The dew point measures the absolute amount of moisture present in the air. It is defined as the temperature to which a parcel of air must be cooled, at a constant barometric pressure, for water vapor to condense into liquid water. This temperature serves as a direct indicator of atmospheric moisture, used by forecasters to gauge the potential for condensation, cloud formation, and precipitation. Understanding the dew point provides a more accurate assessment of moisture content than common humidity percentages alone, making it a key metric for predicting weather systems.
Defining the Dew Point
The air contains invisible water vapor, and the dew point is the precise temperature at which that vapor reaches saturation. Warm air has a greater capacity to hold water vapor than cold air. When the air temperature cools, that capacity decreases, but the actual amount of water vapor remains the same. Once the air temperature drops to the dew point, the air is 100% saturated, and the excess water vapor transforms into liquid water through condensation. The dew point is always equal to or lower than the actual air temperature, serving as a measure of the water vapor concentration.
Dew Point vs. Relative Humidity
Many people confuse the dew point with relative humidity, but the dew point provides an absolute value of moisture content. Relative humidity is a percentage comparing the current water vapor to the maximum amount the air can hold at that specific temperature. This means relative humidity changes constantly as the temperature rises and falls, even if the actual water vapor content remains unchanged. For example, a morning temperature of $50^{\circ}\text{F}$ with a dew point of $50^{\circ}\text{F}$ results in 100% relative humidity. If the temperature later rises to $80^{\circ}\text{F}$, the relative humidity may drop to 40%, yet the dew point remains at $50^{\circ}\text{F}$ because the quantity of water vapor has not changed. Meteorologists prefer the dew point because its measurement is based on the constant mass of water vapor, making it independent of daily temperature fluctuations.
The Role of Dew Point in Weather Phenomena
The dew point is responsible for the formation of several familiar weather occurrences when the ambient temperature drops to this threshold. When air near the ground cools to the dew point, the condensing water vapor forms tiny droplets visible as fog. This often occurs overnight as the ground loses heat and cools the adjacent air layer. If the surface temperature of objects falls to or below the dew point, liquid water forms directly on the surface, which is known as dew. When the dew point is below the freezing point ($32^{\circ}\text{F}$ or $0^{\circ}\text{C}$), the moisture changes directly from water vapor into ice crystals without first becoming a liquid. This process is called deposition, and the resulting formation is frost.
How Dew Point Affects Human Comfort
The dew point is the most accurate measure of how sticky or muggy the air feels. Human bodies cool themselves primarily through the evaporation of sweat, a process that requires the surrounding air to absorb water vapor. When the dew point is high, the air is already saturated with moisture, significantly slowing the rate at which sweat evaporates from the skin. Dew point values below $55^{\circ}\text{F}$ are perceived as dry and comfortable, allowing sweat to evaporate quickly. Once the dew point rises into the $60^{\circ}\text{F}$ to $65^{\circ}\text{F}$ range, the air feels sticky and muggy, making it harder for the body to cool down. A dew point exceeding $70^{\circ}\text{F}$ is considered oppressive, as the body’s natural cooling mechanism is inhibited, making physical exertion difficult.