Relative humidity and dewpoint from psychrometer readings

E. Linacre


A psychrometer’s dry-bulb and wet-bulb temperatures (T and Tw) can be used to calculate the relative humidity, as follows. First, use the readings to calculate the ambient water-vapour-pressure (e), by means of the equation named after Regnault (Section 6.3). The equation requires using the psychrometric chart (or psychrometric table) of saturated-vapour-pressure (svp) versus temperature, to find the svp at Tw. This is done by finding Tw on the left, vertical axis, and reading in the table the corresponding vapour pressure.

Use this chart or table again to find the saturated-vapour-pressure (es) at the ambient temperature T. Then the ratio e/es is the relative humidity, i.e. the ratio of the amount of water present to the amount that there would be if the air were saturated, or the actual over the possible amounts.

The dewpoint (Td) is found by using the psychrometric table once more, in conjunction with the pressure e. Td is the temperature at which e represents saturation. In this case, the table is used ‘inside-out’. In other words, the value within the table equal to e is found, and then the dewpoint read off the temperature scale on the left axis. A web-based Td /RH can be found here.