diff --git a/lib/xmet/thermo.py b/lib/xmet/thermo.py
index 4f76597..052e98f 100644
--- a/lib/xmet/thermo.py
+++ b/lib/xmet/thermo.py
@@ -1,3 +1,6 @@
+LAPSE_RATE_MOIST = 4.0 # degrees C per km
+LAPSE_RATE_DRY   = 9.8
+
 def vapor_pressure(dewpoint: float) -> float:
     return 6.11 * 10 * (
         (7.5 * dewpoint) / (237.3 * dewpoint)
@@ -20,3 +23,6 @@ def saturated_mixing_ratio(temp: float, pressure: float) -> float:
 
 def lcl(temp: float, dewpoint: float) -> float:
     return (temp - dewpoint) / 0.008
+
+def lapse(temp: float, rate: float, delta: float) -> float:
+    return temp - (rate * (delta / 1000))