Why is the air so dry?
Because the air you're breathing hasn't passed over much water lately, or it sank from high in the atmosphere where there is almost none. A dew point below about 40°F is what dry feels like.
See your air right nowWhere it was three days ago, live. No sign-up.→Dry means little water, not low humidity
A humidity reading of 20% sounds dry, and it is, but the useful number is the dew point: how much water the air actually holds. When the dew point drops into the 30s or lower, sweat and moisture evaporate fast. Lips crack, skin tightens, you get static shocks, and wildfire fuels dry out.
Three ways air gets dry
- It crossed land, not water. Air that spent days over deserts or plains never had a chance to pick up moisture.
- It was very cold. Cold air can hold only a little water. Arctic air that warms up as it moves south stays dry, which is why winter air feels so parched indoors once the heater warms it.
- It sank. Air high in the atmosphere is extremely dry. When a high-pressure system pushes it down toward the ground, it arrives warm and nearly waterless. Santa Ana and other downslope winds are the extreme version.
See where your dry air came from
Air Story's trail map colors your air's three-day path from tan (dry) to blue and purple (moist). On a dry day you'll usually see it coming from inland or from the north, and staying tan the whole way.
Questions
What dew point is dry?
Below about 40°F most people notice dry air. Below 20°F, common in deserts and during Santa Ana winds, it's very dry and fire danger rises sharply.
Why is my house so dry in winter?
Cold outdoor air holds very little water. When the heater warms that same air to 70°F, its relative humidity can drop to 20% or less, even though nothing was removed.