From Review of the vapour pressures of ice and supercooled water for atmospheric applications by D. M. Murphy and T. Koop, 29 Dec 2006
The vapour pressure of both ice and liquid water at the triple point is
Pt = 611.657 ± 0.01 Pa at temperature Tt = 273.16 K
(Guildner et al. 1976, Vapor pressure of water at its triple point. J. Res. Natl. Bur. Stand., 80A, 505–521)
The second calibration point is a problem - ref
Perhaps 2 of the formulations have been modified to take this into account
By the way, there is no requirement that a good algorithm has to intersect one, or both, calibration points (though it would be nice). "Improved Magnus` form approximation of saturation vapor pressure" argues that a formulation that produces the smallest maximum error may be "better" than the algorithm with the smallest overall error (determined by, for example, a least squares fit) if the tail values are off by a large amount.
Both programs normally limit the temperature precision to only 2 decimal places. To use additional precision, type the value into the Kelvin field. The only time I have needed this is when checking the *new* boiling point of water at 1 atm - the old standard was 100°C and the new value is 99.974°C .. because the standard temperature scale was redefined in 1990!
For temperatures below 145 K, the partial pressures are so low that they might as well be zero. Also, some of the algorithms produce errors with lower temperatures. As a result, the program just shows zeros for temperatures below 145 K.
The problem was that I had no idea if the values made any sense. Therefore, I added a computation to find the relative humidity (%RH) associated with that mixing ratio. I was surprised to learn that the mixing ratio for 60%RH at the surface (15°C) would be over 2,000%RH at -30°C. I was also concerned that the "over water" equation would provide the wrong value at -30°C. (Not much liquid at that temperature.) It was while investigating the "over ice" algorithm that I discovered that there were many models to compute the saturation vapor pressure.
In addition, I found the following very confusing - when discussing the RH over ice formulations
|The [RH over ice] equations discussed here are mostly of interest for frost-point measurements using chilled mirror hygrometers, since these instruments directly measure the temperature at which a frost layer and the overlying vapor are in equilibrium. In meteorological practice, relative humidity is given over liquid water ... and care needs to be taken to consider this difference. [ref]|
Water_Vapor.exe was written to
Assuming the RH is known
Mixing_Ratio := Saturation_Mixing_Ratio * RH / 100;
RH := Mixing_Ratio / Saturation_Mixing_Ratio * 100
Saturation_Mixing_Ratio := Saturation_Partial_Pressure / Pressure * 1e6;
The temperature inputs and the relative humidities are rounded to 2 decimal points. In the Cloud base and Dew Point Suppression panels, 8 significant figures are displayed. As a result, the numbers may not match exactly.
The program computes the dew point and the frost point - the temperatures where RH reaches 100%. Obviously, frost points above freezing are of no value. However, dew points below freezing are useful - they could indicate the possibility of a freezing rain event, a condition where super cooled liquid water freezes on impact - and occur on a regular basis.
The Cloud Base altitudes are just for fun. Since the temperature and RH are available,
this just determines the height where RH reaches 100% ..
assuming that the air temperature decreases at either
Rework this algorithm - the current algorithm assume that the partial pressure remains constant. It actually decreases with altitude. As air rises, the mixing ratio remains constant, and assuming a constant temperature, the relative humidity decreases. Since both the pressure and the temperature decrease, the cloud condensation level (CCL) [not the correct acronym] will be higher than computed using a constant pressure.
When the mouse is over a temperature, pressure, or altitude control (other than the temperature selector), appropriate conversions will be displayed in the status bar.
The fields with white backgrounds can be edited by the user. Those with grey backgrounds are readonly computed values - they can be copied to the clipboard .. but you can not enter new data.
On the form, 3 parameters are tightly linked
Temperatures are limited to those below the critical temperature - 647.096 K - and above 140 K.
||x ||x ||0.0001
||x ||x ||x
Some of the fields have hard limits
(degrees-K won't go below zero)
and others will turn red outside the expected range