diff --git a/modules/util/util.py b/modules/util/util.py
index 912d6bb438094aa4c998cf49bb5f1951aa911c20..38594e1543ae0b4edd55488daa9f4e44a0d22ae0 100644
--- a/modules/util/util.py
+++ b/modules/util/util.py
@@ -231,7 +231,7 @@ def pressure_to_altitude(pres, temp, prof_pres, prof_temp, sfc_pres=None, sfc_te
 
     if sfc_pres is not None:
         if pres > sfc_pres:  # incoming pressure below surface
-            return -1
+            return -999.0
 
         prof_pres = np.array(pres_s)
         prof_temp = np.array(temp_s)