|HOW IS DEWPOINT CALCULATED?
METEOROLOGIST JEFF HABY
Dewpoint is the temperature air will reach saturation when air cools and maintains a constant pressure
and constant moisture content.
To find dewpoint, the temperature and
wet bulb temperature need to be known. The temperature is read straight
from a thermometer. The wet bulb temperature is the temperature air will cool to when complete evaporative
cooling occurs. Getting the wet bulb temperature requires the use of a psychrometer or hygrometer. Once you have the
temperature and wet bulb temperatures use one of the methods below to determine the dewpoint:
1. Find a chart that will give you dewpoint when temperature and wet-bulb are known. Some charts will require
the wet-bulb depression. The wet-bulb depression is the temperature minus the wet-bulb temperature.
2. On the Skew-T Log-P, plot the temperature and wet-bulb temperature at the pressure level it is being measured.
Draw a line parallel to the dry adiabates through the temperature and draw a line parallel to the moist adiabates
through the wet bulb. These two lines will intersect. At the intersection, drop a line down parallel to the mixing
ratio back down to the original pressure level. This value is the dew point.