Category Weather & Climate

            Meteorologists draw up special weather maps called synoptic charts to show a forecast. The long curved lines — isobars — show areas of equal pressure. Black circles mark the centre of low- and high-pressure areas. Lines of red semicircles indicate a warm front, and a cold front is shown by a line of blue triangles. A combination of triangles and semicircles indicates an occluded front. Ideally, all the observations shown on a synoptic chart should be made at the same time (“synoptic” means “seen together”), but this is rarely possible, so slight variations must be taken into account when interpreting a chart. The synoptic chart illustrated below shows a weather system over north-west Europe.

            The word ‘synoptic’ simply means a summary of the current situation. In weather terms, this means the pressure pattern, fronts, wind direction and speed and how they will change and evolve over the coming few days.Temperature, pressure and winds are all in balance and the atmosphere is constantly changing to preserve this balance. This is why the UK sees such changeable weather.

         The circular lines you see on the chart are isobars, which join areas of the same barometric pressure. The pressure pattern is important because we can use it to tell us where the wind is coming from and how strong it is. It also shows areas of high and low pressure.

            Air moves from high to low pressure along a gradient (similar to squash that is left in a glass of water becoming evenly distributed as it becomes less concentrated). If the difference between areas of high and low pressure is greater then we have a large gradient and the air will move faster to try and balance out this difference. This is shown on a synoptic chart with isobars that are very close together and we feel strong winds as a result.

            In terms of the wind direction, air moves around high pressure in a clockwise direction and low pressure in an anticlockwise direction, so isobars also tell us the direction and speed of the wind.

Cold fronts and warm fronts

            Also on a synoptic chart are the lines, triangles and semi-circles representing ‘fronts’. With the atmosphere trying to balance temperature, pressure and wind there are different sorts of air, known as air masses, circulating around the Earth. The differences are mostly between how warm, cold, dry and moist the air is, and fronts simply mark the boundary between these different types of air.

            A warm front is shown with a red line and red semi-circles and a cold front with a blue line and blue triangles. The way in which the semi-circles or triangles point shows the direction in which the front is moving. The position of a front depends on a number of meteorological factors, such as changes in wind direction or temperature, which we get from our network of weather observation sites. A few things to remember are that warm air follows a warm front and cold air follows a cold front. We also tend to see increased amounts of cloud and rainfall along the front itself.

            Sometimes the red or blue line of a front will be broken by crosses. This indicates that the front is weakening and the difference in the warmth or dryness of the air is becoming less marked.

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            Weather Satellites produce images by interpreting different levels of heat and light. When an area is lit by sunlight, different features — clouds, land, sea, ice, and so on — reflect different amounts of light, which are recorded by the satellite as varying shades of grey. When an area is in darkness, heat emissions are recorded by infrared equipment to produce a similar picture. The information is trans-mitted to a base station, where it is converted into images. Television forecasts often put a series of satellite images together to produce a “movie” of a moving weather system.

          IR or infrared satellite imagery is sort of a temperature map. The weather satellite detects heat energy in the infrared spectrum (infrared energy is invisible to the human eye). The satellite image displays objects(whether clouds, water or land surfaces) based on the temperature of the object. Warm temperatures appear in dark shades. Cold temperatures appear in light shades. A temperature scale(in degrees Celsius) is depicted to the left of the image.

           The chief advantage of IR imagery is that it’s not dependent on sunlight. Visible imagery(like the photos you take with a normal camera) relies on sufficient sunlight reflecting off a surface to be viewable. It’s useless at night, but IR imagery relies on emitted heat energy(detectable day or night if you have the right equipment).

            You can infer relative altitudes of clouds from their temperature. Since temperature, in general, decreases with increasing height, high altitude clouds will appear whiter than low altitude clouds.

            A visible satellite image is created by looking only at the visible portion of the light spectrum and is thus only really useful during daylight hours. The Infrared (IR) image comes from the satellite detecting heat energy in the infrared sepectrum and thus does not depend on visible light. For this reason we switch between the visible and IR images at 1500 GMT (8:00am PDT) and 0200 GMT (7:00pm PDT).

            This is a three letter identifier for each station. Example: ‘SFO’ is San Fransisco. Use the station search engine to find the name of the plotted stations (select the “Call Sign” option on the search form before attempting a search).

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            Detailed, short-range weather forecasts can usually be made for the next 24 hours. However, meteorologists today have access to information that enables them to make a fairly accurate, long-range forecast for up to a week ahead.

           One of the country’s leading commercial forecasters, AccuWeather, said earlier this month that it could predict the weather conditions and temperature three months ahead of time. It’s a bold claim that has been refuted by some meteorologists who say such a 90-day forecast will only be as good as historical averages and not much use to someone planning a hike or outdoor wedding this summer.

            That’s because the Earth’s atmosphere is a chaotic system that doesn’t follow an easily predictable path, according to Keith Seitter, executive director of the American Meteorological Society in Boston.

           “If anybody kept track about how (AccuWeather) did, they would find it’s a pretty horrible forecast,” Seitter said.The best weather forecasters can do now is seven to 10 days. After that, accuracy drops off quickly.

            The good news is that forecasting has gotten better over the years. Improvements in computer technology, data collection and weather models have improved this forecasting number about one day each decade.

            One of the biggest advancements has come in boosting computer power. The National Oceanic and Atmospheric Administration, the parent agency of the National Weather Service, operates supercomputers in Reston, Va., (“Luna”) and Orlando, Fla., (“Surge”) to come up with weather forecasts.

            After a $44 million upgrade in January, each one has the capacity of 2.89 petaflops, or 2.89 quadrillion calculations per second, according to Richard Michaud, director of NOAA’s office of central processing. That’s up from 778 teraflops (1 petaflop equals 1,000 teraflops) of computing power last year.

            Better predict the amount, timing and type of precipitation in both winter storms and thunderstorms Create “water forecasts” and more accurately predict drought and floods Connect the air, ocean and waves to track eight hurricanes at once These supercomputers are the brains behind the weather forecasts you see on TV each night or your smartphone when you wake up.

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          Meteorologists and weather forecasters are employed by national and regional weather centres, as well as by organizations such as the military and by airports. They make forecasts based on their knowledge of weather patterns and information received from local, national and global sources .The forecasts are delivered to the public through television, radio, newspapers and the Internet.

          Meteorologists use a process called numerical weather prediction to create forecasts by inputting current conditions — which they call the “nowcast” — into computer models. The more current and accurate information available to these models, the better the forecast will be. Ground radar, weather balloons, aircraft, satellites, ocean buoys and more can provide three-dimensional observations that a model can use. This allows meteorologists to simulate what the atmosphere is currently doing and predict what will happen in the next few days or, for some models, hours.

          Weather models divide a region, say a single state or even the whole globe, into a set of boxes, or cells. The size of these cells — the resolution of the model — affects its forecasting accuracy. Large boxes mean poor resolution, or the inability to tell what’s happening over small areas, but a broad picture of large-scale weather trends over long timelines. This big-picture forecast is helpful when you want to know how a big storm will move across the U.S. over the course of a week.

          Smaller boxes mean higher resolution, which can forecast smaller storms. These models are more expensive in terms of computing power, and only run to the one- or two-day mark to tell people whether it might storm in their local area. Although all models are based on the same physics, each translates those physics into computer code differently, says Judt. Some models might prioritize certain kinds of data — such as wind speed, temperature and humidity — over others to generate predictions, or simulate physical processes slightly differently than another model. That’s why two models might spit out slightly different results, even with exactly the same starting observations.

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          Television weather forecasts are the most easily understood and widely seen source of weather information for the general public. The weather forecasters may be trained meteorologists who work at a weather centre, or television presenters may read out forecasts provided for them. A detailed forecast is presented as a sequence of weather maps generated on a computer. They usually show temperatures and wind speed and direction, and give some indication of the expected weather conditions for different parts of the country. Local television stations will present a more detailed forecast for their region.

          Television is the main medium through which forecasts are viewed. Television stations use a variety of media to portray forecast information. These may be icons, for example showing a sunshine symbol, or contours, for example showing an area which may be affected by rain.

          Broadcasts are available to view on standard television sets. New technology is changing viewing habits and increasingly weather forecasts are viewable through other methods such as via a desktop computer or downloaded onto a mobile mp4 player.

          Forecast data are often displayed on maps of the area of interest. Before assessing the weather forecast for the area they are interested in, the viewer must be able to ascertain where on the map they are located. As revealed in the survey carried out by Thornes (1992) this is something which the public are not generally able to do with confidence. It does seem that an attempt to pinpoint one’s location to within a general area can be made, but more detailed identification of the location is more difficult.

          During the past few years television weather forecast graphics have evolved, from a ‘hand’ drawing on a weather chart, to fully integrated 3D graphics capable of showing fly?throughs of weather, anywhere in the world. When television weather forecasts were first broadcast, the forecaster would often draw expected conditions directly onto a map using a pen. In the 1970s the BBC introduced magnetic symbols which ‘stuck’ to a base map as the forecaster described the changed weather. These symbols are now perhaps the most well-known of weather symbols.

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          There are many theories about the origin of mysterious patterns that appear in fields of corn around the world – commonly known as crop circles. While some people believe they are the work of alien spacecraft, many of them are known to be man-made. Unusual weather, such as small tornadoes or electrical storms, is thought to be the cause of some of the patterns.

          George Meaden, a meteorologist and physicist, explained crop circles in terms of atmospheric physics, as the effect of a plasma-vortex. Meaden likened the phenomenon to ball-lightning, but larger and longer-lasting. He expanded his theories in later research to include ionisation and electromagnetism. Physicist Stephen Hawking appeared to give credence to this view, writing in 1991 that corn circles are either hoaxes or formed by vortex movement of air.

           Meaden’s ideas were taken up by Ralph Noyes, a senior Ministry of Defence official and an expert in UFO phenomena, who wrote as follows:

          If Meaden is right, our atmosphere is sometimes able to produce a short-lived but vigorously swirling disturbance with strong electrical properties. A layman grasps after the analogy of something between ball-lightning and a mini-tornado. Depending on conditions, this transient energy-form can manifest as a globe of light, often with associated sounds. It may be able to interfere with the ignition system of automobiles and perhaps to affect close witnesses. Descending to earth, it can make a cropfield circle. Acting more vigorously it may well cause more violent circular damage at ground level. In short, a good meteorologist whose sole concern has been to investigate cropfield circles has ended by describing much of the UFO phenomenon!

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