About Satellite Images

Data from Multi-functional Transport Satellites (MTSAT), a series of geostationary satellites operated by the Japan Meteorological Agency (JMA), is the mainstay of the analysis and prognosis system of the Bureau of Meteorology. These satellites are situated over the equator at approximately 140°E (Central Australia) which provide excellent coverage of the Australian Region.

MTSAT data is used extensively within the Bureau to assist in real time analysis and forecasting and the data is fed into the Bureau's Numerical Weather Prediction (NWP) models.

The Bureau also receives data from China's Feng Yun-2 series of geostationary satellites which are situated over the equator at approximately 105°E (Indian Ocean). These satellites provide excellent coverage of the Indian Ocean and the western parts of Australia. For forecasters in Western Australia, this satellite can be more beneficial than MTSAT because the coverage provides a better view of the weather systems approaching the Western Australian coast.

Our Crib Point Satellite Earth Station is the only dedicated weather-related Satellite Earth Station (SES) in Australia, forming a critical part of the Bureau's operations.

The following types of imagery are available to the public as described below.

Infrared Images

Infrared (IR) images are derived from radiation emitted from the Earth and its atmosphere at thermal-infrared wavelengths (10-12 µm). These images provide information on the temperature of the underlying surface or cloud. 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.

Temperatures are represented by a greyscale, where black and white represents the hottest and coldest temperatures in the image, respectively. Given that clouds tend to be cooler than the ground or sea below (not always the case for low-lying clouds), this colour scheme results in clouds appearing as light grey to white, which makes IR images conceptually easier to compare with visible images.

To assist in interpreting greyscale images, methods can be used to colour all pixels representing a particular temperature range. The temperature of clouds is associated with their height, hence highlighting certain temperature ranges is a useful tool in estimating the height of the observed clouds. These temperatures are not absolute, however, so only an indication of the temperature can be obtained in this way (the actual temperature is within one or two degrees Celsius). Images coloured in this way are known as "false colour" images.

Bureau's false colour temperature images use a combination of coloured temperature ranges for colder temperatures (which are mostly associated with clouds) and a blue colour gradient for warmer temperatures. Over the Australian region at temperatures of about 1°C to 10°C it is difficult to determine whether the temperature represents clouds or just cold land. The blue gradient has been chosen to highlight the warmer regions of the imagery whilst not obscuring the cloud/cold landmass boundary. It must also be pointed out that over the alpine areas of SE Australia, the temperatures of the land can go below 0°C and as such, during winter, there may be some areas of the landmass coloured with the light blue colour.

The Bureau also provides false colour infrared images that use the Zehr colour enhancement, which was developed by Ray Zehr from the US National Oceanic and Atmospheric Administration (NOAA). This enhancement only applies temperature colour ranges to the cold end of the scale, which highlights very deep convection that is generally associated with tropical cyclones and thunderstorms. Hence, this type of images can be useful in tracking the movement of tropical cyclones.

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 images. For example, fog appears in visible images, but may not in infrared images. This is due to the temperature of the fog being very close to the temperature of the land below.

The intensity of the image depends on the albedo/reflectivity of the underlying surface or cloud. Hence, visible images are only available during daytime, since during night-time there is no reflected sunlight. Visible images are normally displayed in a manner similar to that seen by the human eye, though using a greyscale. Different shades of grey indicate 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.

Clouds/Surface Composite

The clouds/surface composites are created by combining information from two images. The Earth surface, the land and the oceans, is a static image from NASA's Blue Marble image set. The cloud cover, which is overlaid on top of this surface image, is derived from a greyscale infrared image (see Infrared Images), by removing the temperature range associated with the surface.

While a clouds/surface composite gives a good indication of the current cloud cover, and might be more pleasing to the eye than a greyscale image, it must not be confused with a visible image.

Image Updates and Timestamps

Satellite images are timestamped in "UTC" (Coordinated Universal Time), which is equivalent to GMT or Z time (Greenwich Mean Time). The timestamp on the images is the start time of the reception of the top of the image from the satellite. It takes approximately 25 minutes for MTSAT to complete a scan. Once a scan is completed, the image is generally available on the Bureau's website within 30 minutes.

Eclipse Mode

As geostationary satellites orbit the Earth, there are certain times during the year when the Earth is directly between the Sun and the satellites. This results in the Earth eclipsing the satellites. Such eclipses occur each year, usually between February to April and between August and October.

During these periods, often referred to as "eclipse operations" or "eclipse mode", the satellite observation schedule is altered due to the unfavourable conditions resulting from the eclipse. The satellites enter the Earth's shadow for some time each day, starting with several minutes to over an hour at the peak. Being in the shadow of the Earth, the solar cells do not generate enough power to keep the batteries charged and as such imaging and sounding instrumentation is turned off to preserve battery life. This means that some images or sounding data are unavailable for the period that the satellites are eclipsed.

An additional issue relates to the orientation of the satellites with respect to the Sun. In certain circumstances the Sun can shine directly into the optical path of the satellites' instrumentation causing overload on the sensors. This results in a phenomenon known as "flaring". The satellites are usually shut down to avoid the flaring and as such, a whole image or parts of an image may be missed.