2. Parameters

Moisture/Humidity » Vapor Pressure (e) Skew-T Procedure

From the dewpoint (Td) curve at the given pressure on the sounding, always follow the isotherm to the 622 hPa isobar. The value of the saturation mixing ratio line, read by interpolation if necessary, through this point at 622 hPa gives the vapor pressure in hectopascals (hPa) at the given pressure. In this example, air at a pressure of 850 hPa with a dewpoint of -9°C has a vapor pressure of 3.0 hPa.

Note the procedure to find vapor pressure is quite similar to the one already described to find saturation vapor pressure. The only difference is one starts with the dewpoint to find the vapor pressure, while one starts with the temperature to find the saturation vapor pressure.