diff --git a/lib/xmet/thermo.py b/lib/xmet/thermo.py
index e14dfb6..c4fe55f 100644
--- a/lib/xmet/thermo.py
+++ b/lib/xmet/thermo.py
@@ -5,7 +5,8 @@ from typing import Callable
 from xmet.series import Series
 from xmet.units  import rad, deg, kelvin, celsius
 
-LAPSE_RATE_DRY = 9.8 / 1000 # degrees C per km
+LAPSE_RATE_DRY          = 9.8 / 1000 # degrees C per km
+LAPSE_RATE_MAGIC_FACTOR = 1.09       # adjustment for accuracy
 
 PRESSURE_MAX  = 1050 # millibar
 PRESSURE_MSL  = 1013.25
@@ -130,7 +131,8 @@ def loft_parcel(start_temp: float,
             except OverflowError:
                 break
 
-            temp = lapse(temp, height - last_height, 1.09*rate)
+            temp = lapse(temp, height - last_height,
+                         LAPSE_RATE_MAGIC_FACTOR*rate)
 
             yield temp, pressure