ERA-Interim Pressure and Isentropic (Theta) Levels

Please note that in ERA-Interim, ECMWF uses 1000 hPa as the reference pressure.

Last updated: 2012-07-17 by David Stepaniak (davestep@ucar.edu)

Pressure Levels

The 37 standard pressure levels in ERA-Interim in mb (or hPa) are, from highest isobaric altitude to lowest isobaric altitude in the atmosphere:

1, 2, 3, 5, 7, 10, 20, 30, 50, 70, 100, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 775, 800, 825, 850, 875, 900, 925, 950, 975, 1000

This is the ordering convention of the pressure level vertical coordinate in ERA-Interim. ERA pressure level variables are assigned interpolated values where isobaric surfaces dip below topography. There are no missing values. We do not have specific information about the below topography interpolation scheme used by ECMWF at this time.

Isentropic (Theta) Levels

The 15 standard isentropic (theta) levels in ERA-Interim in K (Kelvin) are, from lowest isentropic altitude to highest isentropic altitude in the atmosphere:

265, 275, 285, 300, 315, [320 — PV only], 330, 350, 370, 395, 430, 475, 530, 600, 700, 850

This is the ordering convention of the theta level vertical coordinate in ERA-Interim. Please note that PV (potential vorticity) is archived on an additional isentropic level, namely 320 K. There are no missing values. We do not have specific information about the below topography interpolation scheme used by ECMWF at this time.

ERA-Interim Pressure and Isentropic (Theta) Levels in Relation to a Standard Atmosphere

Please note that in ERA-Interim, ECMWF uses 1000 hPa as the reference pressure.

For convenience, below we show the ERA-Interim pressure and isentropic levels arranged relative to a standard atmosphere. ERA-Interim pressure levels are shown in bold black digits. ERA-Interim theta levels are shown in bold red digits. Note that the global average tropopause height is indicated ( in blue ~~~) as 12 kilometers — in practice the height of the tropopause varies between 8 kilometers (poles) and 17 kilometers (equator). A stratopause ( in green ~~~) at 50 kilometers is also shown, as well as an ozone maximum near 25 kilometers.

For this representation of a standard atmosphere, potential temperature is computed as where T is the temperature, po the reference pressure (1013.25 mb), p the pressure, R the gas constant for dry air (287.04 J kg-1 K-1), and Cp the specific heat capacity of dry air at constant pressure (1004.6 J kg-1 K-1).

-------------------------------------------------------------------------------------------------------------------
                                      |                  |                              |   Potential    |
                                      |     Pressure     |                              |  Temperature   |
                Height                |            ERA-I |        Temperature           |          ERA-I | Density
 meters      km      feet       mi    |  mb/hPa   mb/hPa |  K         deg C      deg F  |   K †     K    | kg m-3
------------------------------------- |----------------- |----------------------------- |--------------- |---------
- 80000 --- 80.0 -- 262467 --- 49.7 - |   0.010 -------- | 198.6 ---- -74.5 --- -102.1  | 5350.0 ------- | 0.00002
- 70000 --- 70.0 -- 229659 --- 43.5 - |   0.052 -------- | 219.5 ---- -53.6 ---  -64.5  | 3691.6 ------- | 0.00008
- 60000 --- 60.0 -- 196850 --- 37.3 - |   0.220 -------- | 247.0 ---- -26.1 ---  -15.0  | 2750.9 ------- | 0.00030
                                                   
~ 50000 ~~~ 50.0 ~~ 164042 ~~~ 31.1 ~ |   0.798 ~~~~~~~~ | 270.6 ~~~~  -2.5 ~~~  +27.5  | 2085.5 ~~~~~~~ | 0.00100 ← Ave stratopause
                                                     1
- 45000 --- 45.0 -- 147638 --- 28.0 - |   1.491 -------- | 264.1 ----  -9.0 ---  +15.8  | 1702.5 ------- | 0.00200
                                                     2
- 40000 --- 40.0 -- 131234 --- 24.9 - |   2.871 -------- | 250.3 ---- -22.8 ---   -9.0  | 1338.1 ------- | 0.00400
                                                     3
                                                     5
- 35000 --- 35.0 -- 114829 --- 21.7 - |   5.746 -------- | 236.5 ---- -36.6 ---  -33.9  | 1037.0 ------- | 0.00800
                                                     7
                                                    10                                             850
- 30000 --- 30.0 --  98425 --- 18.6 - |  11.970 -------- | 226.5 ---- -46.6 ---  -51.9  |  805.3 ------- | 0.01800
                                                    20                                             700
             
- 25000 --- 25.0 --  82021 --- 15.5 - |  25.492 -------- | 221.5 ---- -51.6 ---  -60.9  |  634.5 ------- | 0.04000 ← Ozone maximum
             
                                                    30                                             600
                                                    50                                             530
- 20000 --- 20.0 --  65617 --- 12.4 - |  55.293 -------- | 216.6 ---- -56.5 ---  -69.7  |  497.3 ------- | 0.08900
- 19000 --- 19.0 --  62336 --- 11.8 - |  64.674 -------- | 216.6 ---- -56.5 ---  -69.7  |  475.6 ------- | 0.10400
                                                    70                                             475
- 18000 --- 18.0 --  59055 --- 11.2 - |  75.652 -------- | 216.6 ---- -56.5 ---  -69.7  |  454.7 ------- | 0.12200
- 17000 --- 17.0 --  55774 --- 10.6 - |  88.497 -------- | 216.6 ---- -56.5 ---  -69.7  |  434.8 ------- | 0.14200
                                                   100                                             430
- 16000 --- 16.0 --  52493 ---  9.9 - | 103.520 -------- | 216.6 ---- -56.5 ---  -69.7  |  415.7 ------- | 0.16600
- 15000 --- 15.0 --  49213 ---  9.3 - | 121.110 -------- | 216.6 ---- -56.5 ---  -69.7  |  397.5 ------- | 0.19500
                                                   125                                             395
- 14000 --- 14.0 --  45932 ---  8.7 - | 141.700 -------- | 216.6 ---- -56.5 ---  -69.7  |  380.1 ------- | 0.22800
                                                   150                                             370
- 13000 --- 13.0 --  42651 ---  8.1 - | 165.790 -------- | 216.6 ---- -56.5 ---  -69.7  |  363.4 ------- | 0.26700
                                                   175                                             350
~ 12000 ~~~ 12.0 ~~  39370 ~~~  7.5 ~ | 193.990 ~~~~~~~~ | 216.6 ~~~~ -56.5 ~~~  -69.7  |  347.4 ~~~~~~~ | 0.31200 ← Ave tropopause
                                                   200
                                                   225
- 11000 --- 11.0 --  36089 ---  6.8 - | 226.990 -------- | 216.7 ---- -56.4 ---  -69.5  |  332.4 ------- | 0.36500
                                                   250                                             330
- 10000 --- 10.0 --  32808 ---  6.2 - | 264.990 -------- | 223.2 ---- -49.9 ---  -57.8  |  327.5 ------- | 0.41300
-  9500 ---  9.5 --  31168 ---  5.9 - | 285.840 -------- | 226.5 ---- -46.6 ---  -51.9  |  325.2 ------- | 0.44000
                                                   300
-  9000 ---  9.0 --  29528 ---  5.6 - | 308.000 -------- | 229.7 ---- -43.4 ---  -46.1  |  322.9 ------- | 0.46700
-  8848 ---  Everest south face (Nepal) - Licensed under Wikipedia Creative Commons Everest (8848 m, 29028 ft)
-  8500 ---  8.5 --  27887 ---  5.3 - | 331.540 -------- | 232.9 ---- -40.2 ---  -40.4  |  320.5 ------- | 0.49600
                                                   350                                             320 — PV only
-  8000 ---  8.0 --  26247 ---  5.0 - | 356.510 -------- | 236.2 ---- -36.9 ---  -34.4  |  318.4 ------- | 0.52600
-  7500 ---  7.5 --  24606 ---  4.7 - | 382.990 -------- | 239.4 ---- -33.7 ---  -28.7  |  316.2 ------- | 0.55700
                                                   400                                             315
-  7000 ---  7.0 --  22966 ---  4.3 - | 411.050 -------- | 242.7 ---- -30.4 ---  -22.7  |  314.1 ------- | 0.59000
-  6500 ---  6.5 --  21325 ---  4.0 - | 440.750 -------- | 245.9 ---- -27.2 ---  -17.0  |  312.0 ------- | 0.62400
                                                   450
-  6000 ---  6.0 --  19685 ---  3.7 - | 472.170 -------- | 249.1 ---- -24.0 ---  -11.2  |  309.9 ------- | 0.66000
                                                   500
-  5500 ---  5.5 --  18045 ---  3.4 - | 505.390 -------- | 252.4 ---- -20.7 ---   -5.3  |  308.0 ------- | 0.69700
-  5000 ---  5.0 --  16404 ---  3.1 - | 540.480 -------- | 255.6 ---- -17.5 ---   +0.5  |  305.9 ------- | 0.73600
                                                   550
-  4500 ---  4.5 --  14764 ---  2.8 - | 577.520 -------- | 258.9 ---- -14.2 ---   +6.4  |  304.1 ------- | 0.77700
                                                   600
-  4000 ---  4.0 --  13123 ---  2.5 - | 616.600 -------- | 262.1 ---- -11.0 ---  +12.2  |  302.1 ------- | 0.81900
                                                   650
-  3500 ---  3.5 --  11483 ---  2.2 - | 657.800 -------- | 265.4 ----  -7.7 ---  +18.1  |  300.3 ------- | 0.86300
                                                   700                                             300
-  3000 ---  3.0 --   9843 ---  1.9 - | 701.210 -------- | 268.6 ----  -4.5 ---  +23.9  |  298.4 ------- | 0.90900
-  2500 ---  2.5 --   8202 ---  1.6 - | 746.910 -------- | 271.9 ----  -1.2 ---  +29.8  |  296.7 ------- | 0.95700
                                                   750
                                                   775
-  2000 ---  2.0 --   6562 ---  1.2 - | 795.010 -------- | 275.1 ----  +2.0 ---  +35.6  |  294.9 ------- | 1.00700
                                                   800
                                                   825
-  1500 ---  1.5 --   4921 ---  0.9 - | 845.590 -------- | 278.4 ----  +5.3 ---  +41.5  |  293.2 ------- | 1.05800
                                                   850
                                                   875
-  1000 ---  1.0 --   3281 ---  0.6 - | 898.760 -------- | 281.6 ----  +8.5 ---  +47.3  |  291.5 ------- | 1.11200
                                                   900
                                                   925
                                                   950
-   500 ---  0.5 --   1640 ---  0.3 - | 954.610 -------- | 284.9 ---- +11.8 ---  +53.2  |  289.8 ------- | 1.16700
                                                   975                                             285
                                                  1000
-     0 ---  0.0 --      0 ---  0.0 - |1013.250 -------- | 288.1 ---- +15.0 ---  +59.0  |  288.1 ------- | 1.22500 ← Surface
                                                                                                   275
                                                                                                   265
-  -420 ---  Dead Sea (Israeli shoreline) - Licensed under Wikipedia Creative Commons Dead Sea (-420 m, -1378 ft)
------------------------------------- |----------------- |----------------------------- |--------------- |---------

Potential temperature is computed as where T is the temperature, po the reference pressure (1013.25 mb), p the pressure, R the gas constant for dry air (287.04 J kg-1 K-1), and Cp the specific heat capacity of dry air at constant pressure (1004.6 J kg-1 K-1).

Only the simplest complete photochemical cycle for ozone (Chapman cycle) is shown. M represents a third molecule needed to conserve momentum and energy in the recombination of O and O2.

Source of standard atmosphere (height, pressure, temperature, and density): Ahrens, C. Donald, 1994, Meteorology Today, An Introduction to Weather, Climate, and the Environment, Fifth Edition, West Publishing Company, St. Paul (Appendix H).