diff --git a/lib/xmet/thermo.py b/lib/xmet/thermo.py
index 49d8515..aaee26b 100644
--- a/lib/xmet/thermo.py
+++ b/lib/xmet/thermo.py
@@ -120,7 +120,7 @@ def loft_parcel(start_temp: float,
 
         if last_height is not None:
             try:
-                rate = lapse_rate(temp, pressure)
+                rate = lapse_rate(temp, pressure-step/2)
             except OverflowError:
                 break