diff --git a/server/src/atlas/region_profile.rs b/server/src/atlas/region_profile.rs
index fa6b14c2f..b21e3d0f0 100644
--- a/server/src/atlas/region_profile.rs
+++ b/server/src/atlas/region_profile.rs
@@ -1171,7 +1171,8 @@ mod tests {
     // T-1024 / D-240 climate derivation tests — planet_class envelope model
     // -----------------------------------------------------------------------
 
-    /// Convenience: derive temperature with seed=0 for deterministic tests.
+    /// Convenience: derive temperature with a fixed seed (0) — deterministic, but
+    /// note seed 0 still applies its (constant, non-zero) nudge like any other seed.
     fn temp(planet_class: &str, atmosphere: &str, lat: f64, elev_km: f64) -> Option<f32> {
         let params = BodyParams {
             planet_class: Some(planet_class.into()),
@@ -1313,21 +1314,25 @@ mod tests {
             let (cold, warm) = climate.envelope(class);
             for atmo in &atmospheres {
                 for &lat in &latitudes {
-                    for seed in seeds {
-                        let params = BodyParams {
-                            planet_class: Some((*class).into()),
-                            atmosphere: Some((*atmo).into()),
-                            region_latitude_deg: lat,
-                            elevation_km: 0.0,
-                            ..Default::default()
-                        };
-                        let t = derive_temperature_c(&params, &climate, seed)
-                            .expect("non-airless body must have temperature");
-                        assert!(
-                            t >= cold && t <= warm,
-                            "class={class} atmo={atmo} lat={lat} seed={seed}: \
-                             temperature {t}°C outside band [{cold}, {warm}]"
-                        );
+                    // Sweep sea level AND high altitude so the lapse term + clamp
+                    // are exercised jointly against the cold end, not just lat alone.
+                    for elev in [0.0_f64, 6.0] {
+                        for seed in seeds {
+                            let params = BodyParams {
+                                planet_class: Some((*class).into()),
+                                atmosphere: Some((*atmo).into()),
+                                region_latitude_deg: lat,
+                                elevation_km: elev,
+                                ..Default::default()
+                            };
+                            let t = derive_temperature_c(&params, &climate, seed)
+                                .expect("non-airless body must have temperature");
+                            assert!(
+                                t >= cold && t <= warm,
+                                "class={class} atmo={atmo} lat={lat} elev={elev} seed={seed}: \
+                                 temperature {t}°C outside band [{cold}, {warm}]"
+                            );
+                        }
                     }
                 }
             }