If you’ve spent a San Antonio August outside, you understand the question every parent of a school-age child eventually asks: how does the school decide when it’s safe to be outdoors? What are they actually measuring, and who made the call?
For most schools, the answer has been the heat index — the “feels like” temperature that combines air temperature and humidity. It’s a familiar number, widely reported in weather apps and news broadcasts, and it’s a reasonable place to start. It also has a significant limitation. Starting this school year, Keystone School has replaced it with a more complete standard: Wet Bulb Globe Temperature, or WBGT.
What WBGT Measures
The heat index captures two variables: temperature and humidity. Wet Bulb Globe Temperature captures more. It combines readings from three thermometers — one measuring air temperature, one measuring how effectively sweat can cool the body in current conditions, and one measuring radiant heat from the sun and surrounding surfaces. It also accounts for wind speed, cloud cover, and sun angle.
The result is a number that more accurately reflects what a child’s body actually experiences during physical activity outdoors. On a hot, overcast, breezy day, the heat index might overstate the risk. On a sunny, low-humidity afternoon with full sun bearing down on an asphalt playground, it might understate it. WBGT is more precise either way.
That’s why the American College of Sports Medicine uses it as the basis for outdoor activity guidelines, and why athletic organizations at all levels have moved toward it as the standard for keeping athletes safe.
What the Policy Means in Practice
Keystone’s new policy sets clear thresholds based on age.
For students in the Little School through 4th grade, students come inside when the WBGT reaches 90 degrees. Younger children are more vulnerable to heat stress than adolescents. They generate more heat relative to their body mass during physical activity, sweat less efficiently, and are generally less aware of the early signs of overheating. The 90-degree threshold for younger students reflects that.
For Middle and Upper School students and athletics, Keystone follows the guidelines of the Texas Association of Private and Parochial Schools, which has also moved to WBGT. Those guidelines require minimum water break frequency per hour and prohibit outdoor practice when WBGT exceeds 92 degrees.
Parents can check current WBGT readings on the Keystone campus directly from the school’s homepage, where a weather widget reflects real-time conditions.
A Decision Made on Evidence
Head of School Billy Handmaker and Director of Athletics D’André Montgomery communicated this change to families directly, explained the reasoning, and pointed parents to a way to monitor conditions themselves. They didn’t have to go to that length. It would have been easy to update the policy quietly and move on.
The fact that they didn’t reflects something worth knowing about how Keystone operates. When the evidence points toward a better approach, the school follows it — and then it explains its thinking to the community. That combination of careful decision-making and transparency shows up across the whole program, from how the school structures academic expectations to how it approaches student wellbeing.
For Texas families evaluating schools, outdoor heat management is a reasonable question to ask. The answer should be specific: what measurement do you use, what are your thresholds at each grade level, and how do you communicate with families when plans change? A school with a clear, evidence-based answer has thought this through. Keystone has.
To learn more about Keystone or to schedule a visit, we’d love to hear from you.
