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Imaggeo On Mondays: Halo

Imaggeo On Mondays: Halo

One of the main perks of being a geoscientist is that, often, research takes scientists all around the globe to conduct their work. While fieldwork can be hard and challenging it also offers the opportunity to see stunning landscapes and experiencing unusual phenomenon. Aboard the Akademik Tryoshnikov research vessel, while cruising the Kara Sea (part of the Arctic Ocean north of Siberia) Tatiana Matveeva was witness to an interesting optical phenomenon, a halo. In today’s post she tells us more about how the elusive halos form and how best to spot them.

It was one of many mornings on the Kara Sea, but the sunrise was very unusual – we saw halo. Because more often than not, the skies over the Arctic seas are covered in cloud, we were very lucky to see a halo!

Halos are produced by ice crystals trapped in thin and wispy cirrus or cirrostratus clouds, which form high (5–10 km) in the upper troposphere. The hexagon ice crystals behave like prisms and mirrors, refracting and reflecting sunlight between their faces, sending shafts of light in different directions.

Halos can have many forms, ranging from colored or white rings to arcs in the sky. The particular shape and orientation of the ice crystals is responsible for the type of halo observed. For example, halos may be due to the refraction of light that passes through the crystals or the reflection of light from crystal faces or a combination of both effects. Refraction effects give rise to colour separation because of the slightly different bending of the different colours composing the incident light as it passes through the crystals. On the other hand, reflection phenomena are whiteish in colour, because the incident light is not broken up into its component colours, each wavelength being reflected at the same angle. The most common halo is circular halo (sometimes called 22° halo) with the Sun or Moon at its centre. The order of coloration is red on the inside and blue on the outside, you can see it in this picture.

Historically, halos were used as an empirical means of weather forecasting before meteorology was developed.

Anecdotally, in the Anglo-Cornish dialect of English, a halo around the Sun or the Moon is called a ‘cock’s eye’ and is a token of bad weather. The term is related to the Breton word kog-heol (sun cock) which has the same meaning. In Nepal, a halo around the sun is called Indrasabha – the Hindu god of lightning, thunder and rain.

To see a halo, don’t look directly into the sun. Block the sun from your view with your hand, so you can just see the clouds around it. And enjoy beautiful optical phenomenon!

By Tatiana Matveeva, researcher at the Moscow State University

 

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

GeoSciences Column: The ‘dirty weather’ diaries of Reverend Richard Davis

GeoSciences Column: The ‘dirty weather’ diaries of Reverend Richard Davis

Researching the Earth’s climate of the past, helps scientists make better predictions about how the climate and our environment will continue to be affected by, change and adapt to rising temperatures.

One of the most invaluable sources of data, when it comes to understanding the Earth’s past climate, are historical meteorological records.

Accounts of weather and climate conditions for the Southern Hemisphere, prior to the 1850s, are particularly sparse. This makes the recently discovered, painstakingly detailed and richly descriptive weather diaries of a 19th Century missionary in New Zealand, incredibly valuable.

Researchers from the National Institute of Water and Atmospheric Research, In Auckland (New Zealand), poured over the contents of the diaries, which provide an eyewitness account to the end of the Little Ice Age (between 1300 and the 1870s winter temperatures – particularly in the Norther Hemisphere- were lower than those experienced throughout the 20th Century). The journals reveal that 19th century New Zealand experienced cooler winter temperatures and more dominant southerly winds when compared to the present day climatic conditions. The researchers present these and other findings in the open access journal of the EGU, Climate of the Past.

Print of a photomechanical portrait of Reverend Richard Davis taken ca. 1860, from the file print collection, Box 16. Ref: PAColl-7344-97, Alexander Turnbull Library, Wellington, New Zealand, sourced from http://natlib.govt.nz/records/23073407 (From A. M. Lorrey et al., 2016).

Print of a photomechanical portrait of Reverend Richard Davis taken ca. 1860, from the file print collection, Box 16. Ref:
PAColl-7344-97, Alexander Turnbull Library, Wellington, New Zealand, sourced from http://natlib.govt.nz/records/23073407 (From A. M. Lorrey et al., 2016).

The diaries were kept by Reverend Richard Davis, born in Dorset (England) in 1790. The Reverend was associated to the Church Mission Society (CMS) of England; an connection which lead him to settle in the blustery Northland Peninsula in the far north of New Zealand, back in 1831.

From 1839 to 1844, and then again from 1848 to 1851, Davis collected over 13,000 meteorological measurements and made detailed notes about the condition of the local environment.

The Reverend’s collection of data is remarkable, not only for its detail, but also because it is the earliest record of land-based meteorological measurements from New Zealand found to date.

He took twice daily temperature measurements – one at 9 a.m. and one at 12 noon – as well as noon pressure measurements. Qualitative observations included information about wind direction and strength, as well as detailed cloud cover descriptions, and notes on the occurrence of hail, frost, rainfall, snowfall, thunderstorms, lightning, sunsets and behavior of wildlife.

The journals also reveal that the Reverend was not keen on particularly gloomy days, when the winds were strong and blustery, cloud cover hung low and was often accompanied by rain.  On 67 separate occasions, Davis’ used the term “dirty weather” to described days like this.

It is important to assess the reliability of the measurements taken by Davis before drawing comparisons between 19th and 20th Century weather patterns for the island of the long white cloud; and especially if the data are to be integrated within past climate and weather reconstructions for New Zealand and the Southern Hemisphere.

To do so, the Auckland based researchers, compared the Reverend’s pressure measurements with observations made by ships travelling through New Zealand waters or stationed on the island (usually completing military operations), during the same time interval. The Reverend’s daily pressure observations are regularly lower (on average by -0:64 ± 0:10 inches of mercury) than those taken on board the ships. The offset is consistent with the change in altitude between the ships anchored in harbour versus the land-based measurements made by Davis; meaning the Reverend’s pressure measurements are robust.

Reverend Richard Davis pressure observation vs. expedition measurements (leader noted in parentheses) from USS Vincennes (Wilkes), the corvettes Astrolabe and Zelee (d’Urville) and the HMS Erebus (Ross). There are 29 pairs of daily observations and so the x axis simply shows the comparisons of Davis’ record to the three ships in a sequence with the specific intervals noted. (From A. M. Lorrey et al., 2016).

Reverend Richard Davis pressure observation vs. expedition measurements (leader noted in parentheses) from USS Vincennes (Wilkes), the corvettes Astrolabe and Zelee (d’Urville) and the HMS Erebus (Ross). There are 29 pairs of daily observations and so the x axis simply shows the comparisons of Davis’ record to the three ships in a sequence with the specific intervals noted. (From A. M. Lorrey et al., 2016).

There is no similar test which would verify the accuracy of Davis’ temperature measurements. However, the researchers argue that the (expected) annual cycles evident in his measurements, as well as the reliability of his other records mean that, at least some, of his readings are faithful to the local conditions. Not only that, Davis’ mean winter temperature anomalies are comparable to the temperatures reconstructed from tree rings and can be used by the researchers to gather information about the local atmospheric circulation at the time.

When compared to modern-day temperature measurements (from the Virtual Climate Station Network, VCSN), the journal data reveals that mid 1800s winters, at the far north of the island, were cooler. At present, atmospheric circulation over Northland means winds from the southwest are common, especially during the winter and spring. During the summer, easterly winds become dominant. There is a higher frequency of records of south and southwesterly winds in Davis’ diaries. Reconstructions of atmospheric flow over New Zealand in the 1800s, made with proxy tree-ring and coral data, also point towards more frequent south and southwesterly winds and cooler temperatures.

Not only that, the timing of monthly and seasonal climate anomalies, recorded both in tree-ring and the Davis diary data suggest that El Niño-Southern Oscillation (ENSO)-like conditions existed, in New Zealand, during the 1839-1851 time period. However, more work (and data) is needed in Australasia to corroborate the findings and define the extent of the ENSO conditions at the time.

With more data, better reconstructions of the atmospheric conditions in the southwest Pacific and Southern Hemisphere can be made. Combined with the newly found Davis’ records, these will make an important impact to the understanding of past weather and climate in the region.

By Laura Roberts Artal, EGU Communications Officer

The Waimate North mission house in the Far North of New Zealand where Davis lived (From: A. M. Lorrey et al., 2016).

The Waimate North mission house in the Far North of New Zealand where Davis lived (From: A. M. Lorrey et al., 2016).

References

Lorrey, A. M. and Chappell, P. R.: The “dirty weather” diaries of Reverend Richard Davis: insights about early colonial-era meteorology and climate variability for northern New Zealand, 1839–1851, Clim. Past, 12, 553-573, doi:10.5194/cp-12-553-2016, 2016.

Imaggeo on Mondays: A look inside a thunderstorm

Imaggeo on Mondays: A look inside a thunderstorm

This week’s contribution to Imaggeo on Mondays is a photograph of a mesocyclone – and its rotating wall cloud – photographed by Mareike Schuster, an atmospheric scientist from Freie Universität Berlin, Germany.

The picture was taken in June 2012 near Cheyenne, Wyoming in the United States during a field campaign, ROTATE, led by the Center for Severe Weather Research, based in Boulder, Colorado. ROTATE stands for Radar Observations of Tornadoes and Thunderstorms Experiment. The mission aimed at probing the inner workings of tornadoes to better understand the intensity and variability of low level winds in these vortices. The experiment setup included several mobile Doppler radars (DOWs) that would measure the convective storm from a distance, and also multiple “mobile mesonet” vehicles – vehicles equipped with weather observation instruments- whose passengers would deploy numerous so called “tornado pods” right in the pathway of a tornado. The instruments of the pods might have been destroyed but the data was saved on “armored black boxes” for later analysis.

What are mesocyclones?

A mesocyclone is a cyclone that is embedded within a convective storm – together they form a supercell. The mesocyclone is characterized through ascending air, that in most cases rotates cyclonic. If the mesocyclone has large vertical extent and persists long enough it can be detected by a Doppler radar and appears as a couplet of motion (of the water droplets in the storm towards and away from the radar) in the data.

What is a wall cloud?

The mesocyclone becomes visible through a persistent lowering of the cloud base – the wall cloud – caused by the condensation of updrafting air. The so called wall cloud typically forms beneath the rain free base of a supercell and indicates the area of the strongest updraft. Wall clouds are inflow clouds, local and tend to slope inward. Please note that they are often mixed-up with shelf clouds (not shown), which in turn are outflow clouds that have a larger extent and are associated with a different atmospheric feature. A wall cloud can also form below a thunderstorm if there is no rotation – but if it rotates, then this indicates the existence of a mesocyclone. So it was the case in this picture.

Sometimes, tornadoes form within the mesocyclone of a supercell. The mesocyclone shown here, however, did not.

The motion of the supercell in the shot is towards the observer. All the dark clouds in the photograph basically show the wall cloud. The storm is already very close to the observer. The rest of the thunderstorm, e.g. the cumulonimbus cloud and the typical anvil are so large, they are far above and behind the observer and did not fit on the frame. Everything’s bigger in the U.S. !  😉

By Mareike Schuster, Institut für Meteorologie, Freie Universität Berlin

 

The picture was taken with a Nikon D5100 and a Tamron 10-24mm wide angle lense.

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

Imaggeo on Mondays: A thermal inversion

Imaggeo on Mondays: A thermal inversion

This week’s Imaggeo on Mondays image is brought to you by Cyril Mayaud, from the University of Graz (Austria), who writes about an impressive hike and layers of cold and warm air.

Thermal inversion is a meteorological phenomenon which occurs when a layer of cold air is trapped near the Earth’s surface by an overlying layer of warmer air. This can happen frequently at the boundary between mountainous and lowland regions such as in Slovenia and last for weeks, obscuring the sun from view to the people living below. When this phenomenon occurs over a large city, the consequence is that it can cause important pollution problems, as the lack of air circulation, prevents the rising and scattering of pollutants in the atmosphere.

The picture was taken at dusk from the top of the Porezen, a 1630 m high mountain located near the town of Cerkno and belonging to the Slovenian Prealps. This mountain is very popular among local hikers because its summit offers an impressive panorama of large parts of Slovenia, comprising the highest peaks of the Julian Alps, the Ljubljana Basin and even Snežnik Mountain located close to the Croatian border. A thick, low altitude, layer of clouds was covering the whole country during the preceding week, but clear, sunny skies prevailed above the clouds, a not too uncommon phenomena.. We made our way to the summit over several hours and spent some time enjoying the panorama and the sunset. As a result of the thermal inversion, the air temperature was warmer at the summit than in the surrounding lowlands. As soon as the sun began to set, the fog slowly started to move forward and cover the narrow valleys below the mountain. By the end of the day, the valley in the foreground was also totally engulfed by fog.

By Cyril Mayaud, Researcher at the University of Graz, Austria

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/