The dew point is the temperature to which air must be cooled (at constant pressure) for water vapour to begin condensing into liquid — forming dew, fog, or clouds. It is an absolute measure of moisture content, unlike relative humidity which varies with temperature. A dew point of 20 °C always means the same amount of moisture in the air, regardless of the actual temperature.
The dew point is extremely useful for weather prediction and human comfort assessment. When the air temperature approaches the dew point, the air is near saturation (RH close to 100 %) and condensation phenomena are likely. At the surface, this means dew or frost (if below 0 °C — then called the frost point); at altitude, it means cloud formation. The level at which rising air reaches its dew point is the lifting condensation level — the cloud base height, which can be estimated as (T − Td) × 125 in metres.
For human comfort, the dew point is more reliable than relative humidity. Below 10 °C the air feels pleasantly dry; between 16–18 °C it becomes noticeably humid; above 21 °C it feels oppressive; and above 24 °C it is dangerously humid as the body can barely cool through sweating. The highest dew points on Earth — around 35 °C — occur near the Persian Gulf and along the Red Sea coast. Meteorologists measure the dew point with a hygrometer or psychrometer (wet-bulb and dry-bulb thermometer pair).