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.