Well, I spent the better part of a day figuring this out and figured I would share.
This is for all of those poor HVAC souls who need wet bulb temp. I would suggest installing TA-wunderground and pull the data into a index. Then you can run the following query to get dry and wet bulb temps. I have compared this against NOAA's online calculator and it's pretty close. Spot on when it's 40 or above and a few tenths of a degree off when freezing.
index=weather | table _time, temp_f, temp_c, pressure_mb, relative_humidity | eval rh=substr(relative_humidity,1,len(relative_humidity)-1) | eval tdc=round(temp_c-(14.55+0.114*temp_c)*(1-(0.01*rh))-pow(((2.5+0.007*temp_c)*(1-(0.01*rh))),3)-(15.9+0.117*temp_c)*pow((1-(0.01*rh)),14),10) | eval e=round(6.11*pow(10,(7.5*tdc/(237.7+tdc))),10) | eval wbc=round((((0.00066*pressure_mb)*temp_c)+((4098*e)/pow((tdc+237.7),2)*tdc))/((0.00066*pressure_mb)+(4098*e)/pow((tdc+237.7),2)),2) | eval wbf=wbc*1.8000+32 | timechart span=15m last(temp_f) as "dry bulb temperature", last(wbf) as "wet bulb temperature"
