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Please note. Forecasts, alerts, Best Times & activity suggestions are provided for general information, as-is & without warranty, and can be wrong or delayed. They are not a substitute for official warnings. In severe weather follow weather.gov & your local officials, and call 911 in an emergency.
The Health tab starts with an outlook card for the day: when the air is the story it says when to be active, otherwise it names the conditions worth a little care and their main causes. Below it, each condition gets a rating (No Impacts, Heads Up or High Impacts) and a sentence on what it means for you. The suggestions are general public-health guidance, not medical advice. Use Manage Health Preferences to choose which of the six conditions you see; the ratings and the headline then cover only those, and your choice is saved in this browser.
The No Impacts, Heads Up and High Impacts ratings are a general outlook built from published research (the studies are listed below), using forecast temperature, humidity, pressure, UV and air quality from Open-Meteo, with fine particles and ozone adjusted to the current AirNow reading. They describe how likely a day is to be hard on a condition for people in general, not what will happen to you, and are not medical advice, a diagnosis or a substitute for your clinician.
Best Times, and the Yes / Maybe / No answer to “Is it a Good Time to Go Outside?”, are calculated by Check the Wind from the forecast, air-quality and rainfall data above, using default limits that you can change with Customize. The activities listed are general suggestions based on those conditions, not safety advice. They are a guide, not a guarantee.
Air Quality & Pollen on the Weather tab shows the latest reading from the nearest AirNow monitor for the Air Quality Index, fine particles and ozone, each on its own card with a five-step level meter. Weeds, trees and grasses are seasonal estimates. On the Health tab, the air quality tile and the ratings also take AirNow’s forecast peak for today into account. When a forecast gives only a category (such as Moderate) instead of a number, the ratings use the bottom of that category.
The Air Quality Index runs from 0 to 500: 0–50 Good, 51–100 Moderate, 101–150 Unhealthy for Sensitive Groups, 151–200 Unhealthy, 201–300 Very Unhealthy and 301 or more Hazardous.
The Air Quality Index, Fine particles (PM2.5) and Ozone are sourced live from AirNow (airnow.gov), EPA — coverage depends on the nearest reporting monitor. The Health Outlook also uses AirNow’s forecast (the expected ozone and fine-particle peaks, set by the forecasters at your local air agency) where AirNow has one for your area; forecasts are preliminary and can change. Where AirNow has no nearby reading, the Air Quality Index falls back to Open-Meteo’s air-quality model and the Fine particles and Ozone cells say no data. Hourly air-quality & recent-rainfall figures used for Best Times come from Open-Meteo. Pollen isn't available from any free public U.S. source, so trees/grasses/weeds levels are a seasonal estimate based on location & time of year, not a measurement.
Weather. In the U.S., wind speed, gusts, direction, temperature, forecasts & weather alerts come live from the National Weather Service (api.weather.gov), NOAA. In Canada they come from Environment and Climate Change Canada’s MSC GeoMet open data (api.weather.gc.ca), used under the ECCC open data licence: current conditions, the city-page hourly & 7-day forecasts, and weather alerts. Coverage is limited to U.S. locations & territories and Canada. Forecasts update hourly at the source; reload to refresh.
Right now. The forecast is not always the same as what is falling this minute, so the current conditions are checked against the nearest official weather station observation (under 90 minutes old), live radar at your location (NOAA radar tiles from the Iowa Environmental Mesonet) and the Open-Meteo model’s current weather. Rain, snow or thunderstorms seen by any of them replace fog or cloud wording, and the temperature and wind shown for the current hour are the station’s reading when one is available. Active warning outlines on the radar (tornado, severe thunderstorm, flash flood, flood and special marine) are the National Weather Service’s storm-based warning polygons, served by the Iowa Environmental Mesonet, for U.S. locations.
Canada. Canadian forecasts come from the nearest Environment Canada forecast location, which can be some distance from the place you searched. Metric values are converted to °F & mph, and temperatures show in °C and wind in km/h for visitors in Canada (judged from the browser’s time zone or language) and for anyone looking at a Canadian place. The city-page hourly forecast covers 24 hours, so Check the Wind fills the hours after that, and the wind on days the 7-day forecast doesn’t mention it (Environment Canada only reports wind of 20 km/h or more), with Environment Canada’s own GEM forecast model, relayed by Open-Meteo. Canadian alerts show each warning, watch or advisory with a link to its full text at weather.gc.ca; special weather statements are not shown. Forecast text is in English only.
Radar. Current radar is NOAA NEXRAD imagery served by Iowa State University's Iowa Environmental Mesonet. The expanded radar uses NOAA MRMS observations & NOAA HRRR model forecasts, which Check the Wind processes and extends a short way ahead using the recent motion of the storms. The forecast part is an estimate and storms can change quickly. In Canada, the radar map loops the last hour of Environment and Climate Change Canada’s national radar images (MSC GeoMet); there is no expanded or forecast radar there yet.
Local weather reports. The Notes, Outlook, Nearby and Story tabs show text and pictures published by your local National Weather Service forecast office (forecast discussion, hazardous weather outlook, local storm reports and weather story), shown as published. Offices do not post every product at every moment, so a tab may say nothing is available. These reports are not available in Canada.
Places, maps & sun times. Place search covers the U.S. and Canada and uses Open-Meteo's geocoding service (GeoNames data, CC BY 4.0). A street address you search for (for example “123 Main St, Denver, CO”) is sent, when you press Enter, to OpenStreetMap’s Nominatim service (© OpenStreetMap contributors) to find its coordinates; Check the Wind does not store it beyond your own recent places in this browser. Place names for “Use my location” come from the National Weather Service or, in Canada, Environment Canada. Map tiles are © OpenStreetMap contributors, shown with Leaflet. Sunrise & sunset times by SUNRISE-SUNSET.ORG.
Your information. Check the Wind has no accounts and no advertising. Your favorites, recent places, Best Times settings, animation preference & last location are stored only in this browser, on your device; clearing your browser's site data removes them. To show you weather, the places you search for — and your approximate coordinates if you tap “Use my location” & allow it — are sent to the services listed above. This page loads its code library & map software from cdnjs and its font from Google Fonts, which can see your IP address as any website you visit can.
Analytics. When you choose Accept on the banner, we use Google Analytics to count visits to Check the Wind and to understand, in aggregate, how the site is used and roughly who uses it. It records things like the pages viewed, how visitors arrive, the type of device & browser, approximate location (city or region, from your IP address), and, when Google has enough data, general age, gender & interest groups. Google Analytics sets cookies in your browser to do this. It is used only for analytics: we don’t use it for advertising, and we don’t sell your data. The places you search for and your saved favorites are not sent to Google Analytics. Learn how Google handles this data at Google’s privacy page, and you can opt out with Google’s browser add-on. If you choose Opt Out, analytics cookies are not set, and your choice is remembered in this browser.
Questions, feedback or a place that looks wrong? Write to contact@checkthewind.com.
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Choose the health concerns that matter to you. Only these show in your Health Outlook and headline. Each shows today’s impact level. Saved in this browser.