The weather and climate of the Earth can be monitored by means of meteorological satellites, also known as weather satellites.
Before these inventions came into use, the weather forecasters had very little knowledge regarding the state of the atmosphere at any given time. With the aid of such satellites, large amounts of useful data can be collected from Earth.
The use of meteorological satellites is not only focused on viewing the layout, structure and location of clouds however for instance, satellite images can be used to predict weather forecasts approximately 6 to 12 hours ahead, as well as serving as a valuable indicator of what is occurring in the atmosphere. Another use is that these satellites can detect and monitor fires, sand and dust storms, snow cover, ice mapping, city lights, effects of pollution, auroras, boundaries of ocean currents, energy flows and much more.
Therefore, with the aid of these satellites, Earth can be monitored and detect changes to try and help prevent and/or decrease the possibility of such disastrous events.
The first ever satellite Sputnik was launched in 1957, however the first meteorological satellite TIROS-1 was launched by the Americans in 1960. Over the years more low elevation orbit satellites based on TIROS were also launched. These were being used for observations and experiments. 
In 1966, the first geostationary meteorological satellite ATS-1 was launched by the Americans. It was then that satellite observation was seen to be effective for weather monitoring and due to this success that such satellites started being used to improve weather forecasting.
In 1963, the World Meteorological Organization (WMO) drafted the WWW (World Weather Watch) Programme and started a meteorological satellite observation network plan covering the globe. It was then that other countries joined in and also launched their own meteorological satellites. Such countries included Russian, China, Japan, India and Europe.
The diagram below displays the satellites around the globe.
Figure . Meteorological layout
Types of satellites
Meteorological observation is the key factor for meteorological satellites due to the capability of observing the whole earth uniformly with a fine spatial density.
There are two basic types of meteorological satellites:Â geostationaryÂ andÂ polar orbiting. (These can also be viewed in Figure 1)
These satellites orbit the Earth above the equator and complete one orbit in 24 hours in sync with Earth's rotation. Thus, they remain over the same location on the equator and can be called stationary.
Figure . Geostationary satellite
The main advantage is that they are able to take continuous images of the entire hemisphere below by means of visible-light and infrared sensors, therefore provide high time-scale resolution of their data. A fresh image of the full earth's disc is available every 30 minutes. However, their disadvantage is that they have limited spatial resolution as compared to the polar orbiting satellites in view of their distance from the earth.
Several geostationary meteorological spacecraft are in operation. Some of the examples of geostationary satellites are GMS (1400 E), GOES-W, GOES-E, INSAT-1 and INSAT-2 Series., GEOS, METEOSAT -5 (Positioned at 64 0 E), METEOSAT-6 etc. 
These weather satellites circle the Earth continuously, passing over the poles in the direction north to south. These satellites complete 14 orbits per day and thus can provide global coverage twice in 24 hours. They vary from geostationary satellites as these are in sync with the sun, therefore they are able to observe any place on Earth and will view every location twice each day with the same general lighting conditions due to the near-constant localÂ solar time.
Figure . Polar orbiting satellite
The main advantage is that they offer a much better resolution than their geostationary counterparts due their closeness to the Earth.
Some of the polar orbiting satellites are NOAA, IRS, ERS-1 &ERS-2, TRMM (low inclination), DMSP, Oceansat-1 etc. 
Uses of satellites
Weather satellites are usually used to forecast and predict the weather. However, these satellites provide Earth with many more usages other than just weather predictions.
Meteorological satellites can assist meteorologists by combining the satellite's data together with human input, such as pattern recognition, to be able to pick the best forecast. There are various types of forecasts and depending on which one is required they can be used to predict the temperature and pressure, wind force, weather warnings and rainfall.
For example in Sierra Nevada snowfield analysis is useful by keeping track of how much snow is available  as well as ice floes, packs and bergs.
Another important usage is that of using the visual and infrared photos taken by the satellites to display areas of pollution - either nature-made or man-made. For instance, aircraftÂ andÂ rocketÂ pollution, as well asÂ condensation trails, can also be spotted. Nowadays, the images are clearer and very accurate compared to the initial images that were taken in the beginning of weather satellite recordings which used to consist of only black and white images. Oil spills can be monitored by information provided by ocean current and low level wind. 
An issue which causes quite a lot of complications are dust and sand storms from the Sahara Desert. When these occur they travel long distances far into regions of the Atlantic Ocean. With the aid of satellites, images can be recorded to observe the storm size and location. GOES-EAST photos enable meteorologists to observe, track and forecast this sand cloud.
Sand clouds suppressÂ hurricaneÂ formation by modifying theÂ solar radiationÂ balance of the tropics. OtherÂ dust stormsÂ inÂ AsiaÂ andÂ mainland ChinaÂ are common and easy to spot and monitor, with recent examples of dust moving across the Pacific Ocean and reachingÂ North America.
One of the most critical uses of a weather satellite is in remote locations where there are only a few observers present. For instance, if a fire occurred and no one realized (for example in woodland away from civilization) this could get out of control causing a catastrophe if left burning for days. Weather satellites can be a life-saving asset in detecting such situations.
Once a fire is detected, the same weather satellites can provide vital information about wind that could fan or spread the fires. These same images from space (also called cloud photos) can predict and inform the authorities when it will rain.
Apart from being able to record images during the day, night images are also available. In such a case they would clearly show the burn-off in the gas and oil fields of theÂ Middle EastÂ and African countries. This burn-off throws large amount ofÂ carbon dioxideÂ into theÂ atmosphere and can be viewed by the thermal and infrared scanners.
Another occurrence is when New York City had a Blackout of 1977. This was captured by one of the night orbiteer DMSP (Defence Meteorological Satellite Program) space vehicles.
Here are a few examples of those uses :
Radiation measurements from the earth's surface and atmosphere give information on amounts of heat and energy being released from the Earth and the Earth's atmosphere.
People who fish for a living can find out valuable information about the temperature of the sea from measurements that satellites make.
Satellites monitor the amount of snow in winter, the movement of ice fields in the Arctic and Antarctic, and the depth of the ocean.
Infrared sensors on satellites examine crop conditions, areas of deforestation and regions of drought.
Some satellites have a water vapour sensor that can measure and describe how much water vapour is in different parts of the atmosphere.
Satellites can detect volcanic eruptions and the motion of ash clouds.
During the winter, satellites monitor freezing air as it moves south towards Florida and Texas, allowing weather forecasters to warn growers of upcoming low temperatures.
Satellites receive environmental information from remote data collection platforms on the surface of the Earth. These include transmitters floating in the water called buoys, gauges of river levels and conditions, automatic weather stations, stations that measure earthquake and tidal wave conditions, and ships. This information, sent to the satellite from the ground, is then relayed from the satellite to a central receiving station back on Earth.
There exist different methods on how to capture images from space.
The first and most simple technique is based on visible-light. Such an image is captured by weather satellites during local daylight and represents the intensity of sunlight reflected from clouds and/or the Earth's surface. These images are quite easy to interpret even by the less technical person, as the high reflectance is bright whilst low reflectance is dark. For example, clouds would be displayed as bright whilst land would be dark.
The structure of the image will consist of clouds, cloud systems (fronts and tropical storms), rivers, lakes, woodland, mountains, snow, fire, pollution and even wind can be determined by means of the cloud patterns, alignments and movements from one successive image to the next.
The disadvantage of visible-light images is the inaccuracy during early morning and evening as the image would be over all darker and would project a false description.
The uses of visible-light images consist of the following:
Distinction between thick and thin clouds:
The reflectance of a cloud depends on the amount and density of the cloud droplets and raindrops contained in the cloud. In general, low-level clouds contain a larger amount of cloud droplets and raindrops and therefore they appear brighter than high clouds. Cumulonimbus and other thick clouds that have developed vertically contain a lot of cloud droplets and raindrops and they appear bright in a VIS image. Through some thin high-level clouds, the underlying low-level clouds and land or sea surface can be seen.
Distinction between convective and strati form types:
Cloud types can be identified from the texture of the cloud top surface. The top surface of a strati form cloud is smooth and uniform while the top surface of a convective cloud is rugged and uneven. The texture of a cloud top surface is easily observed when sunlight hits the cloud top obliquely.
Comparison of cloud top height:
If clouds of different heights coexist when sunlight hits them obliquely, it may happen that the cloud of higher top casts a shadow onto the cloud top of lower height. Comparison of cloud height is possible using this property.
Figure - Visible-light image
Thermal or Infrared
Another technique is based on thermal or infrared images recorded by sensors called scanning radiometers. These images allow meteorologists to determine cloud heights and types, to calculate land and surface water temperatures, and to locate ocean surface features.Â The image itself is represented by a temperature distribution and is at an advantage when compared to visible-light images as these can be observed day or night without any difference.
The portions of low temperature are visualized bright and portions of high temperature dark.
Figure - IR image of a hurricane
The uses of infrared images consist of the following:
Observation of cloud top height:
It is possible to know the cloud top temperature with the infrared image. If the temperature profile of atmosphere is known, the cloud top temperature can be converted into cloud top height. For the estimation of the temperature profile, values by objective analysis or Numerical Weather Prediction (NWP) are often used. In the troposphere, atmospheric temperature is generally lower at the upper layer, and therefore the lower cloud top temperature means a higher cloud top height. It is also possible to monitor the developing level of clouds in the vertical direction by referring to the cloud top temperature.
Measurement of earth's surface temperature:
With the infrared image, it is possible to measure the earth's surface temperature in cloud free areas in addition to the cloud top temperature. This gives useful information on sea surface temperature over the ocean that has sparse area of meteorological observations.
Temperature distribution is also represented in a water vapour image. Such as image is very similar to the infrared images, however focuses on vapour.
Since temperature is high and a large amount of water vapour is near the earth's surface and in the lower layer, the amount of infrared emission is large but most of it is absorbed by water vapour and little emission reaches the satellite.
With increasing height, temperature falls and the amount of water vapour decreases. In the upper atmosphere, the temperature is still lower and the amount of water vapour is still smaller, and almost all infrared re-emission reach the satellite without absorption but the actual amount of radiation reaching the satellite is small.
In dry portions of little water vapour content in the upper and middle layer, the image is dark because the temperature is high due to the contribution of radiation from the lower layer. In wet portions with a lot of water vapour in the upper and middle layer, the image is bright because the temperature is low due to the contribution of the radiation from the upper and middle layer.
Figure - Water vapour image
Grasp of airflow in the upper and middle air
The WV image can also represent radiation from the water vapour content in the upper and middle layer. That is, the airflow in the upper and middle layer can be visualized using water vapour as a tracer even if no cloud is present. The position of troughs, vortices and jet streams in the upper and middle layer can be estimated from the distribution of bright and dark areas on the WV image.
The amount of radiation observed by the satellite is the sum of blackbody radiation from clouds and the earth's surface and reflected sunlight. In the 3.7-Î¼m wavelength band, reflected sunlight is more intense than the radiation from the earth's surface in comparison with the IR1 and IR2 wavelength bands (Figure 1-3-3). Therefore, the image in the daytime is similar to the distribution of reflected sunlight. On the other hand, the image in the night when there is no reflected sunlight differs from that in the daytime. In the night, infrared radiations from clouds and the like are observed. Thus, the appearance of the 3.7-Î¼m image is very different between day and night and therefore it is necessary to take care in using it. At sunrise and sunset, in particular, it is necessary to consider to what extent the influence of sunlight reaches.
Figure - 3.7-um image
Identification of low clouds in the night
Low clouds in the night are difficult to identify in the IR image, however, their identification accuracy is better in the 3.7-Î¼m image. Consider a low cloud (water cloud) above the ocean (Figure 1-3-5). The low cloud can be nearly considered as a blackbody in the IR1 wavelength band. In the 3.7-Î¼m wavelength band, the emissivity of the water cloud is smaller than in IR1 band, and the transmittance is about zero for clouds having enough thickness. The water cloud as observed at 3.7-Î¼m cannot be considered as a blackbody. Therefore, the cloud top is observed lower at 3.7-Î¼m than that observed by IR 1. The sea surface can be considered as a blackbody at both 3.7-Î¼m and 11-Î¼m wavelength. Therefore, for the low cloud, which is a water cloud, the temperature difference between the cloud top and sea surface is larger in the 3.7-Î¼m image than in the IR image and the detection accuracy is improved. Because this relationship holds good for water clouds, whether the cloud detected in the 3.7-Î¼m image is a low or middle cloud can be determined by checking the cloud top height using the IR image at the same time.
Identification of snow/ice area in the day
At 3.7-Î¼m, the reflection of sunlight at the snow and/or ice surface is low just like the ice crystals (Kidder and Wu, 1984). It is difficult to distinguish between a snow and/or ice surface and a cloudy area in the VIS image alone because they both have high reflectance. Using that property, however, it is possible to identify them by comparison with the 3.7-Î¼m image.
e) Images from microwave radiometer such as Special Sensor Microwave/
Imager (SSM/I), and TRMM Microwave Imagers (TMI) can provide a
lot of useful information. Microwave radiation is not affected by the
presence of clouds and that is an important factor in the science of weather.
Microwave observations are widely used for inferring sea surface
temperature, sea surface wind speed and atmospheric water vapor content
(over ocean surfaces), cloud liquid water content, rainfall, and the fraction
of ice/snow particles within the raining systems.
Most remote sensing instruments (sensors) are designed to measure
photons. The fundamental principle underlying sensor operation centers on
what happens in a critical component - the detector. This is the concept of
70 Meteorological satellites
the photoelectric effect (for which Albert Einstein, who first explained it in detail,
won his Nobel Prize). This, simply stated, says that there will be an emission
of negative particles (electrons) when a negatively charged plate of some
appropriate light-sensitive material is subjected to a beam of photons. The
electrons can then be made to flow from the plate, collected, and counted as
a signal. A key point: The magnitude of the electric current produced (number
of photoelectrons per unit time) is directly proportional to the light intensity.
Thus, changes in the electric current can be used to measure changes in the
photons (numbers; intensity) that strike the plate (detector) during a given
time interval. The kinetic energy of the released photoelectrons varies with
frequency (or wavelength) of the impinging radiation. But, different materials
undergo photoelectric effect release of electrons over different wavelength
intervals; each has a threshold wavelength at which the phenomenon begins
and a longer wavelength at which it ceases. Meteorological satellite sensors
can be broadly classified as two types : passive and active (Fig. 2). Passive sensors
do not use their own source of electromagnetic illumination, and depend upon
the radiation emitted or reflected from the object of interest. On the other
hand, active instruments use their own source of electromagnetic radiation
which they use to illuminate the target, and in most cases use the properties
of reflected radiation ( e.g. intensity, polarization, and time delay etc.) to
deduce the information about the target. These sensors can be further subdivided
into the following categories and subcategories :
Equally important is the functional classification of these sensors.
Meteorological satellite sensors may be deployed to obtain one and/or more
of the following characteristics of different objects of the land-oceanatmosphere
(a) Spatial Information : The examples are the extent and temperature of sea
surface, clouds, vegetation, soil moisture, etc. The main objective here is
to obtain the required information over a 2-dimensional plane. The best
suited sensors for this class are imaging radiometers operating in visible,
infrared or microwave frequencies. Active sensors like Synthetic Aperture
Radar (SAR) are also put to effective use for the imaging applications.
(b) Spectral Information : For certain applications, the spectral details of an
electromagnetic signal are of crucial importance. A particular object of
interest, for example an atmospheric layer, or, the ocean surface, interacts
differently with different wavelengths of electromagnetic(EM) spectra. In
most cases, this may be due to the chemical composition of the object.
Absorption, emission, or reflection of an EM radiation from an object is
a function of the wavelength of EM radiation, and the temperature of the
object. Thus, the spectral information can provide details of chemical
composition, and/or the temperature of the object. Meteorological satellite
sensors use this information for sounding applications, where the vertical
structure of temperature, humidity, and in some cases, the atmospheric
gases is retrieved. An example of this sensor is High Resolution IR Sounder
(HIRS), and Advance Microwave Sounding Unit (AMSU) onboard NOAA
series of satellites. Future satellites will carry more advanced sensors like
imaging spectrometers. Geostationary Imaging Fourier Transform
Spectrometer (GIFTS) is a fine example of this new-generation sensor.
GIFTS, when operational, is expected to provide the vertical profiles of
temperature, humidity, and winds at several atmospheric layers in vertical.
(c) Intensity Information : The intensity of EM radiation can provide several
clues about the object of interest. In most cases, the satellite sensors
measure the intensity of the radiation reflected from the object to know
the dielectric properties and the roughness of the object. By the use of
suitable algorithms these parameters can be translated to the properties
of geophysical parameters like soil moisture, ocean surface roughness,
ocean surface wind speed, and wind direction, etc. The sensors that use
this information are radar, scatterometer, and polarimeters.
Finally, Earth has gained a great deal by the usage of meteorological satellites. However, despite that the current information is an aid to the meteorologist it is essential that new techniques and new ways of analysing data are established over the years.
Currently, even with such a great and expansive effort, only spotty information can be provided which by its very nature lacks one quality of observation most necessary to synoptic meteorology, that of complete continuity in time and space.
One aim for the future would be to be able to record all the meteorological data from around the globe - hence including parts of the world which are not presently being scanned. Thus, satellites will be continuously capturing an overall picture of Earth, day and night so no matter how minimal a change it will be recorded and noted.
http://rammb.cira.colostate.edu/wmovl/vrl/Texts/SATELLITE_METEOROLOGY/CHAPTER-1.PDF last assessed on: 12/03/2011
C.M. Kishtawal, "Meteorological Satellites", Atmospheric Sciences Division, Meteorology and Oceanographic Group, Space Application Centre (ISRO), Ahmedabad
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