenon is known as noctilucent cloudNoctilucent cloud
Night clouds or Noctilucent clouds are tenuous cloud-like phenomena that are the "ragged-edge" of a much brighter and pervasive polar cloud layer called polar mesospheric clouds in the upper atmosphere, visible in a deep twilight. They are made of crystals of water ice. The name means roughly night...
s. From satellites, PMCs are most frequently observed above 70°-75° in latitude and have a season of 60 to 80 days duration centered about a peak which occurs about 20 days after the summer solstice
Summer solstice
The summer solstice occurs exactly when the axial tilt of a planet's semi-axis in a given hemisphere is most inclined towards the star that it orbits. Earth's maximum axial tilt to our star, the Sun, during a solstice is 23° 26'. Though the summer solstice is an instant in time, the term is also...
. This holds true for both hemispheres. Great variability in scattering is observed from day-to-day and year-to- year, but averaging over large time and space scales reveals a basic underlying symmetry and pattern. The long- term behaviour of polar mesospheric cloud frequency has been found to vary inversely with solar activity.
When mesospheric clouds are viewed above the atmosphere, the geometrical limitations of observing
from the ground are significantly reduced. They may be observed ‘edge-on’ against the comparatively
dark sky background, even in full daylight. The photometer field of view must be well baffled to avoid
interference from the very bright Earth about a degree beneath the cloud layer. It is a much more
difficult task to observe the clouds against the bright background of the illuminated Earth, although
this has been achieved in the ultraviolet in the 200 to 300 nm spectral region, because of the very
small albedo of the earth in this part of spectrum.
American and Soviet astronauts observed the phenomenon from space as early as 1970. Most
observations are reported from the night side of the orbit and the observer is looking towards the
twilight sector. At this time the observer's eye is dark-adapted and polar mesospheric clouds would
appear with maximum contrast against a comparatively dark background. Soviet astronauts have
reported sightings of mesospheric clouds even when the Sun is above the horizon.
Satellite observations allow the very coldest parts of the polar mesosphere to be observed, all the way
to the geographic pole. In the early 1970s, visible airglow photometers first scanned the atmospheric
horizon throughout the summer polar mesospause region (Donahue et al., 1972). This experiment,
which flew on the OGO-6 satellite, was the first to trace noctilucent-like cloud layers across the polar
cap. The very bright scattering layer was seen in full daylight conditions, and was identified as the
poleward extension of noctilucent clouds. In the early 1980s, the layer was observed again from a
satellite, the Solar Mesospheric Explorer (SME) . On board this satellite was an ultraviolet
spectrometer, which mapped the distributions of clouds over the time period 1981 to 1986. The
experiment measured the altitude profile of scattering from clouds at two spectral channels (primarily)
265 nm and 296 nm (Thomas and MacKay, 1985). This phenomenon is now known as Polar
Mesospheric Clouds.
The general seasonal characteristics of polar mesospheric clouds are well established from the five
years of continuous SME data. Over that period, data for four cloud ‘seasons’ in the north, and five
‘seasons’ in the south were recorded. In both hemispheres, the season begins about one month
before summer solstice and ends about two months afterwards. Since there are no biases due to
such factors as changing number of hours of visibility, weather conditions, etc. this is a ‘true’
behaviour. It is believed to be a result of the fact that summertime mesopause region becomes
coldest during this period causing water-ice to form, in contrast to most other regions of the
atmosphere which are warmest in summer. Temperatures at latitudes equatorward of the boundary
of detection never get low enough for water-ice to form.
Polar mesospheric clouds generally increase in brightness and occurrence frequency with increasing
latitude, from about 60 degrees to the highest latitudes observed (85 degrees). So far, no apparent
dependence on longitude has been found, nor is there any evidence of a dependence on auroral
activity (Thomas and Olivero, 1989). This indicates that control of polar mesospheric clouds is
determined by geographical rather than geomagnetic factors. The brightness of polar mesospheric
clouds and noctilucent clouds appears to be consistent at the latitudes where both are observed, but
polar mesospheric clouds near the pole are much brighter than noctilucent clouds, even taking into
account the lower sky background seen from space. Polar mesospheric cloud observations have
revealed that the well-known phenomenon of the northward shifting with latitude of date of peak
noctilucent cloud occurrence is partly due to the increased number of hours of noctilucent cloud
visibility with latitude and partly due to an actual northward retreat of the boundary towards the end of
the season.
See also
- Iridescent Cloud
- Noctilucent clouds
- Polar stratospheric clouds
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