MonthWeather

Tonight, at your house

Should I open my windows tonight?

Put in your ZIP. We’ll read tonight’s National Weather Service forecast for you — the low, the dew point, the humidity — against the three numbers that decide whether open windows cool the house or just leave it clammy.

Tonight’s check

Your thermostat reading is optional — without it we’ll tell you what the night is doing, just not how it compares to your living room.

The one number this turns on

Not the temperature — the dew point. It is the temperature the air would have to cool to before the moisture in it starts condensing, and unlike a humidity percentage it doesn’t move around as the night cools. It is simply how much water is in the air, written as a temperature. The National Weather Service publishes what each range feels like:

Dew point tonightWhat the Weather Service calls itWhat that means at the window
55°F or below Dry and comfortable Open everything. This air dries the house out instead of damping it down.
55–65°F Becoming sticky, with muggy evenings Your call: you get the cool, and the damp comes with it.
65°F and up Lots of moisture in the air, becoming oppressive Shut, mostly. Whatever cooling you gain arrives wrapped in moisture.

Why the humidity percentage misleads you here

The Weather Service’s own example makes it plain: air at 30° with a dew point of 30° is at 100% relative humidity, while air at 80° with a dew point of 60° is only at 50% — and it is the 80° air that feels humid. Relative humidity is a percentage of what the air could hold at whatever temperature it happens to be, so it climbs all evening as the air cools without a drop more water arriving. The dew point is the water itself, and it is what walks through an open window.

What open windows are actually doing

The Department of Energy calls it fresh air cooling, or night flushing: night-time air is used to cool a home and all of its thermal mass. That last part is the point. You are not cooling the air in the room — that re-warms in minutes — you are cooling the plaster, the floors, the furniture, and often that carries the house through much of the next day. Two habits make it work:

  • Open low and high at once. Air enters through lower windows where it picks up heat, rises, and leaves through the windows upstairs. That convection current is the stack effect, and it runs without a fan.
  • Open across, not on one side. Windows on opposite sides let a breeze push straight through the house; windows on one wall let it stall.

It works best, the DOE says, in climates with cool nights and regular breezes — which is exactly why the answer to this question depends so much on where you live and what month it is.

When to leave them shut

  • When outside is no cooler than inside. The whole mechanism runs downhill. If tonight’s low matches your thermostat, the windows have nothing to give you.
  • When the dew point is at or above 65°. The EPA’s guidance for indoors is to keep humidity below 60 percent relative humidity, ideally between 30 and 50 percent. Muggy night air is how a closed-up house sails past that, and an air conditioner then spends the morning wringing it back out.
  • When the air outside is bad. The Department of Energy puts it in one line: natural ventilation should not be used when outdoor air quality is poor. On a smoke day or a poor-air day, the current reading is at AirNow.

Reading it yourself, without this page

The checker is a shortcut for three things you can read yourself at forecast.weather.gov — find your town, then open tonight’s period and its hourly table:

  1. The overnight low. Compare it with what your house is sitting at right now.
  2. The dew point in the hourly forecast. Compare it with 55 and 65.
  3. The wind. A little of it is what turns an open window into moving air.

And then the thing no forecast knows: your street. A ground-floor room on a shaded side runs cooler than the grid square it sits in, and an upstairs bedroom under a hot roof runs warmer.

The version of this question you can plan around

Tonight is a forecast. The month you’re travelling in, or the month you’re dreading at home, is a pattern — and that’s the rest of this site. Every destination page carries what the air actually does there, month by month: whether nights cool off, whether the humidity ever lets go, and what to pack for either answer.

Or start from the top: all destinations, state by state, or a month at a time.

Common questions

What dew point is too humid to open the windows?
The National Weather Service calls a dew point of 55°F or below dry and comfortable, 55–65°F sticky with muggy evenings, and 65°F and up "lots of moisture in the air, becoming oppressive". Above 65° you are trading a few degrees of cool for a houseful of damp, which is the night most people regret the open window by morning.
Why does dew point matter more than the humidity percentage?
Relative humidity is a percentage of what the air could hold at its current temperature, so it swings all night while the actual moisture stays put. The Weather Service’s own example: 30°F air with a 30°F dew point is 100% relative humidity, while 80°F air with a 60°F dew point is only 50% — and the second one is the one that feels muggy. The dew point is the moisture itself.
How does opening windows at night actually cool a house?
The Department of Energy calls it night flushing: night air is pulled through the house and cools not just the air but the walls, floors and furniture, which then hold that cool through much of the next day. Open low and high at once and the stack effect does the work for you — cool air enters at the bottom, warm air leaves at the top.
When should the windows stay shut even on a cool night?
When the air outside is no cooler than the air inside, there is nothing to gain. When the dew point is at or above 65°F, you are importing moisture the EPA would rather you kept out — its guidance is to keep indoor humidity below 60% relative humidity, ideally 30 to 50%. And the Department of Energy is blunt about air quality: natural ventilation should not be used when outdoor air quality is poor.

Where these numbers come from