Science Facts - HITRAN Data Plot

I have had trouble using (mostly, understanding) the HITRAN data. I never could make sense of the plots provided on their web site, and my computers do not run Java. (All their "universal" software is written in Java. Adding Java to an MS Windows system causes every other program on the computer to run slower ... all of them.)

At any rate - I wrote my own program. It run on MS Windows (which means that it should work on almost any computer). Just download and unpack it. It contains one *.exe and two data files. Just run the program - it does not modify the registry or make any changes to your system. (This will not work with Vista.)

Basics | Data Range | Spectra | Mean Path Tab | Mean Path Equations | Absorbed Energy | Atmospheric Pressure | Discussion


When the program runs, it automatically reads the provided HITRAN spectra files. These files contain the older 100-character records. (I plan to provide the newer data later.)

The initial display is the CO2 spectra around 15,000 nm (666.67 cm-1).

There are 2 checkboxes (future designs will contain more series) - use these to hide and display the plots. Double click the associated color swatches to change the colors. Sometimes, one series will obscure the other, click the "Transparent plots" checkbox to see where the overlaps are.

There are several ways to present spectra - by wavelength, by wavenumber, by frequency, and by energy. Typically, UV and visible data are presented by wavelength and IR data is presented by wavenumber. Because the HITRAN data is provided as an absorbance at a specific wavenumber, the plots in this application are by wavenumber. A bi-directional calculator is provided to convert between these units.

As you type in either box, the other will be computed. In addition, if you click on the graph, the conversion will be made automatically.

Data Range

There are several ways to control how much data is displayed. The amount of data depends on the molecule - there is a big difference in the ranges of water vapor and CO2. Most of the blackbody radiation from the surface of the Earth is in the IR band from 500 cm-1 to 2,500 cm-1. When investigating Global Warming, this is the range to evaluate. The rest of the spectra are there to observe, but it is of (almost) no importance when trying to understand the atmosphere and radiative forcing.

Changing the minimum and maximum values will have no "visible" effect if the graph is currently zoomed. (It does affect the available scroll amount.) If you change the values and nothing appears to happen, just remove the zoom (drag the mouse from the lower-left to upper-right).

Currently, one of the 2 plot buttons must be clicked before some changes are displayed, others are automatic.

There is no way to save any configuration changes you make.


The spectral lines are dimensioned as I am still trying to determine how to interpret this. These are supposedly vacuum spectra. It is not clear how much is measured and how much is theoretical (computed from quantum theory). At any rate, the spectra consists of very fine (narrow) lines, apparently much finer than can be measured.

The - (molecule/cm2) - is easy to understand. The total absorption of a column of gas is assumed to be based on the total number of molecules in the column. Therefore, the HITRAN spectra are scaled to represent the absorption of a single molecule. To determine the actual absorption in the atmosphere, it is necessary to multiply the value by the number of molecules in the column.

The basic assumption is that the column does not contain enough molecules so that all the photons are absorbed. Once the total number of molecules passes some threshold, the computations fail to give the correct results. (Actually, a different model is necessary.)

A column of air at 14.7 psi contains about 2.149E+25 molecules/cm2 (from the surface to the edge of space, assuming that I have done the math right). The first HITRAN report (1973) says that

Since the height of atmosphere at constant pressure is 8,496 m, this gives a value of 2.29E+25 for the total column. (About a 6% difference.)

CO2 is about 350 ppmv (parts per million by volume). Water vapor varies from 5 ppmv in the stratosphere, to about 4% (40,000 ppmv) at the surface. [350 ppmv is 0.0350%, therefore, 4% = 40,000 ppmv] For a default, I have set CO2 as 350 ppmv and water vapor as 10,000 ppmv (I assume that this is typical - the references provide many different values). These values can be set on the General tab.

The absorbance plot simply multiplies the HITRAN values by 1E+20 ... it makes the y-axis scale look better. This allows direct comparisons of the water vapor and CO2 spectra. However, in the atmosphere, the relative concentration is also important. The Scale spectra checkbox on the Absorbance tab causes the water vapor spectra to be multiplied by the ratio of the concentrations.

I suggest setting the spectra interval to 500 cm-1 to 2500 cm-1 and toggle the scaling. This should make it obvious that CO2 is a stronger absorber of IR radiation but that the concentration of water vapor means that water vapor absorbs more energy. These plots actually just indicate that that may be true, but it is still necessary to compute the actual energy absorbed to be sure. This is because the fine structure and line widths are obscured by a graph at this resolution.

It should also be stressed that the absorbance plots do not represent the energy absorbed in a specific column length but present "per molecule" and "per molecule times relative concentration" spectra.

Mean Path Tab

The Global Warming arguments are based, in part, on whether or not the CO2 absorption spectra is saturated. Unfortunately, none of the references I've seen present data in a way that I can understand it. The "Mean Path" plots are my attempt to present the data in a better way. (And I am still working on this. I am open to suggestions.)

The idea is to compute how much energy (or the percent of photons) is absorbed in 10 meters, 100 meters, and so forth. Rather than "plot percent absorbed", I have decided to plot "distance to absorb x%". The plots use 2 different models

Both models produce the same basic results. The program allows you to select which method you prefer, to enter a value for h/k, and then plots the distance required to absorb the associated amount of radiation. (I know this is not very clear.)

h/k Percent Absorbed
1 - (1/2)^h/k 1 - exp(-h/k)
1 0.5 0.632120559
2 0.75 0.864664717
3 0.875 0.950212932
4 0.9375 0.981684361
5 0.96875 0.993262053
6 0.984375 0.997521248
7 0.9921875 0.999088118

Thus, using the exponential formula, 6-sigma means that 99.7% of the available energy has been absorbed. Enter h/k into the Multiplier field. (For no particular reason), the field only allows values between 0.1 and 100.

I am still having a problem figuring out how to handle the line widths. As I learn more, I will be adding this to the code.

Mean Path Equations

The equations assume that if half the energy is absorbed in 100 meters, then if you double the energy in, half of that will still be absorbed in the same 100 meters. This assumption ignores that fact that the absorbed energy will change the temperature of the air layer and will therefore also change the net energy absorbed.

The value - 2.5294256E+15 - is the number of molecules in a column one meter long with a cross section of 1 cm2 assuming a concentration of 1 ppmv and a column pressure of 1 atm at stp. To obtain the number of molecules at a specific concentration, the value is multiplied by F_Concentration (the value the user types in). The formula is a bit obscure The line width (cm-1) is ignored and assumed to be equal to one. It may be more accurate to multiply H (height) by 0.07, or perhaps 0.14, but I am not sure yet.

At any rate, if the multiplier is set to 6, and the exponential model is selected, the plot shows how far radiation will travel before 99.75% is absorbed. The lower edge of the graph indicates the level where that percent of photons are absorbed. When "Mean Free Path" is selected and the multiplier is set to one, the graph shows how far radiation goes before one half of it is absorbed.

This is the type of data I wanted - I want to know how far radiation goes before it is 100% absorbed. (6-sigma is close enough to 100% for this analysis.) It should be obvious that for many frequencies, 100% of the radiation is absorbed within a few feet of the ground. Likewise, other frequencies require 100 meters, 1,000 meters, and so forth. With this program, you can double the concentration and actually see the effect. (I call that a success.)

To simulate the effect of a 0.1 cm-1 line width, simply multiply the multiplier by 0.1 (a 6-sigma multiplier becomes 0.6). As far as I can tell, any line below 100 meters should be considered fully saturated. Because the program does not consider atmospheric pressure, the thickness of the entire atmosphere is only 8,496 m. Therefore, you should consider 5,000 meters to be the height of the tropopause (not the normal 10,000 meters).

The factor of 4 * pi is a little shaky - the HITRAN data is based on radiation through a column having an area of 1.0 cm2. However, radiation from the surface of the Earth goes in all directions. Since there are 2 * pi steradians in one half a sphere, that is where that part comes from. Using a layered model of the atmosphere produces an average photon path length that is twice the thickness of the layer. That is where the second two comes from. Because the Earth is a sphere, a value of two is too large, but it is close enough until I find a better model. Basically, the effective path from the surface is longer than the thickness and 300 watts from a 1.0 cm2 surface area will not go directly to space, but will spread out through all the atmosphere between the surface and space. As a result, the distance an average photon travels before being absorbed is closer to the ground than expected without accounting for the geometry.

Absorbed Energy

Because the current plot does not use a blackbody energy distribution to determine accurate results, the plots are instructive but not realistic. Looking at the energy absorbed over a narrow band will provide a better comparison than looking at bands that are "far" apart.

An accurate accounting of line widths is also necessary for computing the actual energy absorbed.

As a result, this program will show what is known and how changing the concentrations will affect the atmosphere, but it can not determine how many additional watts are absorbed.

Atmospheric Pressure

All the calculations assume one atmosphere (1 atm) of pressure. However, the pressure decreases with altitude. This has two effects The first effect can be accounted for by changing the concentrations on the General tab.

The second will require program modifications. When (if) those are made, a way to set the pressure will be added.

Therefore, even though the concentration of CO2 (350 ppmv) remains fairly constant from the surface up to the mesopause, its ability to absorb radiation changes considerably. Specifically, the pressure adjusted concentration at the stratopause is 0.35 ppmv, so that is the value to enter in the program.

In addition, the thickness of these upper layers plays a part in understanding what actually happens. At the surface, the effect of the lower 100 meters is important to weather (and Global Warming). At the stratopause, some (not many) of the CO2 bands are still saturated. (Near 4.2 um) As a result, evaluating absorption "per kilometer" makes more sense than "per meter" used at the surface. Therefore, the concentrations should be multiplied by 1,000 (canceling out the pressure adjustment) before evaluating the results.

(If this stuff was simple, then there would be no arguments over Global Warming and such.)


There are plenty of errors and approximations in this program. I have tried to identify those that I know of. However, the intent of this program is to show the principles behind radiative forcing, and the known errors don't affect that.

Please have fun with this and let me know what needs to be fixed.

The intent is to teach, not to disprove Global Warming. (I have already done that.)

Author: Robert Clemenzi
URL: http:// / Science_Facts / HITRAN / HITRAN_Data_Plot.html