diff --git a/src/layers/weather/infraredAlpha.js b/src/layers/weather/infraredAlpha.js
index f42bda603..287e985e0 100644
--- a/src/layers/weather/infraredAlpha.js
+++ b/src/layers/weather/infraredAlpha.js
@@ -1,22 +1,29 @@
 export const INFRARED_ALPHA_T0 = 0.4;
 export const INFRARED_ALPHA_T1 = 0.7;
 
-/** Return display pixels without changing RGB or the source RGBA bytes.
+function infraredAmount(strength) {
+  return strength === 'full'
+    ? 0
+    : strength === 'filtered' || !Number.isFinite(strength)
+      ? 1
+      : Math.max(0, Math.min(100, strength)) / 100;
+}
+
+/** Scale each pixel's alpha in place; RGB is left as it is. The scale depends
+ * only on the brightest channel, so it is worked out once for each of the 256
+ * byte values instead of once per pixel: a 4096×2048 image has 8.4 million
+ * pixels and the power curve per pixel held the page for over 100 ms. The
+ * table holds the same numbers the per-pixel formula produced, so the output
+ * bytes are unchanged.
  * `strength` is 0–100: zero is the full image and 100 is the cloud key.
  */
-export function infraredAlpha(rgba, strength = 'filtered') {
-  const pixels = new Uint8ClampedArray(rgba);
-  const amount =
-    strength === 'full'
-      ? 0
-      : strength === 'filtered' || !Number.isFinite(strength)
-        ? 1
-        : Math.max(0, Math.min(100, strength)) / 100;
+export function applyInfraredAlpha(pixels, strength = 'filtered') {
+  const amount = infraredAmount(strength);
   if (amount === 0) return pixels;
-  for (let i = 0; i < pixels.length; i += 4) {
+  const scale = new Float64Array(256);
+  for (let value = 0; value < 256; value++) {
     // Match Cesium's czm_srgbToLinear conversion in draped imagery.
-    const linearMax =
-      (Math.max(rgba[i], rgba[i + 1], rgba[i + 2]) / 255) ** 2.2;
+    const linearMax = (value / 255) ** 2.2;
     const t = Math.max(
       0,
       Math.min(
@@ -26,7 +33,21 @@ export function infraredAlpha(rgba, strength = 'filtered') {
       ),
     );
     const filtered = t * t * (3 - 2 * t);
-    pixels[i + 3] = rgba[i + 3] * (1 - amount + filtered * amount);
+    scale[value] = 1 - amount + filtered * amount;
+  }
+  for (let i = 0; i < pixels.length; i += 4) {
+    const r = pixels[i];
+    const g = pixels[i + 1];
+    const b = pixels[i + 2];
+    const max = r > g ? (r > b ? r : b) : g > b ? g : b;
+    pixels[i + 3] = pixels[i + 3] * scale[max];
   }
   return pixels;
 }
+
+/** Return display pixels without changing RGB or the source RGBA bytes.
+ * `strength` is 0–100: zero is the full image and 100 is the cloud key.
+ */
+export function infraredAlpha(rgba, strength = 'filtered') {
+  return applyInfraredAlpha(new Uint8ClampedArray(rgba), strength);
+}
diff --git a/src/layers/weather/infraredAlpha.test.mjs b/src/layers/weather/infraredAlpha.test.mjs
index 950271a9d..d2c20cc6e 100644
--- a/src/layers/weather/infraredAlpha.test.mjs
+++ b/src/layers/weather/infraredAlpha.test.mjs
@@ -1,6 +1,7 @@
 import test from 'node:test';
 import assert from 'node:assert/strict';
 import {
+  applyInfraredAlpha,
   infraredAlpha,
   INFRARED_ALPHA_T0,
   INFRARED_ALPHA_T1,
@@ -61,3 +62,43 @@ test('full infrared preserves every channel including partial and zero alpha', (
   ]);
   assert.deepEqual(infraredAlpha(pixels, 'full'), pixels);
 });
+
+test('the lookup table gives the per-pixel curve byte for byte, in place', () => {
+  // The transfer as it was written per pixel, before the table.
+  const perPixel = (rgba, amount) => {
+    const out = new Uint8ClampedArray(rgba);
+    for (let i = 0; i < out.length; i += 4) {
+      const linearMax =
+        (Math.max(rgba[i], rgba[i + 1], rgba[i + 2]) / 255) ** 2.2;
+      const t = Math.max(
+        0,
+        Math.min(
+          1,
+          (linearMax - INFRARED_ALPHA_T0) /
+            (INFRARED_ALPHA_T1 - INFRARED_ALPHA_T0),
+        ),
+      );
+      out[i + 3] = rgba[i + 3] * (1 - amount + t * t * (3 - 2 * t) * amount);
+    }
+    return out;
+  };
+  // Every brightest-channel value in each channel, against a spread of alphas.
+  const bytes = [];
+  for (let value = 0; value < 256; value++)
+    for (const alpha of [0, 1, 37, 128, 200, 254, 255]) {
+      bytes.push(value, 0, 0, alpha);
+      bytes.push(value >> 1, value, 0, alpha);
+      bytes.push(0, value >> 2, value, alpha);
+    }
+  const source = new Uint8ClampedArray(bytes);
+  for (const strength of [1, 33, 50, 99, 100, 'filtered', Number.NaN]) {
+    const amount = Number.isFinite(strength) ? strength / 100 : 1;
+    const pixels = source.slice();
+    assert.equal(applyInfraredAlpha(pixels, strength), pixels);
+    assert.deepEqual(pixels, perPixel(source, amount), `strength ${strength}`);
+  }
+  for (const strength of [0, 'full']) {
+    const pixels = source.slice();
+    assert.deepEqual(applyInfraredAlpha(pixels, strength), source);
+  }
+});
diff --git a/src/layers/weather/infraredImage.js b/src/layers/weather/infraredImage.js
index de57ab3b9..3a017171b 100644
--- a/src/layers/weather/infraredImage.js
+++ b/src/layers/weather/infraredImage.js
@@ -6,7 +6,7 @@ import {
   drawWeatherPixels,
   closeWeatherPixels,
 } from './source.js';
-import { infraredAlpha } from './infraredAlpha.js';
+import { applyInfraredAlpha } from './infraredAlpha.js';
 
 export const MAX_MOSAIC_BYTES = 4 * 1024 * 1024;
 
@@ -31,7 +31,8 @@ export function processInfraredImage(
 
 function applyInfraredTransfer(context, canvas, mode) {
   const pixels = context.getImageData(0, 0, canvas.width, canvas.height);
-  pixels.data.set(infraredAlpha(pixels.data, mode));
+  // In place: a copy of a 4096×2048 image is 33 MB, made and written back.
+  applyInfraredAlpha(pixels.data, mode);
   context.putImageData(pixels, 0, 0);
 }
 
diff --git a/src/layers/weather/infraredImage.test.mjs b/src/layers/weather/infraredImage.test.mjs
index fb06607e4..a3fe08308 100644
--- a/src/layers/weather/infraredImage.test.mjs
+++ b/src/layers/weather/infraredImage.test.mjs
@@ -26,6 +26,28 @@ function canvas() {
   return output;
 }
 
+test('the transfer is applied to the canvas pixels in place, with no copy written back', () => {
+  let read = null;
+  const output = {};
+  output.getContext = () => ({
+    drawImage() {},
+    getImageData: () => {
+      read = { data: new Uint8ClampedArray([194, 0, 0, 200]) };
+      // A second full-size buffer copied over the first is what this guards.
+      read.data.set = () => {
+        throw new Error('the canvas pixels were overwritten from a copy');
+      };
+      return read;
+    },
+    putImageData: (pixels) => {
+      output.pixels = pixels;
+    },
+  });
+  processInfraredImage({ width: 1, height: 1 }, 'filtered', () => output);
+  assert.equal(output.pixels, read);
+  assert.deepEqual([...read.data], [194, 0, 0, 98]);
+});
+
 test('decoded pixels are processed once on a same-sized canvas', () => {
   const image = { width: 1, height: 1 };
   const output = processInfraredImage(image, 'filtered', canvas);