How the numbers work
Every number has a source.
Sideline Sky does no weather forecasting of its own. It reads the National Weather Service forecast for a point, puts the hours side by side and compares them with a published table. This page says exactly how.
1. The point
A ZIP code becomes the centre point of its ZIP Code Tabulation Area in the U.S. Census Bureau's 2025 Gazetteer (public domain). A ZIP code area can be large, so for a field far from the middle of its ZIP, type the field's latitude, longitude (for example 41.45, -81.63) or use your browser's location. The result page shows the point used and the place name the Weather Service gives it.
2. The forecast
For that point the Weather Service names a 2.5 km forecast grid square (the grid square is looked up for the point rounded to 0.01 degree, about 1 km, so nearby fields share one lookup). Sideline Sky reads that square's gridded forecast (api.weather.gov/gridpoints) for the next 7 days and keeps, hour by hour: temperature, relative humidity, heat index, wet bulb globe temperature (WBGT), probability of thunder, probability of precipitation, wind speed and wind gust. Celsius is converted to Fahrenheit (one decimal) and km/h to mph. A forecast is refreshed at most every 45 minutes; if the Weather Service cannot be reached, a copy up to 3 hours old may be shown, marked as such, and nothing older is ever shown. Active alerts for the point rounded to 0.001 degree, about 100 m (api.weather.gov/alerts), are refreshed every 5 minutes. Times are the field's local time.
A forecast is not a reading. WBGT measured on site with a meter can differ from the forecast, for example on artificial turf or in still air. Where your policy requires an on-site reading, take one.
3. The activity tables
Each hour's forecast WBGT is rounded to 0.1 °F and given a level from 1 to 5. A value exactly on a boundary takes the higher level. The three regional tables are the Korey Stringer Institute's categories from Grundstein et al., "Regional heat safety thresholds for athletes in the contiguous United States," Applied Geography (2015), as reproduced in the NAIA championship heat and humidity policy:
| Level | Category 1 | Category 2 | Category 3 |
|---|---|---|---|
| Level 1 | under 76.2 °F | under 79.8 °F | under 82.0 °F |
| Level 2 | 76.2 to 81.0 °F | 79.8 to 84.6 °F | 82.0 to 86.9 °F |
| Level 3 | 81.1 to 84.0 °F | 84.7 to 87.6 °F | 87.0 to 90.0 °F |
| Level 4 | 84.1 to 86.0 °F | 87.7 to 89.6 °F | 90.1 to 91.9 °F |
| Level 5 | 86.1 and over °F | 89.7 and over °F | 92.0 and over °F |
In the source, Level 1 is "activities proceed as normal" and Level 5 is the band where outdoor activity is cancelled or stopped. The levels between bring more rest breaks, shorter practices and equipment limits, and those rules differ by state association and league, so Sideline Sky names the level and leaves the rule to your policy. Category 1 has the lowest numbers and is the default here because it is the most cautious; pick your own region's table from KSI's map, or type your association's four numbers (where Levels 2, 3, 4 and 5 start) as "your table".
4. Heat index
The heat index is shown as the Weather Service forecasts it, with the Weather Service's bands: Caution 80 to 90 °F, Extreme Caution 90 to 103 °F, Danger 103 to 124 °F, Extreme Danger 125 °F or higher (a value on a boundary takes the higher band). The Weather Service notes that its heat index values are for shady places and that full sunshine can raise the heat index by up to 15 °F. WBGT already accounts for sun, wind and humidity, which is why the tables use it.
5. The best window
For each day, the finder looks at every start on the hour between your earliest start and your latest finish, for a practice of your length rounded up to whole hours. A window counts only if every hour in it has a forecast WBGT and a forecast thunder chance, and no hour's thunder chance is over your limit. Of those, it picks the one with the lowest peak WBGT; a tie goes to the earlier start. It is a planning aid for tomorrow and the days after, not a promise about the weather.
6. The lightning clock
The National Weather Service: "experts agree that people should wait at least 30 minutes after the last thunder before resuming outdoor activities," and if you hear thunder, "it's a safe bet that the storm is within ten miles" (weather.gov/safety/lightning-sports). The Weather Service also recommends that organisers have a lightning safety plan and follow it without exception, with a lightning monitor who is not the coach, umpire or referee. Each press of "Thunder heard / lightning seen" restarts the 30 minutes from that moment for everyone with the clock's link; presses less than 5 seconds apart count once. When 30 minutes pass with no press, the clock says the wait is over; it never says it is safe. Check the sky and the radar before play restarts.
7. The conditions log
For each weekly session on a Team Desk, once an hour the site writes down what the forecast says for that session's hours: peak WBGT and its level on the venue's table, peak heat index and temperature, the highest thunder and rain chances, the active alerts, the time the Weather Service issued the forecast, and when the row was written. It is a record of the forecast, useful for your season file; it is not a measurement.