Size & scale

The rainbow colour map

One degree buys seventeen units of colour here and half a unit over there.

a.k.a. jet · the rainbow palette · the spectrum scale · hsv · cmap="jet" · the stoplight gradient · non-perceptual colour scale · rainbow considered harmful

A colour ramp is an axis: it converts a quantity into an appearance, and the reader converts it back. Lightness is the one channel with an order everybody already agrees on — hue has none until a convention supplies one — and a rainbow ramp does not spend its lightness in order. Jet climbs from dark blue through cyan to a peak in the yellow, then falls away into dark red, so the brightest thing on the chart is a value from the middle of the range and both ends of the scale arrive drawn dark. The rate is wrong too: equal steps of data buy wildly unequal steps of colour, fast where two hues meet and slow where one hue runs on, so the ramp invents hard edges in smooth data and flattens real gradients elsewhere. None of it shows up in the usual audit, because the scale is linear, the legend is printed, no bin edge was chosen and no value was touched.

How to spot it

Find the brightest patch and read its value off the legend. On a well-behaved ramp that patch is at one end of the scale; on a rainbow it is usually somewhere in the middle.
Ask which channel is carrying the order. Hue has no order — nothing tells a reader whether cyan outranks green — so if lightness is not climbing from end to end, the ranking is being supplied by convention or by nothing.
Convert it to greyscale, or just print it. If the picture stops making sense, the colours were doing work the lightness was not.
Look for a boundary that is too sharp for the quantity. Rainbows accelerate where hues meet, so a smooth field grows a crisp contour there — and a reader will name it, schedule around it, and ask what happened at that value.
Look for the flat stretch too: a broad region of one hue where the data is still moving. Detail lost to it never announces itself.
Red–green ramps at all. Around one man in twelve — more among readers of northern European descent, fewer elsewhere — reads the two ends as much the same hue, and a simulation takes ten seconds.
The names: jet, rainbow, hsv, turbo, spectral, or a hand-rolled red-amber-green gradient. Rarely a default any more, but still the setting in GIS packages, older ParaView, thermal-camera firmware, Excel’s three-colour scale and every chart inherited from a colleague who typed it years ago.

The fix

Choose the ramp for the job and let lightness carry the order. For magnitude that means one hue running light to dark, or a multi-hue ramp built so that lightness only ever climbs — viridis, cividis, magma, or the Scientific colour maps Crameri publishes and which he, Shephard and Heron argue for in their 2020 paper on the misuse of colour. Keep a diverging pair with a neutral middle for data that has a real zero to diverge about, and never put a hue at that midpoint. Then run two checks rather than trusting one, because they catch different things: desaturate the finished picture and see whether it still reads, then simulate colour-vision deficiency and look again. They are not the same test — a dichromat is not a monochromat, and keeps a full blue–yellow axis — and both take about ten seconds. Give the ramp the things an axis gets: a legend with numbers on it, its two ends labelled, and the name of the map in the caption the way you would print a unit. Where a specific level genuinely matters, draw a labelled contour rather than hoping a colour boundary lands on it. None of this makes rainbows useless. Where a rainbow accelerates it separates values a single-hue ramp would flatten, and readers are accurate at looking a single value up against a rainbow key; Ware, Stone and Szafir make that case in IEEE CG&A in 2023. What a rainbow is poor at is ranking a whole field at a glance, which is what most heat maps are for — so know which task you have, and do not assume a smoother rainbow settles it, because turbo is smoother than jet and its lightness still turns over in the middle.

In the gallery

Dark navy facilities-dashboard slide, masthead “Peak Load Desk · Campus Energy Services · Thermal & battery dispatch review · Phoenix, Arizona · Calendar year 2025”, headlined “Every hour of the year on one map — and the stress band runs through the mild hours”. A standfirst says there are 8,760 hourly air-temperature values averaged into one cell for each hour of the day in each month, 288 cells with every value in exactly one of them; that the colour scale below the map is linear and unbroken and runs from the coldest cell to the hottest, 3.60 °C to 41.83 °C, with nothing binned, capped, smoothed or set aside; and that the bright band is wide, sits either side of the mild hours, and that 281 of the 288 cells read lighter than the darkest cell of the afternoon block. The main panel is a heat map labelled “Mean 2 m air temperature, °C · hour of day (columns) × month (rows) · ERA5-Land reanalysis · 2025”: twenty-four columns of clock hours 00 to 23 across, twelve rows of months January to December down, drawn on the jet rainbow colour map. January and December run deep blue; the spring and autumn afternoons and the summer nights are cyan, green and vivid yellow; and the middle of the summer — June, July and August from about noon to seven in the evening — is a block of orange deepening to dark red at its centre. A horizontal colour bar beneath the map is ticked 5, 10, 15, 20, 25, 30, 35 and 40 and captioned “°C · minimum cell 3.60, maximum cell 41.83”, with a note that the scale is linear, has no bins and no cap, and that every one of the 8,760 hourly values is inside exactly one cell, 28 to 31 values per cell. A table headed “The year, month by month · °C” gives each month’s coolest hour, the hour it falls at, the monthly mean, the warmest hour and the hour it falls at: January 3.6 at 08:00, mean 11.3, 18.6 at 16:00; February 8.7 at 07:00, 17.6, 24.9 at 16:00; March 10.1 at 07:00, 17.9, 24.3 at 16:00; April 13.7 at 06:00, 22.7, 29.4 at 16:00; May 19.3 at 06:00, 27.8, 34.0 at 16:00; June 25.0 at 06:00, 33.6, 40.3 at 16:00; July 28.1 at 06:00, 35.4, 41.1 at 16:00; August 28.9 at 06:00, 36.0, 41.8 at 16:00; September 24.4 at 07:00, 30.6, 35.9 at 16:00; October 16.1 at 07:00, 24.1, 30.7 at 16:00; November 11.6 at 07:00, 18.6, 25.1 at 15:00; December 7.8 at 08:00, 14.6, 22.3 at 15:00. A second table, “Five of the brightest cells on the map”, lists July 06:00 at 28.07 °C, April 13:00 at 27.91, May 22:00 at 27.93, September 02:00 at 27.69 and June 04:00 at 27.37, each with a swatch of the vivid yellow they are drawn in, captioned that the brightest cells are the ones the eye finds first and these are five of them. Down the right-hand side, a gold-bordered card carries “281 / 288” over “hour-cells inside the elevated band — brighter than the map’s darkest afternoon”. A card headed “Read-out for the facilities board” says the load does not peak where the calendar says it should, that the bright band runs along the shoulder hours and the shoulder months while the August afternoons sit at the dark end of the scale alongside the January nights, and that two hard edges frame the band with a broad flat map between them. A “Dispatch note” card says to hold the battery for the bright band, let it ride through the dark blocks, and book the plant-room maintenance window for August, 15:00 to 18:00, because on this map it is the calmest cell of the year. A card headed “Why a rainbow scale” argues that a full spectrum uses the whole visible range so the eye has more to work with than a single-hue fade — more distinguishable steps and a colour name for every level — that it is the standard engineering map, and that readers are accurate at looking a value up against its key. A card headed “What we did not do” says no axis is truncated because there is no axis to truncate, nothing is binned so no bin edge was chosen, no cell is dropped, no series is indexed, no value is smoothed, and the scale is printed under the map with its two ends marked. A card headed “Definition — the elevated band” defines it as cells whose colour reads lighter than the darkest cell of the afternoon block, notes that 281 of 288 qualify, and says the band is therefore reported by its edges rather than its extent. A card headed “What a cell is” explains that one cell is the mean of that clock hour across that month, 28 to 31 hourly values, with columns as local clock hours 00 to 23 and rows as the twelve months, January at the top. A final card, “Basis of measurement”, names ERA5-Land hourly 2 m air temperature at 33.4 °N 112.1 °W, elevation 333 m, local time, 1 January to 31 December 2025, via the Open-Meteo historical weather API. Three ticked boxes run along the foot: “Linear, unbroken scale”, “Every value plotted” and “One source, one place, one year”. A footnote gives the reanalysis, the retrieval date, the coordinates and the grid point, records that Open-Meteo statistically downscales to a 90 m elevation model and that these are reanalysis values — model output for the grid cell, not instrument readings from a weather station — names the colour map as matplotlib 3.11.1 jet applied linearly between the minimum and maximum cell, and says the slide is a demonstration drawn by us in the manner of a facilities dashboard, that the desk and the dispatch note are invented, and that every temperature on it is real.

The heat map that paints its hottest hour darkest

The rainbow colour map