How this works
Every hour, the tool pulls the current and forecast weather for the location you pinned and asks five simple questions:
- Is the outdoor temperature below your comfort target? (You set this — default 75°F)
- Is the dewpoint reasonable, so opening windows won't make the house clammy? (You also control what counts as a good dewpoint — default 65°F)
- Is the wind calm enough that windows won't slam shut or blow dust in?
- Is air quality (AQI) safe to breathe?
- No rain blowing in, no NWS advisory active?
If all five are yes, you should open your windows — that hour gets a green block. If any one is no, you should keep them closed — that hour gets an amber block with the specific reason ("dewpoint 68°F — humid air" or "AQI 142 — smoke advisory").
💨 Tip: when it says open, opening more than one window — ideally on opposite sides of the house — sets up a cross-breeze and cools far better than a single open window.
The result is a 24-hour color-coded timeline. At a glance you know when to open, when to close, and why. The "next change" banner tells you the next time you'll need to act.
Why dewpoint, not relative humidity?
Relative humidity by itself is misleading. 60°F at 80% RH feels fine. 75°F at 80% RH feels swampy. Dewpoint measures the absolute moisture in the air, independent of temperature. Above about 65°F dewpoint, opening windows brings sticky air into the house — even if the temperature seems comfortable. The tool blocks window-opening on that threshold so you don't lose the dryness you've built up indoors.
The "open all day" sweet spot
Mild days where outdoor stays below your comfort target for the whole 24 hours are a free win — A/C off all day, windows open, no work needed. The tool tells you "windows open all day" outright when that applies. In dry climates this can happen 30-40 days a year; in coastal mild climates, far more often.
Time-of-use rates change the math
If your utility charges more for electricity during certain hours (a "peak rate" or "TOU window" — often 4-9 PM in summer), running A/C during that window can cost 3-6× more than off-peak. The tool's Expensive Hours control lets you mark those hours so the timeline shows when to not run A/C regardless of outdoor temperature — pre-cool earlier when rates are cheap, let the house ride through peak, recover after.
This works for any reason A/C is "expensive": utility TOU rates, generator fuel cost, off-grid battery depletion windows. The tool is reason-agnostic — just mark the hours you want to avoid.
Air quality and wildfire smoke
The tool checks hourly US Air Quality Index (AQI) for your location. When AQI exceeds 100 (the EPA's Moderate-to-Unhealthy crossover), open windows let smoke and particulates into the house. Western US summers increasingly bring wildfire smoke into clean-temperature weather — you can have a perfect 70°F evening that should be a windows-open night, but the AQI says otherwise. The tool surfaces this so you don't pay the lung cost of a single beautiful night.
Precooling: how to set your programmable thermostat
If your power costs more during a peak-rate / time-of-use window (common on EV-friendly plans), you can keep your home at your normal comfort temperature through that window without running the A/C during it. The trick is precooling: cool the house a few degrees extra before the expensive hours, then let it coast. The house holds the cool it banked and drifts slowly back up to your normal target — so the A/C stays off while electricity is most expensive.
Set the peak-rate window in the tool above and you'll get a personalized plan with exact times and temperatures. Here's how to put it on any programmable thermostat — it's just three schedule periods:
- Pre-cool period — a couple of hours before your peak window (more on a hot day, since the A/C cools the house more slowly when it's scorching outside — the plan above figures the exact start time from today's forecast), set the temperature a few degrees below your normal target. This over-cools the house so it has cool "in the bank."
- Peak period — for the expensive hours, set the temperature a few degrees higher than you actually need (a few degrees above your normal target). That keeps the A/C from turning on at all. Because the house started cold, it stays comfortable and just drifts slowly up toward your normal target — without the compressor ever kicking on while power is expensive.
- Normal period — once the peak window ends, return to your usual comfort setting and let the A/C run normally again.
Most programmable and smart thermostats let you add multiple time periods per day — look for "Schedule," "Periods," or "Add a time" in the menu. Set the three periods above to the times and temperatures the tool gives you. If your thermostat has a "hold" or "vacation" override, you can also just punch in the pre-cool temperature manually a couple hours before peak and switch it back afterward.
How deep to pre-cool depends on how long your peak window is and how well your home holds temperature — a longer window or a leakier house needs a bit more head start. The plan above estimates this for you; adjust by a degree or two based on how your home actually feels.
Common questions
Should I open my windows at night to cool the house?
Usually yes — if outdoor temperature drops below your comfort target (around 75°F), dewpoint is under 65°F, and wind is calm. Overnight open-window cooling can pre-cool a house enough to skip morning A/C entirely. The tool tells you the exact hour to open and the exact hour to close before sunrise heat starts climbing.
Can I turn off my A/C today?
Pin your location and look at the timeline. If all 24 hours are green, yes — windows do the work, A/C off all day. If the afternoon hours are amber (outdoor exceeds your comfort cap), the A/C needs to run for part of the day. Pre-cool early in the morning while it's free, then let the house drift through peak.
Will it be cool today or hot?
The tool shows the next 24 hours color-coded for your exact location. Green = comfortable enough for open windows. Amber = hot, humid, smoky, windy, or rainy — close them. You'll know within 5 seconds of pinning the map.
Does opening windows make my A/C less efficient?
Only if outside is hotter or more humid than inside. The right strategy is alternating: open when outside is cooler and dry, close when outside is hotter or humid. The tool tells you exactly when to switch — no guesswork.
How does humidity affect my decision?
Dewpoint matters more than relative humidity. A dewpoint above 65°F means the air feels sticky and clammy even at comfortable temperatures. The tool blocks window-opening on that threshold automatically — you don't need to think about it.
When should I close windows because of wildfire smoke or dust?
Whenever AQI exceeds your threshold (default 100). The tool checks hourly US AQI and adds active NWS dust storm, fire weather, and smoke advisories. If any of those flag, the timeline tells you "keep windows closed" regardless of how nice the temperature is.
What is a TOU peak rate and does it apply to me?
Time-of-use (TOU) rate plans charge more during peak demand hours — usually evenings. If you're on a TOU plan (especially an EV- owner plan), peak rates can be 3-6× off-peak. Check your most recent utility bill. Even non-TOU users sometimes care about "expensive hours" — generator fuel cost, off-grid battery depletion windows. The tool's Expensive Hours setting works for any reason.
How accurate is the forecast?
Weather and AQI come from Open-Meteo, which aggregates national weather service data (in the US, NWS) plus EPA AirNow. Same quality as your phone's weather app. For more than 24 hours out the tool intentionally trims accuracy — beyond that horizon, replan when you check tomorrow.
Is my location stored anywhere?
Only if you want it to be. Used anonymously, your pinned location lives only in the URL — bookmark the page after pinning and your settings come back, with no cookies and no account needed. If you create an optional account, we save your settings so they follow you across devices. See the privacy section below.
Why do I see Amazon product suggestions on the page?
Affiliate links pay for the server. We show items that match the day's actual forecast — weatherstripping when rain is blowing in, air purifiers when AQI is bad, smart thermostats year-round. If you buy through them we earn a small commission at no cost to you. Full disclosure right below the suggestions.
Privacy
Using the tool anonymously requires no cookies and no account — your pinned location lives only in the URL of the page (which you can bookmark or clear), and no per-user history is retained. Weather data is fetched from Open-Meteo and cached on the server until the top of the next hour (up to about 45 minutes on average).
Accounts are optional. If you choose to create one, we store the email and password you provide and the settings you save (location, comfort temperatures, peak-rate hours, etc.) so they come back on your next visit. That's the only personal data we keep, it's yours, and you can stop using it at any time. We never sell or share it.
The Amazon affiliate links use Amazon's standard cookie-based attribution (24 hours, set by Amazon, not us). If you click an affiliate link, Amazon sees the click; if you then buy something there in the next 24 hours, we earn a small commission. We do not see what you bought, only aggregate click counts to know which products people find useful. We never sell or share your data.
Server access logs (IP, page requested, timestamp) are kept for 30 days for routine security monitoring then deleted. No analytics services, no fingerprinting, no third-party trackers run on the page.