Description on images:
Notices to user:
- True Colour image is available at day time only. The image likens the view as seen by naked eyes from space.
- Sea Surface Temperature image presents the water temperatures at the sea surface. The image is generated from using observations from infra-red channels of MODIS. The colour scale provides a read-out of the temperature. As clouds may obstruct the observation, the image is usually constructed from a number of satellite observations over the same region. If an area has been covered by clouds for a long period, the sea temperature over the area on the image may represent observation days ago.
- Aerosol optical depth (AOD) is a measure of the opaqueness of air, and high values of AOD indicate poor visibility. Generally speaking, higher values of AOD are represented in red and yellow colours and lower values of AOD are depicted in blue and green colours.
- Water droplets in the atmosphere including cloud, fog and mist may affect AOD measurement by the satellite. In generating the AOD images, an automatic means is in place to filter the affected data to the extent possible. The area with removed data will be shown in white colour in the AOD image. Some thin clouds and mist may however escape the filter and appear to be aerosol in the image. Care should be taken in interpreting the AOD images. Please refer to the true colour image to see if the area is actually covered by clouds. Please also cross-check the actual surface visibility observations, such as those in the webpage on visibility in Hong Kong Waters.
- Area with no data will also appear in white colour.
- Due to dissimilar surface optical characteristics of land and sea, different methods are used to derive the AOD values over land and those over the sea. Both methods produce AOD values with reference to the wavelength of 550 nm (greenish-yellow light). However, the accuracy of AOD values over the coastal ocean may be less optimal than those over green vegetated land.
- AOD image is available at day time only.
- Acknowledgement: The plotting software for AOD images is adapted from a software originally developed by the Center for Coastal and Atmospheric Research of the Hong Kong University of Science and Technology. The processing of true colour images is adapted from a software originally developed by the Space Science and Engineering Centre of the University of Wisconsin-Madison, USA.
- The images are captured by MODIS, an acronym for MODerate Resolution Imaging Spectroradiometer, which is a sensor on board of Terra and Aqua satellites of the Earth Observing System (EOS) operated by the U.S. National Aeronautics and Space Administration (NASA).
- Terra and Aqua are polar-orbiting satellites. They revolve around the Earth along paths passing over the polar region. As they move, they take pictures of different parts of the Earth. Two images are normally received in Hong Kong from each satellite every day.
- MODIS has 36 observational channels, covering a wide frequency spectrum ranging from visible to infra-red radiation, making it valuable data source for a number of disciplines such as meteorology, oceanography as well as environmental monitoring. The high-resolution images from MODIS are particularly useful for monitoring fine features or phenomena like hill fire and haze.
- There may be occasional signal interference and blockage by obstacles during image reception, causing black lines to appear in the image.
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- Anyone wishing to further disseminate these satellite images should seek permission from NASA.