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About the Satellite ImagesInfrared (IR) images | Visible images | Water Vapour Channel 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. 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). See below for the colour scheme we use. Colours In the colour Infrared IR satellite images, artificial colours have been added to indicate different temperatures. For this particular type of image, temperatures at the base of each colour band are:
IR Sensors IR imagery is available on: |
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MTSAT-1R IR1 10.3-11.3µm 
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MTSAT-1R IR2 10.5-11.5µm 
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| NOAA 16 Channel 4 10.3-11.3 µm |
NOAA 16 Channel 5 11.5-12.5 µm |
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| NOAA 17 Channel 4 10.3-11.3 µm |
NOAA 17 Channel 5 11.5-12.5 µm |
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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] Visible imagery is available on:
MTSAT-1R VIS 0.55-0.8 µm 
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] The water vapour channel is available on MTSAT-1R and FY-2C . Image Updates The time stamp on the images is the start time of the reception of the top of the image from the satellite. It takes approximately 30 minutes for MTSAT-1R to complete a scan and the image is generally available on the Web within 30 minutes of completion, i.e an hour after the time stamp. Sometimes an image is missed due to scheduled maintenance outages or a systems failure. The images remain available for 7 days. Time Stamps Satellite images are time stamped in "UTC" (Coordinated Universal Time), which is equivalent to GMT or Z time (Greenwich Mean Time). This is the start time of the reception of the top of the image from the satellite. It takes approximately 30 minutes for MTSAT-1R to complete a scan and the image is available on the Web within 30 minutes of completion, i.e an hour after the time stamp. The colour enhanced imagery web pages also have the equivalent time in Eastern Standard Time (EST). One hour needs to be added during daylight saving. Below is a comparison of current UTC to current Eastern Standard Time (EST), Central Standard Time (CST) and Western Standard Time (WST) or the Daylight Saving equivalent. Sunday 5 July 2009 01:55 UTC -- 5 July 2009 12:55 EDT -- 5 July 2009 12:25 CDT -- 5 July 2009 09:55 WST Visible images are only available during daylight hours, approximately EST 7am-7pm (21Z-09Z). To convert UTC times to local Australian times, add the time difference between your Australian time zone and UTC. For daylight saving add an extra hour to the difference between Australian times and UTC. During daylight saving, the difference for (EDT) i.e. New South Wales, Victoria and Tasmania is 11 hours instead of 10, and the difference for South Australia (CDT) is 10.5 hours instead of 9.5.
Browsing the Archive A browse service provides free access to browse images for Visible (1:32 resolution) or Infrared (1:8 resolution) channels to assist with the selection of required dates and times for ordering higher resolution images or radiances from the Bureau's Satellite Data Archive.
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