The dew point is a measurement that reveals the actual amount of invisible water vapor suspended in the air. It is defined as the temperature to which a parcel of air, at constant pressure, must be cooled to become completely saturated with water vapor, at which point condensation begins to form liquid water, or dew. This temperature acts as a direct measure of the air’s absolute moisture content. Unlike relative humidity, which is expressed as a percentage and changes with air temperature, the dew point remains constant unless the moisture content changes. A high dew point indicates a large quantity of water molecules present in the air and a greater potential for condensation.
The Direct Factor Absolute Moisture Content
The dew point temperature is determined by the absolute amount of water vapor in the atmosphere, often described in terms of the partial pressure of water vapor. Air is a mixture of gases, and the water vapor within that mixture exerts its own pressure. The dew point is the temperature at which this partial pressure equals the saturation vapor pressure—the maximum amount of water the air can hold.
This relationship explains why the air temperature itself does not alter the dew point value. If the air temperature rises, the air’s capacity to hold water vapor increases, but the absolute number of water molecules remains the same. Consequently, relative humidity decreases, but the dew point does not change. Conversely, if the air cools above the dew point, the relative humidity rises because the air’s capacity for moisture shrinks, but the absolute moisture content stays constant.
A high dew point, such as 70°F (21°C), signifies a high concentration of water vapor, requiring only a small temperature drop for saturation. A low dew point, like 30°F (-1°C), means the air contains very little moisture, and the temperature must fall significantly before condensation can occur.
Sources of Moisture in the Atmosphere
The factors that cause the dew point to change are environmental processes that introduce or remove water vapor from an air mass. Evaporation is a major source, transforming liquid water from oceans, lakes, and rivers into a gaseous state. Globally, the vast majority of atmospheric water vapor, approximately 90%, is generated through evaporation, with the oceans being the largest contributor.
Another mechanism for adding water vapor is transpiration, where plants release water vapor into the air through small pores on their leaves. This process is often combined with evaporation from the soil, termed evapotranspiration, and accounts for roughly 10% of total atmospheric moisture. The density of vegetation, such as a large forest, directly influences the local transpiration rate and the dew point.
Beyond local moisture generation, the movement of air masses, known as advection, plays a role in altering the dew point. When warm, moist air from a source region, such as the Gulf of Mexico, is transported inland by winds, it raises the absolute moisture content of the air it replaces. This geographical shift can rapidly increase the dew point far from the original water source.
How Dew Point Affects Comfort and Condensation
The dew point is a reliable indicator of human comfort because it determines how effectively the body can cool itself through sweat evaporation. When the dew point is high, the air is close to saturation, meaning sweat evaporates slowly or not at all, leaving a person feeling sticky and muggy. Most people find a dew point below 55°F (13°C) to be dry and comfortable, as sweat evaporates quickly.
As the dew point rises into the range of 55°F to 65°F (13°C to 18°C), the air feels noticeably humid. Once the dew point exceeds 65°F (18°C), the environment becomes increasingly oppressive. Values above 70°F (21°C) are considered uncomfortable.
Condensation occurs whenever air comes into contact with a surface that is at or below the air’s dew point temperature. This phenomenon is seen outdoors when fog forms, or indoors on a cold glass of water. In buildings, a high indoor dew point, typically above 60°F (15°C), increases the risk of moisture forming on uninsulated surfaces like windows or pipes.
Condensation provides the moisture necessary for the proliferation of mold, which can thrive when a surface remains damp. Mold formation often occurs in hidden areas, such as within wall cavities, where the surface temperature drops below the dew point of the indoor air. Controlling the dew point indoors, often through dehumidification, is a direct strategy for preventing condensation and mitigating structural moisture issues.