diff --git a/src/layers/weather/shellRendering.js b/src/layers/weather/shellRendering.js
index b20a3711a..bff4705ae 100644
--- a/src/layers/weather/shellRendering.js
+++ b/src/layers/weather/shellRendering.js
@@ -2,6 +2,7 @@ import { acquireWeatherImage } from './infraredImage.js';
 import { imageryHostStatus } from './imageryHost.js';
 import { WEATHER_DETAIL_SIZE, WEATHER_IMAGE_SIZES } from './source.js';
 import { msg } from '../../i18n/index.js';
+import * as geo from '../../geo/index.js';
 
 /** Metres above the ellipsoid. Lower shells draw first, so lightning draws last. */
 export const WEATHER_SHELL_HEIGHTS = Object.freeze({
@@ -317,7 +318,8 @@ export function createShellSurface({
 }
 
 function fitTexture(cesium, { width, height }) {
-  const limit = cesium.ContextLimits?.maximumTextureSize;
+  // Cesium's limit once its context exists; a Navara shell has no SDK.
+  const limit = cesium?.ContextLimits?.maximumTextureSize;
   // Halve, down to 1024 px wide, on devices with a smaller texture limit.
   while (limit > 0 && width > limit && width > 1024) {
     width /= 2;
@@ -574,7 +576,7 @@ export function createWeatherShell({
   /** Recompute the window from the camera's ground footprint; returns whether it changed. */
   function computeView() {
     const rectangle = detailed ? viewRectangle() : null;
-    const degrees = cesium.Math.toDegrees;
+    const degrees = geo.math.toDegrees;
     const footprint = rectangle && {
       west: degrees(rectangle.west),
       south: degrees(rectangle.south),
@@ -688,7 +690,15 @@ export function createWeatherShell({
       viewer,
       cesium,
       height,
-      rectangle: cesium.Rectangle.fromDegrees(west, south, east, north),
+      // A Navara surface reads the four radians; Cesium gets its Rectangle.
+      rectangle: cesium
+        ? cesium.Rectangle.fromDegrees(west, south, east, north)
+        : {
+            west: geo.math.toRadians(west),
+            south: geo.math.toRadians(south),
+            east: geo.math.toRadians(east),
+            north: geo.math.toRadians(north),
+          },
       onSettled: () => commit(frame),
     });
     next.bounds = frame.bounds;
diff --git a/src/layers/weather/shellRendering.test.mjs b/src/layers/weather/shellRendering.test.mjs
index ad65dcef2..e56a57c5f 100644
--- a/src/layers/weather/shellRendering.test.mjs
+++ b/src/layers/weather/shellRendering.test.mjs
@@ -1502,3 +1502,42 @@ test('global infrared keeps one full-extent image; detail images halve for small
   });
   small.shell.clear();
 });
+
+test('a Navara shell runs without the Cesium SDK and hands its surface the extent in radians', async () => {
+  const surfaces = [];
+  const shell = createWeatherShell({
+    product: 'radar',
+    viewRectangle: () => ({ west: -1.8, south: 0.5, east: -1.6, north: 0.6 }),
+    moveEnd: { addEventListener: () => () => {} },
+    requestRender: () => {},
+    fetchImpl: async () => response(),
+    decodeImage: async () => ({
+      width: 4096,
+      height: 2048,
+      close() {},
+    }),
+    createCanvas: canvas,
+    createSurface: (options) => {
+      surfaces.push(options);
+      return {
+        setAlpha() {},
+        setImage() {},
+        setWindow() {},
+        setDetail() {},
+        setHidden() {},
+        destroy() {},
+      };
+    },
+  });
+  shell.setFrame(snapshot('radar'), '2026-11-04T12:00:00Z');
+  await flush();
+  assert.equal(surfaces.length, 1);
+  assert.equal(surfaces[0].cesium, undefined);
+  const { bounds } = snapshot('radar');
+  for (const side of ['west', 'south', 'east', 'north'])
+    assert.equal(
+      surfaces[0].rectangle[side],
+      Cesium.Math.toRadians(bounds[side]),
+    );
+  shell.clear();
+});