They photograph sunlight reflected by clouds in daytime and measure the heat clouds give off at all hours, using several wavelengths at once.
A weather satellite is a camera that sees more colours than your eyes. Each colour, or channel, shows something different about the clouds below. India's newest weather satellite, INSAT-3DS, carries a six-channel imager. It was launched on 17 February 2024 on GSLV-F14 and is funded by the Ministry of Earth Sciences.
The visible channel works like an ordinary photograph. Clouds reflect sunlight and look white; land looks grey and water looks dark. The US National Weather Service points out the obvious limit: visible pictures only work during the day.
Everything warm gives off infrared light. Infrared channels measure that heat, so they work day and night. Clouds are usually colder than the land or sea beneath them, which makes them easy to pick out. The higher a cloud top, the colder it is. Strong thunderstorms tower high into the atmosphere, so they have very cold tops, and forecasters use this to judge how strong a storm is.
A water-vapour channel shows how much moisture is in the upper atmosphere, roughly 4.5 to 9 km up, even where there are no clouds. Moist areas appear white and dry areas dark. Forecasters use it to spot where heavy rain could develop.
INSAT-3DS's first images, taken on 7 March 2024, used visible, short-wave infrared, mid-wave infrared and thermal infrared bands. ISRO says the imager helps capture clouds, aerosols, land surface temperature, vegetation health and water vapour. Because the satellite is geostationary, it watches the same part of the Earth all the time, so forecasters can follow a cloud system through the day.
Look at INSAT-3DS's position: it stays over the equator south of India, watching the same view of the subcontinent all day. Track India's weather satellites
Why infrared? Every object radiates, and the wavelength where it radiates most is set by its temperature (Wien's law):
λ_max = 2,898 μm·K / T
Earth's surface at about 300 K: λ_max = 2,898 / 300 ≈ 9.7 μm. A high cloud top at −60 °C (213 K): λ_max = 2,898 / 213 ≈ 13.6 μm. The Sun at about 5,800 K: λ_max ≈ 0.5 μm, in the visible.
So sunlight reflected by clouds is strongest in the visible, while the Earth and clouds glow mainly in the thermal infrared, around 10 μm. A thermal channel measures this glow and converts it to a brightness temperature. A cold reading means a high cloud top; a warm reading means low cloud or clear ground.
The water-vapour channel is chosen at a wavelength that water vapour itself absorbs strongly. Radiation from the ground cannot get through, so the satellite sees only the glow of the moist air high up. Where the upper air is dry, the satellite sees deeper and warmer layers, which appear dark.
Mixing channels makes products. Combining visible and infrared at the same moment tells low fog (bright in visible, warm in infrared) from high cirrus (thin in visible, cold in infrared). This is why INSAT-3DS's first images were shown as false-colour composites of several bands.
Visible images record reflected sunlight; at night there is no sunlight to reflect.
A tall thunderstorm's top is very cold, while low cloud is warmer, close to the surface temperature.
The amount of moisture in the upper atmosphere, even where there are no clouds.