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Section 7 4 clouds
The CLOUDS layer puts the weather on the globe, live, from NOAA/NESDIS's Global Mosaic of Geostationary Satellite Imagery — GOES-East and GOES-West, both Meteosats and Himawari, stitched into one picture of the planet every hour.
Like the aurora it is drawn as a depth of air rather than as a picture stuck on a sphere, and for the same reason: that is what it is. What makes that possible is the infrared channel. Brightness in the infrared is cloud-top temperature, and temperature is altitude — so the renderer is handed a height field taken from measurement, and a thunderhead stands fifteen kilometres tall over the stratus beside it because it really does. The Sun lights the tops and they shade their own undersides, which is what makes a deck read as three-dimensional rather than as fog, and the limb shows the deck standing off the surface because a grazing line of sight crosses a great deal more of it.
Two channels are fetched. Infrared is the backbone and works in the dark. Visible is a correction on the sunlit half only: low warm cloud is nearly invisible to infrared — the top of a marine stratus deck is within a few kelvin of the sea under it — and obvious in visible light, so where the Sun is up that channel fills in what the first cannot see.
What is real and what is not. The cloud field is measured. The lightning is simulated — and the readout along the bottom of the window says so, because a globe that flickers with plausible-looking strikes must not be read as showing strikes. What comes from the data is where the thunderstorms are, how large, how tall, and how often each should flash: cold-top area is the oldest satellite proxy there is for flash rate and a good one. What is invented is which millisecond a given stroke fires. No free worldwide feed of individual strikes exists to use instead. The flashes light the cloud from inside rather than being drawn as marks on it, so an anvil goes bright from below.
Four honest limits, all of them stated in the readout or visible in the picture:
- Nothing is known above about 73°. A ring of geostationary satellites cannot see the poles, so the layer fades out towards them rather than guessing. The aurora owns those latitudes anyway.
- The picture is an hour or more old. The mosaics are published hourly and run about an hour and a quarter behind the clock. The readout gives the hour the picture is of, never the moment it was fetched.
- Cloud-top height is a fit, not a retrieval. The mosaic is a rendered image rather than a calibrated field, so the brightness-to-temperature ramp is a fit to the standard infrared enhancement. The shapes are exactly what the satellites saw; the heights are the right heights to within a kilometre or two.
- A cloud field is a difference. Cloud is measured as brightness above a locally estimated clear-sky background — which is what stops the cold winter hemisphere and the polar night being read as an overcast, and what makes the deserts come out clear. The cost is at the other end: an overcast broader than the window used to estimate it sets its own background and reads thinner than it is.
The vertical scale is exaggerated about six times. Eighteen kilometres on a six-thousand-kilometre planet is a quarter of one per cent of the radius, and a hairline cannot be volumetric; six times over is enough for a storm to stand up out of the deck and shallow enough that nobody would mistake the result for a mountain range. Altitudes are fractions of the radius the globe is drawn at, so the deck stays glued to the surface at any setting of the body scale.
Cloud rendering on the UI settings tab (6.3) chooses how the deck is drawn. Layered stacks slices through the troposphere and is the cheap option. Volumetric walks a ray through it instead, so the Sun casts the cloud tops onto the deck below and a flash glows out through the storm making it rather than only brightening its outside — at several times the cost per pixel. Both draw the same weather.
From the User Manual. Home.