diff --git a/refet/daily.py b/refet/daily.py
index 33e3c75..90bd65c 100644
--- a/refet/daily.py
+++ b/refet/daily.py
@@ -97,7 +97,7 @@ def __init__(self, tmin, tmax, ea, rs, uz, zw, elev, lat, doy,
                 continue
             elif unit.lower() not in [
                     'k', 'kelvin', 'f', 'fahrenheit',
-                    'pa',
+                    'pa','rh','relative humidity',
                     'langleys', 'w m-2', 'w/m2',
                     'mph',
                     'ft', 'feet',
@@ -121,6 +121,12 @@ def __init__(self, tmin, tmax, ea, rs, uz, zw, elev, lat, doy,
             elif variable == 'ea':
                 if unit.lower() in ['pa']:
                     self.ea /= 1000.0
+                elif unit.lower() in ['rh','relative humidity']:
+                    if self.ea.max() > 1.0: # check for percentage format
+                        self.ea /= 100
+                    # use Teten's formula
+                    self.ea = self.ea * (0.61078 ** (17.27 * ((self.tmin+self.tmax)/2) / (
+                        ((self.tmin+self.tmax)/2) + 237.3)))
             elif variable == 'rs':
                 if unit.lower() in ['langleys']:
                     self.rs *= 0.041868
diff --git a/refet/hourly.py b/refet/hourly.py
index a4d8d10..b66b024 100644
--- a/refet/hourly.py
+++ b/refet/hourly.py
@@ -91,7 +91,7 @@ def __init__(self, tmean, ea, rs, uz, zw, elev, lat, lon, doy, time,
                 continue
             elif unit.lower() not in [
                     'k', 'kelvin', 'f', 'fahrenheit',
-                    'pa',
+                    'pa','rh','relative humidity',
                     'langleys', 'w m-2', 'w/m2',
                     'mph',
                     'ft', 'feet',
@@ -109,6 +109,11 @@ def __init__(self, tmean, ea, rs, uz, zw, elev, lat, lon, doy, time,
             elif variable == 'ea':
                 if unit.lower() in ['pa']:
                     self.ea /= 1000.0
+                elif unit.lower() in ['rh','relative humidity']:
+                    if self.ea.max() > 1.0: # check for percentage format
+                        self.ea /= 100
+                    # use Teten's formula
+                    self.ea = self.ea * (0.61078 ** (17.27 * self.tmean / (self.tmean + 237.3)))
             elif variable == 'rs':
                 if unit.lower() in ['langleys']:
                     self.rs *= 0.041868