At 7.57 am today, the India Meteorological Department’s Twitter handle posted this lovely image of fog over North India on January 21, as captured by the INSAT 3DR satellite. However, it didn’t bother explaining what the colours meant or how the satellite captured this information. So I dug a little.

At the bottom right of the image is a useful clue: “Night Microphysics”. According to this paper, the INSAT 3D satellite has an RGB (red, green, blue) imager whose colours are determined by two factors: solar reflectance and brightness temperature. Solar reflectance is a ratio of the amount of solar energy reflected by a surface and the amount of solar energy incident on it. Brightness temperature has to do with the relationship between the temperature of an object and the corresponding brightness of its surface. It is different from temperature as we usually understand it – by touching a glass of hot tea, say – because brightness temperature also has to do with how the tea glass emits the thermal radiation: at different frequencies in different directions.

INSAT 3D’s ‘day microphysics’ data component studies solar reflectance at three wavelengths: 0.5 µm (visible), 1.6 µm (shortwave infrared) and 10.8 µm (thermal infrared). The strength of the visible signal determines the amount of green colour; the strength of the shortwave infrared signal, the amount of red colour; and the strength of the thermal infrared signal, the amount of blue colour. This way, the INSAT 3D computer determines the colour on each point of the screen to produce an image like this:

CB is short for ‘cumulonimbus’

According to the paper:

The major applications of this colour scheme are an analysis of different cloud types, initial stages of convection, maturing stages of a thunderstorm, identification of snow area and the detection of fires.

The authors also note that the INSAT 3D is useful to image snow: while the solar reflectance of snow and the clouds is similar in the visible part of the spectrum, snow absorbs radiation of 1.6 µm strongly. As a result, when the satellite is imaging snow, the red component of the colour scheme becomes very weak.

The night microphysics is a little more involved. Here, two colours are determined not by a single signal but by the strength of the difference between two signals. The computer determines the amount of red colour according to the difference between two thermal infrared signals: 12.0 µm and 10.0 µm. The amount of green colour varies according to the difference between a thermal infrared and a middle infrared signal: 10.8 µm and 3.9 µm. The amount of blue colour is not a difference, and is determined by the strength of a thermal infrared signal of wavelength 10.8 µm.

And voila:

Perhaps a more explanatory image might help understand the each colour:

The ‘K’ denotes the temperature differences in kelvin. This image shows three kinds of clouds. A mature cumulonimbus cell, possibly part of a tropical storm, hangs over West Bengal and is visible mostly in red (but whose blue component indicates it is also very cold). Somewhere north of Delhi, flecks of green dominate, indicating a preponderance of lower clouds. Even further north, a the sky is dominated by a heavy, high cloud system that encompasses lower clouds as well.

By combining day and night microphysics data, atmospheric scientists can elucidate the presence of moisture droplets of different shapes and temperature differences over time, and in turn track the formation, evolution and depletion of cyclones and other weather events.

For example, taking advantage of the fact that INSAT 3D can produce images based on signals of multiple wavelengths, the authors of the paper have proposed day and night microphysics data that they say would indicate a thunderstorm impending in one to three hours.

Both INSAT 3D and INSAT 3DR use radiometers to make their spectral measurements. A radiometer is a device that measures various useful properties of radiation, typically by taking advantage of radiation’s interaction with matter (e.g. in the form of temperature or electrical activity).

Both satellites also carry atmospheric sounders. Despite the name, these instruments have nothing to do with sound. Instead, they measure temperature and humidity and study water vapour as a function of their heights from the ground.

Scientists combine the radiometer and sounder measurements to understand various atmospheric characteristics.

According to the INSAT 3DR brochure, its radiometer is an upgraded version of the very-high-resolution radiometer (VHRR) that the Kalpana 1 and INSAT 3A satellites used (launched in 2002 and 2003, respectively).

The Space Application Centre’s brief for INSAT 3A states: “For meteorological observation, INSAT-3A carries a three channel Very High Resolution Radiometer (VHRR) with 2 km resolution in the visible band and 8 km resolution in thermal infrared and water vapour bands.” The radiometers onboard 3D and 3DR have
“significant improvements in spatial resolution, number of spectral channels and functionality”.

The Kalpana 1 and INSATs 3A, 3D and 3DR satellites aided India’s weather monitoring and warning services with the best technology available in the country at the time, and with each new satellite being an improved as well as better-equipped version of the previous one. So while Kalpana 1 had a launch mass of 1,060 kg and carried a early VHRR and a data-relay transponder, INSAT 3DR had a launch mass of 2,211 kg – in 2016 – and carried an upgraded VHRR, a sounder, a data-relay transponder and a search-and-rescue transponder.

India deactivated Kalpana 1 in September 2017, after 15 years in orbit. The INSAT 3A, 3D and 3DR satellites are currently active in a geostationary orbit around Earth, at inclinations respectively of 93.5º, 82º and 74º E longitudes.

Featured image: INSAT-3DR satellite in a clean room, with its solar panel deployed, ahead of launch in August 2016. Credit: ISRO.