Derived Variables

Derived variable details

Dew Point Calculation

This value is computed locally at the module level every time a data sample is taken.

Reference: https://www.weather.gov/media/epz/wxcalc/wetBulbTdFromRh.pdf

Dew Point: The higher the dew point rises, the greater the amount of moisture in the air

The dew point is the temperature the air needs to be cooled to (at constant pressure) in order to achieve a relative humidity (RH) of 100%. At this point the air cannot hold anymore water in the gas form. If the air were to be cooled even more, water vapor would have to come out of the atmosphere in the liquid form, usually as fog or precipitation. (NOAA)

Heat Index Calculation

The computation of the heat index is a refinement of a result obtained by multiple regression analysis carried out by Lans P. Rothfusz and described in a 1990 National Weather Service (NWS) Technical Attachment (SR 90-23).  The regression equation of Rothfusz is

HI = -42.379 + 2.04901523*T + 10.14333127*RH - .22475541*T*RH - .00683783*T*T - .05481717*RH*RH + .00122874*T*T*RH + .00085282*T*RH*RH - .00000199*T*T*RH*RH
where T is temperature in degrees F and RH is relative humidity in percent.  HI is the heat index expressed as an apparent temperature in degrees F.

This value is computed locally at the module level every time a data sample is taken.

Please note there is a correction factor if the RH is greater than 85% and the temperature is between 80 and 87 degrees F that is not currently included in our heat index calculation.

This information may be of particular interest for historic buildings without HVAC systems, and especially for 2-3 story structures and those with upper floors that can be accessed regularly. It may be a source of insights on potential low-impact ventilation solutions.