An expression of humidity defined as the temperature to which air must be cooled to cause condensation of its water vapor content (dew formation) without the adding or subtracting of water vapor or changing its pressure. At this point the air is saturated and relative humidity becomes 100%. When the dew point temperature is below freezing it is also referred to as the frost point. The dewpoint is a conservative expression of humidity because it changes very little across a wide range of temperature and pressure, unlike relative humidity which changes with both. Dew points, however, are affected by water vapor content in the air. High dew points indicate large amounts of water vapor in the air and low dew points indicate small amounts. Scientists measure dew points in several ways: with a dew point hygrometer; from known temperature and relative humidity values; or from the difference between dry and wet bulb temperatures using tables. They use this measurement to predict fog, frost, dew, and overnight minimum temperature.