There is a huge range of weather and climate products that are available through different institutions and organisations. A list of products that can be accessed online is given below. Please note that this is a simple description of the general product. For specific information on any product you can contact the source institution directly through their respective websites.
The various products available online may be classified into three main groups; Historic, Near-real time and Forecast. In each of these categories there may be a number of products produced by various sources. Some of these are listed below.
HISTORICAL DATA
Observed records
This is the recorded rainfall, temperature, wind speed and direction, etc, of a particular weather station.
Climatology
This is the description and scientific study of climate. Descriptive climatology deals with the observed geographic or temporal distribution of meteorological observations over a specified period of time. Scientific climatology addresses the nature and controls of the earth's climate and the causes of climate variability and change on all timescales. The modern treatment of the nature and theory of climate, as opposed to a purely descriptive account, must deal with the dynamics of the entire atmosphere–ocean–land surface climate system, in terms of its internal interactions and its response to external factors, for example, incoming solar radiation. Applied climatology addresses the climate factors involved in a broad range of problems relating to the planning, design, operations, and other decision-making activities of climate sensitive sectors of modern society. Climatology often refers to the long term averages of a station or area.
(Source: http://amsglossary.allenpress.com/glossary/search?id=climatology1)
NEAR REAL-TIME
Real-time data or observations are when the reporting or recording of events are nearly simultaneous with their occurrence. Sometimes observations are made available a short time after their actual recording to allow for retrieving and post-processing of the data. Such information is then referred to as near real-time data.
Synoptic chart
In meteorology, this is any chart or map on which data and analyses are presented that describe the state of the atmosphere over a large area at a given moment in time. The possible variety of such charts is almost limitless, but in meteorological history there has been a more or less standard set of synoptic charts, including surface charts and the constant- pressure charts of the upper air. Other synoptic charts include isentropic charts and constant- height charts, both used for upper-air analysis. There are a number of auxiliary and special- purpose synoptic charts, including thickness charts, tropopause charts, stability charts, change charts, continuity charts, etc., that have useful applications for preparing forecasts of weather events at various locations.
(source - American Met. Soc.: http://amsglossary.allenpress.com/glossary/search?p=1&query=synoptic+chart&submit=Search )
Another good description of synoptic charts is given on the following weblink: http://www.metoffice.gov.uk/education/secondary/students/charts.html
Radar
Weather radar is a remote sensing instrument using microwave energy between X-band (3 cm wavelength) and S-band (10 cm wavelength). A short pulse of high power microwave energy is produced by a magnetron in the transmitter system and this energy is focused by an antenna system into a narrow beam. This pulse of energy travels through the atmosphere at the speed of light (3 x 10 8 ms-1). When a target such as a raindrop is encountered, some of the energy is scattered of which a minute fraction is in the direction back to the antenna system were a sensitive receiver system is used to process and amplify this received power into useful data. From the azimuth and elevation information on the pointing direction of the antenna, the time between transmitting and receiving and the power of the received signal, the target location can be determined as well as its intensity or reflectivity. dBZ is the unit used we use for reflectivity in meteorology. dBZ is related to the number of drops per unit volume and the sixth power of their diameter and it can be related to rainfall rate through an empirical relationship called a Z-R relationship. In the table below a guideline on the interpretation of dBZ factors are given.
In the table below a guideline on the interpretation of dBZ factors are given.
Please take note that various atmospheric and environmental conditions can negatively affect radar data and caution should be exercised when interpreting the information. Some of these effects include:
• return from mountains and other non-meteorological targets, • attenuation of the radar signal when viewing weather echoes through areas of intense precipitation(with C-band radars), • temperature inversions in the lower layers of the atmosphere which bend the radar beam in such a way that ground clutter is observed where normally not expected, • the "bright band" which is a layer of enhanced reflectivity caused by the melting of ice particles as they fall through the 0oC level in the atmosphere and which can result in over-estimation of rainfall.
(Source: http://www.weathersa.co.za/References/RadarInfo.jsp)
Info on the national radar network (Location, lat/long, height above sea level, etc) http://www.weathersa.co.za/References/RadarNetwork.jsp
Satellite images
Satellites provide a huge variety of information. They carry instruments that relay telecommunications signals (telephone messages, TV pictures, emergency messages from ships and aircraft, etc.), help in navigation, measure changes in vegetation or movements in the earth's surface and observe the atmosphere. Those that observe the atmosphere are known as weather satellites and the information they provide is used by weather forecasters, as well as others with an interest in the weather. Most people are now familiar with the pictures that are shown on the TV Weather Forecast, but there are other types of observation being made in the atmosphere.
The first successful weather satellite was called TIROS1 and was launched on 1 April 1960. The subsequent launch of other observing systems has resulted in the creation of an imaging network on a truly global scale. Information is now available for inhospitable land areas and the oceans, where weather data were previously largely unavailable.
The advent of weather satellites has also provided a continuous, automatic feed of data, with a coverage and resolution (horizontal, vertical and temporal) not possible by any other means. Therefore, we can now 'look down' and record what is happening, and the information from satellites helps in the prediction of changes in the weather. (Source: http://www.metoffice.gov.uk/education/secondary/students/satellites.html )
Types of 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 remain 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
Satellite instrumentation
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. (source: http://www.metoffice.gov.uk/education/secondary/students/satellites.html )
Satellite receiving bands
1) Infrared images Thermal Infrared (IR) images show the temperature of the land, the sea or the tops of the clouds above them. Warm temperatures (0-30° C) generally mean land or sea without cloud cover. As the temperature decreases it implies that clouds are getting higher and denser. Very cold temperatures mean that cloud tops are very high, which can imply strong convective storm activity.
IR imagery is derived from emission from the Earth and its atmosphere at thermal-infrared wavelengths (10-12 µm) and provides information on the temperature of the underlying surface or cloud. However, since the emitted radiation must traverse the Earth's atmosphere before reaching the satellite, it is modified during passage by atmospheric absorption and re-emission.
The conventional displaying of IR images in black and white is to present them so they are consistant with the appearance of visible images by having the clouds appear in white shades against the darker background of the Earth. Since the temperature normally decreases with height, the IR radiation with the lowest intensity is emitted by the highest and coldest clouds and these appear whitest. This is convenient but is the reverse of the procedure used for VIS images where the lowest reflectivities appear black. Quantitative measurements of the temperature of an emitting surface needs to take account of absorption and emission within the window. However for qualitative interpretation the atmosphere can normally be considered as transparent in the window region. The only exception to this is in the very warm, high dew point air in the tropics where imagery of cloudless air may show patterns of grey shades that are related to the humidity distribution. [Source Bader et al, 1995]
IR images are available 24 hours per day because temperatures can always be measured, regardless of day or night. (This is in contrast to Visible images which are only available during the day. See above.)
The temperatures can be represented in a grey-scale (black is no-cloud, and increasing white means higher colder clouds), or in a colour scheme (dark-blue for land/sea and low cloud, through various colours for mid temperatures to very light shades for very cold high clouds). (Source: http://www.bom.gov.au/weather/satellite/about_satpix.shtml#IR )
2) Visible images Visible (VIS) images are a record of the visible light scattered or reflected towards the satellite from the Earth and clouds. i.e. you can 'see' the clouds. Visible images give meteorologists extra information that may not appear on Infrared temperature images. For example, fog appears in Visible images, but may not in Infrared images when the fog and the land are at the same temperature.
The intensity of the image depends on the albedo/reflectivity of the underlying surface or cloud. Visible images are only available during the daytime, because at night the world looks black. Early morning 7am VIS images show the sunlight rising in the east, and the 7pm VIS images show the sun setting in the west.
VIS images are normally displayed in a manner similar to that seen by the human eye. Using a black and white colour scale, with different shades of grey indicating different levels of reflectivity, the brightest and most reflective surfaces are in white tones and the least reflective in black. In general, clouds are seen as white objects against the darker background of the earth's surface. The brightness also depends on the intensity of the reflectivity and the relative positions of the sun and satellite with respect to the earth. Shadows and highlights can be seen where the sun shines obliquely on to cloud. [Source Bader et al, 1995] (Source: http://www.bom.gov.au/weather/satellite/about_satpix.shtml#IR )
3) Water Vapour channel
Water Vapour imagery is derived from radiation emitted by water vapour at wavelengths around 6-7 µm. This is not an atmospheric window but a part of the spectrum where water vapour is the dominant absorbing gas. The centre of the absorbing band is 6.7µm.
Emissions from water vapour low in the atmosphere will not normally escape to space. If the upper troposhpere is moist, the radiation reaching the satellite will mostly originate from this (cold) region and be displayed in white shades, following the IR imagery colour convention. Only if the upper atmosphere is dry will will radiation originate from water vapour at warmer, mid-troposheric levels and be displayed in darker shades on the image. In normally moist atmosphere , most of the WV radiation recieved by the satellite originates in the 300-600 hPa layer, but when the air is dry some radiation may come from layers as low as 800hPa. Because of the general poleward decrease of water vapour content, the height of the contributing layer gets lower towards the poles. [Source Bader et al, 1995] (Source: http://www.bom.gov.au/weather/satellite/about_satpix.shtml#IR )
Useful satellite Acronyms
EUMETSAT - this is the European organisation that designs, builds and launches satellites 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) (Source: http://www.metoffice.gov.uk/education/secondary/students/satellites.html)
FORECASTS
Daily to weekly forecast
A daily forecast is simply a probability of atmospheric condition that may prevail during that day. Using various methods (described in the section below), this type of forecast can have a high accuracy and skill. The skill usually deteriorates as we move to the longer time range.
Mid-range Forecast (Weekly to Monthly)
Forecast extending from one week to one month give probabilities of atmospheric systems that may influence the weather. They describe the likelihood of these systems occurring during the period.
Seasonal Forecast
A seasonal forecast is made for time periods of one to three months. The lead time for this forecast may vary it can be one month to six months depending on the variable being forecasted. Some common questions asked about seasonal forecasts are answered in the section below;
Is the seasonal forecast the same as a weather forecast? No. The seasonal forecast is a CLIMATE forecast. There is a distinct difference between weather and climate. An area’s climate can be described as the “average weather” calculated over a long period of many years. Climate forecasts are different from weather forecasts since the former involves the prediction of deviations in the seasonal average of the weather. The weather at particular locations and at specific times can therefore seem to contradict the seasonal forecasts.
What is a probability forecast? The seasonal climate forecasts are expressed probabilistically. Forecasts are made for three equiprobable categories of below-normal (dry conditions), near-normal (around average) and above normal (wet conditions). A probability is assigned to each category, indicating the chance of the particular category to occur during the target season. The subsequent forecast probabilities indicate the 1) direction of the forecast and 2) amount of confidence in the forecast. Forecast users should be particularly aware of the probabilities of the non-favoured categories, as these probabilities are never small enough to disregard.
How is the confidence in the forecast reflected in the probabilities? The higher the confidence in the forecast, the higher the assigned probability (chance) will be for that specific category to occur. When there is no confidence in the forecast, climatological probabilities (33%) are assigned to the three categories.
What is near-normal? Near-normal is NOT the average, but an interval for a particular region. The near-normal interval for a specific region is calculated statistically for each season, e.g. the near-normal for January for Gauteng is 95 to 129 mm, and usually contains the average value.
Which area is covered? Seasonal forecasts are only relevant for large areas and give an indication of the expectation for a long period of time, e.g. a season. Seasonal forecasts are NOT suitable for small, localized areas for specific days. In addition to this, the boundary between forecast regions (the thick grey line) should be seen as a transition zone and not an absolute boundary.
Does an above-normal forecast mean that we are going to have a “good” season? A forecast for above-normal rainfall does not necessarily imply a “good” season since the timing of the rainfall events within a season remains unknown. The same argument holds true for below-normal rainfall and a “bad” season.
(Source: South African Weather Service)