Two numbers, two different questions
Relative humidity and dew point both describe moisture in the air, but they do not describe it in the same way. Relative humidity tells you how close the air is to saturation at its current temperature. Dew point is the temperature to which the air would have to cool, at constant pressure, to become saturated.
That distinction matters because relative humidity can change even when the actual amount of water vapour changes very little. Cool the air and its relative humidity rises; warm the same air and its relative humidity falls. The dew point remains a steadier clue to the moisture present.
This is why a cool morning can show 95% relative humidity without feeling tropical, while a hot afternoon at a lower relative humidity can feel much more humid. The percentage is describing nearness to saturation, not giving a simple count of water molecules.
What the dew point tells you
A higher dew point means more water vapour is present. When the air temperature falls to the dew point, the air is saturated and condensation can begin if the other conditions are suitable. Dew on grass, fog and low cloud can all involve air cooling toward saturation, although their formation also depends on wind, surface conditions and the structure of the atmosphere.
The gap between air temperature and dew point is often useful. A wide gap points to relatively dry air. A narrowing gap tells you the air is approaching saturation. It does not guarantee fog or rain: moisture can be uneven with height, wind can mix drier air downward, and cloud formation needs the right cooling and lifting.
Dew point is also a useful guide to how muggy the air may feel, especially in warm weather. Comfort is personal and depends on acclimatisation, wind, sunshine, clothing and activity, so a single universal comfort threshold is too blunt. The practical comparison is simple: when the dew point rises from one day to the next, there is more moisture in the air.
Why relative humidity changes through the day
Relative humidity commonly peaks near the coolest part of the day and falls as the temperature rises after sunrise. That daily swing can happen without a matching surge or loss of atmospheric moisture.
Imagine air with the same dew point through the morning. Before sunrise, the air temperature may sit only a little above that dew point, producing high relative humidity. By afternoon, solar heating widens the temperature-dew-point gap and the relative humidity drops. Reading the percentage alone could make it appear that the moisture vanished; comparing the dew point shows a more complete picture.
Rain, evaporation, a change of wind or the arrival of a new air mass can change the dew point itself. A sharp rise may signal that moister air is moving in. A fall after a front can mark drier air, though any interpretation should be checked against the wider forecast rather than one observation.
Humidity is not a rain forecast
High relative humidity does not automatically mean it will rain. Air near the ground can be humid while the atmosphere above is too dry or too stable for precipitation. Rain requires cloud processes and enough lift and moisture through a deeper layer.
The reverse can also be misleading. A modest surface relative humidity during the afternoon does not rule out later showers if moisture and instability are increasing elsewhere in the atmosphere. Use the hourly precipitation forecast and official warnings for that question.
A practical reading order
Start with air temperature because it sets the context. Then compare the dew point: the closer the two values, the nearer the surface air is to saturation. Use relative humidity to see that relationship expressed as a percentage at the current temperature. Finally, scan the next several hours rather than treating one value as a promise.
For outdoor comfort, watch the dew-point trend together with temperature, wind and sunshine. For fog, frost, storms or heat hazards, use the responsible public weather service. Humidity numbers are useful ingredients, but they are not warning products on their own.