Humidity ratio calculator
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#' rh relative humidity, ratio of actual water mixing ratio to saturation mixing ratio kg/kg) ratio of water mass / total air mass #' qair specific humidity, dimensionless (e.g. #' from Bolton 1980 The computation of Equivalent Potential Temperature
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#' NCEP surface flux data does not have RH #' converting specific humidity into relative humidity In addition to laboriously working through a few textbooks and online references, I have compared the results to a psychrometric chart.Īs you have likely noticed, it is a challenge to keep these terms and units straight - I wish I had answer then! #' Convert specific humidity to relative humidity Here is an implementation that I wrote in R (with documentation in the header). $T_0$ reference temperature (typically 273.16 K) (K)
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If you have specific humidity, which is the mass mixing ratio of water vapor in air, defined as: Comfort, in reality, is not a static thing and the assumption that a single polygon can represent the dynamism of your thermal preference is inherently flawed.Relative humidity is just $e/e_s$, the ratio of the vapor pressure to saturation vapor pressure or $w/w_s$, the ratio of mass mixing ratios of water vapor at actual and saturation values. To assess comfort over long periods of time, you should really be running the PMV comfort component and you should probably account for how the wind and MRT change over time as well as people’s clothing. In the end, the psychometric chart is just a means to help you visualize temperature and humidity and it is not really a tool for measuring how comfortable a space is over a long time period. I can just tell people that the assumption of a single MRT and windSpeed is a limitation of the one-polygon psych chart methodology. Nor can I accurately plot the strategy polygons. Alternatively, I can plot two comfort polygons showing the range from warmest comfort to coldest comfort across the 8760 values but, with this, I can’t allow the user to plot multiple metRate or cloValue polygons. The simplest option would just be to not allow the user to plot any polygons on the chart in this case. The issue here becomes what to do with comfort and strategy polygons. If someone connects up epw data or a list of 8760 values to either MRT or windSpeed of the psych chart, I assume that the user wants to align the data and I just pass the aligned values through the PMV function. I have realized that this discrepancy can be confusing to people and I can think of 2 ways of addressing it: This is because the PMV component assumes that the MRT is the same as the dry bulb temperature by default while the psych chart needs to assume that there is only one MRT value for all hours in order to draw only one comfort polygon. At present, when you plug in epw values to both the psych chart and the PMV comfort component, you do not get the same percentage of comfortable hours out of the two components. You should also note that you can connect up the chartHourPoints output of the psych chart to a native GH “Point” component to see the point in Rhino.įinally, I would like to take this opportunity to get your thoughts on another topic that Mostapha mentioned to me earlier. Still, if you make a case here that exposing such an input would help you better coordinate your point values, I can do it. I originally did not expose this parameter as an input to the component because the psychrometric chart was already so large and it currently has the most inputs of all components in Ladybug. Because humidity ratio values are so small, I multiply them by a scaling factor that is set inside the component: Alternatively, you can also find it from the mixing ratio of air. You can also use it to calculate the relative humidity if you know the dew point temperature. I have fixed it in the copy of the components on the github and in the attached GH file.Īs for the second issue of aligning points in Rhino space with the chart, you have already realized that the temperature is plotted with 1 Rhino unit equal to 1 degree temperature. If you would like to know more about the relative humidity formula, check out our dew point calculator. Thank you for finding it and reporting it.