From 49a43650d84da10acff2aa332b2ff66eb4ba18de Mon Sep 17 00:00:00 2001
From: Douglas Greenshields <douglas.greenshields@levellingup.gov.uk>
Date: Thu, 4 Apr 2024 16:05:11 +0100
Subject: [PATCH] correct specific external conditions arithmetic polarity

---
 src/external_conditions.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/external_conditions.rs b/src/external_conditions.rs
index fd57f60..3348daf 100644
--- a/src/external_conditions.rs
+++ b/src/external_conditions.rs
@@ -1025,7 +1025,7 @@ impl ExternalConditions {
                 + 0.6514 * (1.0 - (p2_finR / (p1_finR.powi(2) + p2_finR.powi(2)).sqrt())))
                 / 2.0;
             let f_w_o = 0.3282 * (1.0 - (p2_ovh / (p1_ovh.powi(2) + p2_ovh.powi(2)).sqrt()));
-            let f_w_sky = (1.0 - (alpha + beta + 90.0f64.to_radians()).sin()) / 2.0;
+            let f_w_sky = (1.0 - (alpha + beta - 90.0f64.to_radians()).sin()) / 2.0;
 
             // # Calculate denominators of eqns F.9 to F.14
             let view_factor_sky_no_obstacles = (1.0 + beta.cos()) / 2.0;