|Key Stage 4 - Satellites|
There are two types of satellite providing weather data.
Geostationary - these are positioned at a height of 35,780 km above the equator, and 'hang' over the same spot on the Earth's surface all the time. Meteosat, the geostationary satellite operated by European countries, is positioned over the equator on the Greenwich meridian and covers Africa, Europe, the Middle East, much of the Atlantic Ocean and the western Indian Ocean. The present satellite is called MSG and provides pictures every 15 minutes. It is possible to receive images with a resolution that is similar to that usually available from the much lower polar-orbiting satellites, although a very powerful computer is needed to process the data for much more than a relatively small area.
Polar-orbiting - these pass over the Earth from pole to pole. The NOAA satellites, operated by the USA, orbit at a height of 830 km and take 1 hour and 42 minutes to complete each orbit. During this time, the Earth has turned by about 25 degrees, so the satellite views a different part of the surface each time it passes. Metop, a European satellite due to be launched in 2006, will replace one of these satellites. As the orbit is much lower than that of the geostationary satellites, the images provide detailed information about the cloud structure. The UK receives images from a set of three passes, twice a day, from each satellite. The first pass is over the eastern Mediterranean, the second virtually over the UK, and the third over the eastern Atlantic. One set of passes occurs during the day and the other at night. There are also instruments that measure the temperature vertically through the atmosphere along the path of the satellite. The data from these is fed into numerical forecasting models, helping with the analysis of the state of the atmosphere and hence with the weather forecast.
Satellites carry a variety of instruments. Some of the instruments provide the images, with which most people are familiar - these are known as radiometers. Others measure the temperature and humidity vertically through the atmosphere - these are spectrometers and interferometers. Such remote sensing instruments are called passive because they measure the radiation being emitted by various parts of the atmosphere. Active remote sensing instruments are also used. These emit radiation from a transmitting device, such as radar, towards either the earth's surface or objects in the atmosphere, like clouds or falling rain, which reflect the radiation. The target attenuates the radiation pulse, making the reflected radiation different from the outgoing, and this difference can be measured. Such measurements are then used to assess surface wind speed, rates of rainfall and other useful parameters.
The information from spectrometers and interferometers is not available, even to weather forecasters. It is only used by numerical weather prediction models. However, the images that are created from the radiometers' data are of immense value in both analysing and forecasting the weather, and many of them are readily available to anyone with the appropriate equipment.Types of satellite images
Satellite images are available from a number of different channels which are used individually or in combination to reveal information about the atmosphere and surface. Two of these channels are commonly referred to as Visible and Infra-red.
A combination of visible and infrared images is very useful and can help distinguish between high and low cloud. For example, if a bright area appears on both the infrared and visible images in the same place, it is likely to be thick, high cloud. However, if the area appears bright on the visible image but dark on the infrared one, it is probably low cloud or perhaps fog. On the other hand, high-level cirrus cloud is readily detected on an infrared image but, unless quite thick, is barely detectable on a visible image.
Satellite images provide a 'real-time' view of weather systems and are available from many web sites, including Dundee University. Many schools and colleges also have systems that provide access to live weather satellite images and allow a time-lapse sequence of images to be displayed showing how weather systems develop over time.
(i) Analysing cloud patterns
In general, the clouds shown in satellite pictures can be classified as layer clouds or convective clouds. Layer clouds tend to cover large areas and are indicated on a satellite picture by an area of uniform brightness. This type of cloud is formed by either widespread condensation at low levels, often under an inversion, or by large-scale rising motion in the atmosphere, often associated with depressions or fronts. Convective clouds are usually formed by air being heated from below. Rising bubbles of air generate cloud while the surrounding descending air is cloud free. The individual clouds can be identified on a satellite picture, and it is sometimes possible to look at the build-up of thunderstorm cells.
Satellites and their instruments require a significant level of investment in order to be designed and built, while their launch has a higher risk of failure than installing other observational platforms. Operational costs are also high, but as these notes show, the benefits of satellite technology images are huge, providing large quantities of usable and relevant data on a global scale.
With each new generation of satellites, a new opportunity is presented in using the latest instrument technology. There is now a new generation of Meteosat satellites, known as Meteosat Second Generation (MSG). The first of these MSG-1 which was launched in 2002, the second, MSG-2 was launched on 21 December 2005. The later of these will be renamed Meteosat 9 once it is operational. Its radiometers can provide images with a similar resolution to those on polar orbiting satellites. There are additional instruments to measure the earth's radiation budget, which will be essential for climate studies, and to provide Search and Rescue (SAR) communications.
EUMETSAT - this is the European organisation that designs, builds and launches satellites, and the United Kingdom is represented by the Met Office.
MSG - Meteosat Second Generation
GMS - Geostationary Meteorological Satellite (Japanese)
GOES - Geostationary Operational Environment Satellite
INSAT - Indian National Satellite
NOAA - National Oceanic and Atmospheric Administration (US Department of Commerce)
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