Dewpoint is the temperature air must be cooled to, at constant pressure and constant water vapor content, in order to reach saturation. It is a direct measure of how much moisture is actually in the air.
What dewpoint is measuring
Warmer air can hold more water vapor than cooler air. If you take a parcel and cool it while keeping the moisture content and pressure the same, at some temperature the air becomes saturated and any further cooling forces water vapor to condense. That temperature is the dewpoint.
Because the value depends on how much vapor is present, not on how hot the air feels, dewpoint moves much less through the day than relative humidity does. A summer afternoon can warm from 75 to 95 degrees Fahrenheit while the dewpoint stays near 72, telling you the moisture reservoir has not changed even though the air feels less humid.
How forecasters read it
Surface dewpoint is one of the first fields a severe weather forecaster checks. It defines where the moist air is, where it is missing, and where the boundary between the two sits. A tightening dewpoint gradient often marks a dryline or a moisture axis, both of which focus thunderstorm development.
Dewpoints in the mid to upper 60s (Fahrenheit) or higher across the warm sector are commonly cited as supportive of severe convection in the Plains, but the useful number varies with elevation and season. In the higher terrain of the Southern Rockies, mid-50s dewpoints can support strong storms because the surface pressure is lower and the depth of moisture matters more than the raw value.
Important limits
Dewpoint is a surface or single-level quantity. It says nothing about how deep the moist layer extends, and a shallow moist layer can mimic a rich environment while producing little rain when storms fire. To know the depth, forecasters read the entire moisture profile on a sounding rather than trusting a single dewpoint value.
Dewpoint also carries no information about lift or instability. Very high dewpoints under a warm cap can sit for hours with no storms at all, and lower dewpoints in a cold, unstable airmass can still produce severe weather. Treat dewpoint as one ingredient among several, never as a switch.