#Code table 4.5: Fixed surface types and units
0 0 Reserved
1 sfc Ground or water surface 
2 2 Cloud base level 
3 3 Level of cloud tops 
4 4 Level of 0 degree C isotherm 
5 5 Level of adiabatic condensation lifted from the surface
6 6 Maximum wind level 
7 sfc Tropopause 
8 sfc Nominal top of the atmosphere 
9 9 Sea bottom
10 10 Entire atmosphere
11 11 Cumulonimbus (CB) base (m)
12 12 Cumulonimbus (CB) top (m)
# 13-19 Reserved
20 20 Isothermal level (K)
#21-99 Reserved
100 pl Isobaric surface (Pa)
101 sfc Mean sea level
102 102 Specific altitude above mean sea level (m)
103 sfc Specified height level above ground (m)
104 104 Sigma level (sigma value)
105 ml Hybrid level
106 sfc Depth below land surface (m)
107 pt Isentropic (theta) level (K)
108 108 Level at specified pressure difference from ground to level (Pa)
109 pv Potential vorticity surface (K m2 kg-1 s-1)
110 110 Reserved
111 111 Eta level
# 112-116 Reserved
117 117 Mixed layer depth (m)
118 hhl Hybrid height level
119 hpl Hybrid pressure level
# 120-159 Reserved
160 160 Depth below sea level (m)
#161-191 Reserved
#192-254 Reserved for local use
255 255 Missing