Relative humidity, absolute humidity, dew point — three numbers, one air
The weather app on my phone says 70% humidity in July and 70% humidity in January and I feel completely different in each. It took me years to understand that relative humidity is a ratio, not an amount.
Warm air can hold more water vapour than cold air. A lot more. At 30°C, saturated air holds about 30 grams of water per cubic metre. At 0°C, saturated air holds about 5. So 70% relative humidity at 30°C means 21 g/m³ of actual water in the air. 70% RH at 0°C means 3.5 g/m³. Six times less water, same percentage.
This is why the same relative humidity feels totally different across seasons. Summer at 70% is sweaty and stifling because your sweat can barely evaporate — the air is already close to saturation. Winter at 70% (measured outdoors, before heating dries the indoor air) feels fine because the actual moisture load is tiny.
Dew point is the honest number
Meteorologists prefer dew point because it's an absolute measurement, not a ratio. Dew point is the temperature to which air would have to cool for its current moisture to condense — i.e., the temperature at which its relative humidity would hit 100%. Air at 30°C with a 20°C dew point has less moisture than air at 30°C with a 25°C dew point, regardless of what the RH percentage says.
The comfort ranges I've internalised: below 10°C dew point is dry (winter indoor air, desert climates). 10-15°C is comfortable. 15-18°C is starting to feel humid. 18-21°C is muggy. Above 21°C is oppressive — sleep gets hard, sweat doesn't evaporate. Above 24°C is genuinely dangerous for outdoor work.
The rule of thumb for outdoor summer weather in a humid climate: if dew point is above 20°C, you're going to be uncomfortable regardless of the actual temperature. If it's above 24°C, cancel the outdoor exercise plans.
The indoor problem
Heating cold outside air to indoor temperature keeps the absolute moisture the same but crashes the relative humidity. Take that 0°C, 70% RH outdoor air (3.5 g/m³) and heat it to 21°C. Saturation capacity at 21°C is about 18 g/m³, so 3.5 grams becomes 3.5/18 = 19% RH. That's why winter indoor air feels desert-dry and your skin cracks. Humidifiers add back some of the lost moisture.
Summer air conditioning works in reverse — cooling saturates the air, water condenses out on the cold coils (that's the drip from the outdoor unit), and the air comes into the room drier as well as cooler. This is why AC feels so much better than a fan on a humid day: it removes water, not just heat.
Absolute humidity vs specific humidity vs mixing ratio
Three ways to say the same thing slightly differently. Absolute humidity is g of water per m³ of air. Specific humidity is g of water per kg of moist air. Mixing ratio is g of water per kg of dry air. Meteorologists use mixing ratio (grams per kilogram) because it doesn't change when air rises and expands. HVAC engineers use specific humidity. General public sees dew point and RH. All measure the same underlying quantity in different units.
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FAQ
Why do I feel colder in humid winter air?+
Moist air conducts heat away from skin more readily than dry air. Same reason humid summer air feels hotter — humidity accelerates heat exchange in both directions.
What's a good indoor humidity target?+
30-50% RH at typical room temperature. Below 30% is dry (static shocks, cracked skin). Above 60% risks mould in cold corners.
This article was written by The Turbo Unit Converter engineering desk (Applied metrology & engineering units) and last reviewed on 2026-07-21 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.