OUTDOORS → INDOORS
Same air.
A different humidity.
Cold air can feel damp outside and turn surprisingly dry inside. See what happens when you bring it in.
WHEN THAT AIR REACHES 70°F
relative humidity indoors
Yep. That’s really dry air.
At the same air pressure, with no moisture added or removed.
Why does the humidity change?
Relative humidity compares the water vapor in the air with the amount needed for saturation at that temperature. Warm the air without adding moisture, and its relative humidity drops. The dew point stays the same.
We use the Magnus/Bolton approximation to find vapor pressure and dew point, then divide that vapor pressure by the saturation vapor pressure at your indoor temperature. This assumes unchanged air pressure. Values are approximate and use relative humidity over liquid water, including below freezing.
If you cool air below its dew point, moisture condenses. We show 100% humidity and flag the condensation, because the assumption of no moisture loss can no longer hold. At 0% humidity, there is no finite dew point.
Formula reference: NOAA / National Weather Service ↗