GeoLog

Rainfall

Imaggeo on Mondays: Isolated storm

Imaggeo on Mondays: Isolated storm

Clouds and storms are formed when warm, moist air rises. This causes the air to expand and cool: forming clouds as the moisture condenses onto particles suspended in the air (called cloud condensation nuclei). Normally, air rises from surface heating, or when warm and cold air pockets collide, or if air is pushed upwards when passing over hills or mountains. If this heating, and subsequent rising, is rapid enough then thunderstorms can form.

This imaggeo on Mondays photo shows an isolated thunderstorm roughly 50 km North of Vienna. The difference between an isolated thunderstorm and scattered storms is how much coverage the clouds have over a given area. If less than 10-20 % is covered then these storms are described as isolated. Scattered storms occur when coverage is at least 30-50 %. These storms can lead to downpours lasting a few minutes that then leads to sunny spells, only to have another rain storm occur again shortly afterwards.1

Globally, there are roughly 16 million thunderstorms each year, and at any given moment, there are ~2,000 thunderstorms in progress.2 The visible dark grey anvil shape and the fact that the lighter clouds above appear to be being ‘pulled into’ the storm suggests that this is a ‘severe’ thunderstorm. This means that the storm is self-supporting3 and can cause more extreme impacts than a normal thunderstorm. Rainfall is more intense and can cause flash flooding. In some cases, hail over 2.5 cm large can fall and tornados can even be formed.4 For more information about severe thunderstorms please check out the further reading list below.

By Sarah Connors, EGU Science Policy Officer

Further reading / sources

[1] – Aerostorms Scattered vs. Thunderstorms – http://www.aerostorms.com/scattered-vs-isolated-thunderstorms-what-is-the-difference/

[2] – Thunderstorm Basics – http://www.nssl.noaa.gov/education/svrwx101/thunderstorms/

[3] – Royal Meteorological Society Thunderclouds presentation – http://www.met.reading.ac.uk/~sgs02rpa/CONTED/WEATHER09_thunder.pdf

[4] – Frequently Asked Questions About Thunderstorms – http://www.nssl.noaa.gov/education/svrwx101/thunderstorms/faq/

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: Landslide on the Cantabrian coastline

Shimmering blue seas, rocky outcrops and lush green hills sides; this idyllic landscape is punctuated by a stark reminder that geohazards are all around us. Irene Pérez Cáceres, a PhD student at the University of Granada (Spain) explains the geomorphology behind this small scale landslide on the Asturian coastline.

Landslide on the Cantabrian Sea. Credit: Irene Pérez Cáceres (distributed via imaggeo.egu.eu)

Landslide on the Cantabrian Sea. Credit: Irene Pérez Cáceres (distributed via imaggeo.egu.eu)

This picture was taken in May 2011 in the coast of Llanes (Asturias, Spain). I was living in Oviedo (Asturias, Spain) doing my Master in the structural geology of the Axial Zone of the Pyrenees. Thus, geomorphology and geohazards are not my specialty or area of expertise. However, the landslides are well known and studied in this region, and people from Asturias call them Argayos.

This argayo is situated in Niembru Mountain, over the San Antolín beach, constantly affected by waves and swell of tides of the Cantabrian Sea, and continuous rain typical in the region. It was defined as a rotational landslide with two fracture surfaces, possibly conjugated in wedge shape. It is approximately 50 meters high and 60 meters width at its base. The slide volume is calculated at 45000 m3. It is carved in quartzite altered by the water rain infiltration through crevices in the surface. The initial displacement was between 10 and 15 meters in the scar. Experts say this landslide is still active, moving and evolving continuously. It is an imminent risk for the swimmers, but it is very difficult to control it, due to the size and the slope, and the technical requirements to stabilize the rock. On the other side of this mountain, further landslides are evident, as a result of the building of a road.

These natural geomorphological processes are very common in the north of Spain, mainly in riverbeds, as well in other nearby beaches. The main causes are the abundant (and sometimes heavy) rainfall, the typically clay rich soils, steep slopes, building works that destabilize the slopes, and the absence of vegetation in some areas. They vary in in size and volume, and can sometimes have important material consequences and can pose a significant risk for the local inhabitants. The annual economic cost for repairing the damage caused by these processes is estimated to be 66 million of euros in this region.

Studies carried out in the Department of Geology of the University of Oviedo (Mª José Domínguez and her group), indicate that 70% of the landslides in Asturias happen when it rains over 200 mm during over a period of a minimum of three days. Research has also been carried out to try and predict when landslides might happen, examining numerous landslides over the last 20 years approximately. It seems that one conditioning factor is the exact location of new buildings, being that ancient constructions used to be in secure zones, probably because people observed more minutely to the nature, but the new ones are more vulnerable.

To conclude, detailed geological and geomorphological studies are always recommended to carry out before constructions. Thereby it is possible to minimise this common geohazard in Asturias.

By Irene Pérez Cáceres, PhD Student, Granada University.

 

If you pre-register for the 2015 General Assembly (Vienna, 12 – 17 April), you can take part in our annual photo competition! From 1 February up until 1 March, every participant pre-registered for the General Assembly can submit up three original photos and one moving image related to the Earth, planetary, and space sciences in competition for free registration to next year’s General Assembly!  These can include fantastic field photos, a stunning shot of your favourite thin section, what you’ve captured out on holiday or under the electron microscope – if it’s geoscientific, it fits the bill. Find out more about how to take part at http://imaggeo.egu.eu/photo-contest/information/.