diff --git a/Trajectories/cost_function.py b/Trajectories/cost_function.py
index 6e4000cbca532b8e18b3068145a9ae9d6dae0fb8..58f0a782b4bcd6e4f27bbacaa4192d1a720d432a 100755
--- a/Trajectories/cost_function.py
+++ b/Trajectories/cost_function.py
@@ -245,13 +245,13 @@ for edge in g.edges():
 
     # Rossby numbers:
     if first_av_ros[target_node] and last_av_ros[source_node]:
-        Delta_Ro = last_av_ros[source_node] - first_av_ros[target_node]
+        Delta_Ro = first_av_ros[target_node] - last_av_ros[source_node]
     else:
         # At least one of the rossbies is invalid.
         Delta_Ro = 0
 
     # R_Vmax 1 and 2 already exist, just get the delta
-    Delta_R_Vmax = last_av_rad[source_node] - first_av_rad[target_node]
+    Delta_R_Vmax = first_av_rad[target_node] - last_av_rad[source_node]
 
     # Calculate the cost and assign to the edge:
     g.ep.cost_function[edge] = math.sqrt(