1. Making Color Images
Meteorological satellite images are originally gray-scale images with intensity information, just like black-and-white photographs. To use color information in addition to intensity information, we make color images from gray-scale satellite images using following methods.
Using a color image as a background image
This is the method to use a color image as a background image, and illustrate clouds as opaque objects covering the background. Moreover, to differentiate the opacity of clouds based on their types, cloud types can be represented on the image. This is the method used in this web site. In addition, we actually use two types of background images as follows.
Background image using the digital elevation map (GTOPO30)
This is a background image whose coloring is based on the elevation of the place derived from the digital elevation map. This scheme is almost identical to the coloring of a map and does not represent the true color of the earth. Like a map, if we assume a virtual illumination from some direction, we can get the 3D representation using shading. Moreover, if we set the direction of illumination as the real direction of the sun at some particular date and time, we can create a background image correct in the sense of illumination by the sun.
Background image using true color data (Blue Marble)
This is a background image whose coloring is based on the true color of the earth derived from the Blue Marble. Wallpaper images (Earth / Typhoon) and AMeDAS images use this type of background images with a minor modification. In comparison with digital elevation map, we can create an image which looks similar to what we can obtain from observing the earth from space.
In addition, the following methods are not used in this web site, but are popular in other places.
Combining multi-spectral observations
Meteorological satellites are making
observations in multiple spectral ranges (bands) such as visible and infrared. Each image is a gray-scale image, from which we can create a color image by assigning appropriate color (RGB) to each band or the combination of bands. The purpose of this representation is to improve the significance of information regarding the difference of bands. This does not mean that we can obtain an image which looks similar to real colors.
Using pseudo color for temperature
A pseudo color image, or a color enhanced image, assigns conspicuous color on important values to attract human's attention to clouds with low cloud top temperature, for example. The low temperature of clouds means high and thick clouds, so it may directly lead to very heavy precipitation. Hence red or purple is usually assigned for clouds with low temperature. This representation is not often used in Japan, but is very popular in the United States and others. For example, color enhanced images (BD enhancement and NHC enhancement) provided in the typhoon image database is created from the conversion rules actually used in meteorological organizations in USA and others.