The dew point is the temperature at which air is saturated with water vapor. If the temperature goes below the dew point, water will be begin to condense on solid surfaces, including dust or salt in the atmosphere, causing fog to form.
As the graph from PicoLog 6 screenshot clearly shows, dew point is closely related to relative humidity (dew point is the middle trace, relative humidity is the lower trace). It is often cited as a more precise way of quantifying "comfort" of the air because it is an absolute measurement (degrees C) unlike relative humidity which is a percentage.
To measure dew point, a DrDAQ provides an ideal cost-effective solution for measuring temperature using a DD100 temperature sensor, humidity using a PP163 humidity sensor, and therefore dew point in degrees Celsius using the following formula in PicoLog 6's math channel equation editor: