You open a weather app and see two numbers — relative humidity at 65% and a dew point of 18°C. They both sound like they measure moisture. So why are there two of them? And which one actually tells you whether it's going to feel sticky and uncomfortable outside? The answer might surprise you: meteorologists trust dew point far more than humidity when it comes to how the air actually feels on your skin.
Relative humidity (RH) is the percentage of moisture in the air compared to the maximum amount the air could hold at that temperature. So if relative humidity is 70%, the air is holding 70% of the water vapour it could theoretically contain before it becomes fully saturated.
The catch is that the maximum amount of moisture air can hold changes dramatically with temperature. Warm air holds far more water vapour than cold air. This makes relative humidity a moving target — it shifts throughout the day even if the actual amount of moisture in the air stays exactly the same.
For example, on a typical day the temperature rises from morning to afternoon. As the temperature goes up, the air's capacity to hold moisture increases — so the relative humidity drops, even though no moisture has left the air. By late afternoon the humidity might read 45%, feeling comfortable, even though the actual moisture content is identical to the morning reading of 80%.
The dew point is the temperature at which air would need to be cooled before the water vapour in it starts to condense into liquid — forming dew, fog, or eventually rain. It is a direct measure of the actual amount of moisture present in the air, expressed as a temperature rather than a percentage.
Unlike relative humidity, dew point does not change as the temperature rises or falls during the day. If the dew point is 18°C at 7am, it will still be around 18°C at 3pm — assuming no new moisture has been added to or removed from the air. This stability makes it a far more reliable indicator of how muggy the air actually is.
Your body cools itself by sweating. Sweat evaporates from your skin and carries heat away. The dew point determines how easily that evaporation can happen. When the dew point is high, the air is already carrying a large amount of moisture, which slows evaporation and makes your body's cooling system less effective.
Relative humidity alone can be misleading. A 90% relative humidity reading sounds oppressive — but if the temperature is only 5°C, the actual moisture content is very low and you will feel perfectly fine. On the other hand, a 55% relative humidity on a 35°C day means the air holds far more moisture than a cold 90% day, and you will feel distinctly uncomfortable.
The dew point cuts through this confusion. A dew point above 18°C feels noticeably humid to most people. Above 21°C it starts to feel oppressive. Above 24°C it becomes genuinely uncomfortable and poses real health risks during physical activity.
| Dew Point | How It Feels | What to Expect |
|---|---|---|
| Below 10°C | Dry, crisp | Very comfortable, air feels fresh |
| 10°C – 15°C | Comfortable | Pleasant, most people feel good |
| 16°C – 18°C | Slightly humid | Noticeable but still acceptable |
| 19°C – 21°C | Humid | Sticky, sweating less effective |
| 22°C – 24°C | Very humid | Oppressive, uncomfortable outdoors |
| Above 24°C | Extreme | Dangerous for outdoor activity, heat stress risk |
Imagine two cities on the same summer afternoon:
City A: Temperature 38°C, relative humidity 25%. Sounds dry and manageable. But the dew point is around 15°C — and it is. The low moisture in the air means sweat evaporates quickly. It feels hot, but the dry heat is tolerable.
City B: Temperature 32°C, relative humidity 70%. Sounds much more humid. The dew point is around 26°C. Despite being 6 degrees cooler than City A, City B feels far worse. Sweat barely evaporates. The air feels heavy. Heat exhaustion becomes a real risk.
Relative humidity said City B was manageable at 70%. Dew point told the truth — City B is dangerous.
Dew forms when surfaces cool down to the dew point temperature overnight. Grass, car windscreens, and metal surfaces radiate heat into the clear night sky and cool rapidly. When they drop to the dew point, the water vapour in the air immediately above them condenses into liquid droplets — dew. This is also how fog forms: when the air itself cools to its dew point, the moisture condenses into tiny suspended droplets.
For a quick glance at how muggy a day will feel — especially if you are planning outdoor exercise, gardening, or a summer event — check the dew point, not the humidity percentage. A dew point below 15°C means you will feel comfortable. Above 20°C, plan accordingly: exercise early in the morning, stay hydrated, and take breaks in the shade or indoors.
ClearCast displays both relative humidity and dew point alongside the feels-like temperature so you can see the full picture at a glance.