The Earth Story

Calling all budding meteorologists! Come play with this! This website (http://earth.nullschool.net/) offers a full colour, global wind map in real time. The visualisation, which looks quite tranquil really, is generated by a supercomputer, similar to those used to establish weather forecasting. It is also interactive, allowing the user to rotate the globe and zoom in and out and to change the altitude. -Jean

A 19th century instrument, still in use

How does one measure the length of the day and intensity of sunlight at the same time? A Scottish author and Celtic folklore expert called John Campbell in 1853 wanted to find out in a quest to record 'the nature and power of sunshine' (as he put it a quarter century later), and his desire turned into a meteorological instrument that is so simple that it remains of use. Further refined by physicist Gabriel Stokes in 1879 they are now known as Campbell Stokes sunshine recorders and proved a mainstay of meteorology for decades.

A hollow sphere of glass is used to focus the rays of the sun, at first onto a bowl of marked wood, and as anyone who has fooled around with a magnifying glass and a piece of paper on a sunny day knows, focussed sun is hot. As the world rotates and the solar orb appears to move across the sky, it burns a line across the wood whose depth records the changing intensity of sunlight. Stokes' main refinement was to replace the wood with metal and to add in a removable graduated thick card to record the information. His inspiration came from the classics, mainly the ancient tales of Archimedes' use of a burning glass to set fire to a Roman fleet that was attacking his home city of Syracuse. He made a homemade version and tried it out on his window sill, and the recorder was born.

Modern versions use a 10cm glass sphere on a stand focussing the rays on a card, which can be held in one of 3 sets of grooves, depending on the season and how high the sun is in the sky. Several cards carry the measurement through the year (eg in the northern hemisphere winter cards go from 15 October-29 February, while there are two shorter equinox cards (when the length of day is changing fastest) running 1 March-11 April and 3 September-44 October. Each card is gradated by hour. The instrument is set to record maximum light, ie pointing south in the northern hemisphere and vice versa in the southern and away from interfering shadows, whether from buildings or trees.

While several astronomical observatories still use them (including Hong Kong and Blue Hill in Massachusetts) alongside meteorologists (particularly outside the USA), the examples in the second two photos sit at the South Pole, on the roof of Amundsen Scott Research Station where they record nothing in the dark months, and both get burned through in the summer as the sun seems to circumnambulate the sky without setting. It is so simple and maintenance free that it can be used anywhere, though near the Poles two are needed. Its main disadvantage is that when the sun is too low in the sky, it may not have enough power to burn the card, and hence won't be recorded (mostly at dawn and dusk). Interpretation can be tricky too on days when sun and cloud alternate.

Loz

Image credit: 1: Iain Farrell 2: Akulovz 3: Eli Duke

http://bit.ly/1VhDqBp

This animated video from the New York Times tells the story of meteorologist Alfred Wegener, whose name appears in almost every geology textbook today as the person who, in the early 20th century, proposed the idea that continents drifted around the earth's surface based on geologic evidence.

Shelf cloud Known to meteorologists as arcus clouds, they form at the leading edge of a thunderstorm when descending air flows outwards from the storm front at the base of the cumulo-nimbus supercell. As the cool air hits the land, it spreads out as the powerful wind preceding the storm and pushes warmer air upwards, where the moisture within the column then condenses into the shelf cloud, that roils with the shearing air currents. As they pass over, the winds shift and change, shortly followed by violent rain or hail. They tend to be very low, highly turbulent, and obstruct the view of the storm in a dark and menacing manner while indicating an approaching squall.  They are often confused with wall clouds (which form at the trailing edge of storms), and most usually occur in lines of storms rather than solitary anvils. Shelf clouds often seem to rotate about a horizontal axis, while wall clouds will do so in the vertical one, and the former are associated with rain while the latter are a sign that the storm has passed.  Loz Image credit: Thalia Traianou via EPOD http://www.strangephoton.com/ http://www.weather.com/news/shelf-cloud-photos-20130412 http://www.accuweather.com/en/features/trend/what-are-shelf-clouds/5506892 http://www.crh.noaa.gov/lmk/?n=shelfcloudversusawallcloud http://eo.ucar.edu/webweather/cloudpic1.html