When Does It Get Too Hot for People?

Climate change is pushing San Antonio into heat conditions that have never been experienced before. Recent heat waves are not just hotter: they are longer, more humid, and more dangerous, especially at night when bodies need relief. Planning for this new climate means asking a fundamental question: when does heat become unsafe for people?

Too often, we rely on simple air temperature, which tells only part of the story. Human heat stress depends on multiple factors: temperature, humidity, sunlight, wind, and even the heat radiating from pavement and buildings. Selecting the right measure of thermal stress is critical to make decisions about San Antonio’s future.

The Wet Bulb Globe Temperature (WBGT), developed by the U.S. military in the 1950s to reduce heat-related illness and death during training, and similar indices were designed specifically to assess when outdoor conditions become hazardous. Some of the organizations using the WBGT include Joint Base San Antonio, Northside Independent School District, Texas High School Coaches Association, Occupational Safety and Health Administration, and National Collegiate Athletic Association.

The WBGT is a single number measured in degrees that combines air temperature, humidity, wind speed, and radiated heat from the sun and other sources. Human health threat levels have been developed, and levels for San Antonio are shown here. I took some readings around San Antonio during recent summers, and you can see that I found places with elevated, moderate, and even high threat levels.

Graphic by Bill Barker

As San Antonio adapts to a hotter future, using accurate heat-stress metrics will help guide better decisions about parks, transit stops, work schedules, and public safety. In a warming world, measuring heat correctly can save lives.

Graphic by Bill Barker

Reversing Falls, Pembroke Maine

Another interesting spot we might imagine visiting in Downeast Maine. For intro info about the whole area there was an article in the November 2025 issue of this newsletter. This one is set in Pembroke, which is 20 miles northwest of the eastern-most spot in the US (February 2020). An excellent area map was mentioned in the November article. Left of center at the top of the map we can see Leighton Neck in Pembroke with Cobscook Bay to the east.

At the south end of that neck we can see the little Reversing Falls Park with a parking spot labeled. Just southeast of the parking spot there is a circle of arrows around a tiny island. The map legend says about these circles: “DANGER Extreme Currents & Whirlpools”.

Reversing Falls at high tide just left of upper center, the waves resulting from the submerged island. Picture taken from the shore just below the parking spot, looking SE.

So that spot is what this park is named for; the picture above is at high tide. The island is entirely submerged, with the waves across it's top when the tide has begun moving in or out. Below are pictures at low and middle tides.

So as the map notes, extreme currents and whirlpools. We might imagine swimming out there and trying to stand on the submerged island at high tide so a buddy can take a picture. But surely don't want to approach up-current at high tide in a boat.

From the same spot as the prior picture, at low tide. The fully visible island is of upper center.

The picture below is from the northernmost spot in Race Point Preserve (again, on the map linked above a Cobscook Shores preserve, about those in the November 2025 issue), just SE of the prior pics. Taken at a spot the same distance from the island as the previous two, in exactly the opposite direction. Looking at the map we can see Race Point Preserve with Reversing Falls trail along the north shoreline.

The map shows Falls Island, which is upper left in the previous pictures and upper right in the next one. The map says it is owned by The Nature Conservancy. Looking further SE we can see Denbow Neck with Hog and Long islands on the E side, also TNC. I couldn't find a reference to Falls Island, likely few visitors, though we might imagine getting there in a boat and wandering through the thick forest.

From the TNC Where-We-Work Maine webpage, “TNC has helped protect more than 2.4 million acres in Maine, which benefit wildlife, protect unique habitats, encourage nature-based recreation and support sustainable industries.” TNC's impact is very important. I send them a donation every year.

Just SE of Reversing Falls at maybe mid-tide. The island is just left of upper center, the cleared areas barely visible behind are Reversing Falls Park. Falls Island is upper right. From Cobscook Shores Race Point Preserve looking NW. Got my dog to pose for me. The rock is slate.

Profound Books About Nature and Adventures by Bill Streever

We met Bill and his wife at the library in Pembroke, Maine. He is a biologist with published academic works and the list of books below. His wife is a marine biologist, they live on their sailboat and are building a house in Pembroke.

The books are personal accounts of experiences, plus extensive though basic explanations of the ecosystems and the biology, geology, geography involved and the chemistry and physics behind that. And human history. All very lucid and readable, even if we don't have much knowledge about the bits.

From the Cold preface, last in the list: “The world warms, awash in greenhouse gases, but forty below remains forty below. … Cold is part of everyday life, but we often isolate ourselves from it, hiding in overheated houses and retreating to overheated climates, all without understanding what we so eagerly avoid.... It is time to enjoy an occasional shiver as we worry about a newly emerging climate likely to melt our icecaps, devour our glaciers... It is time to embrace and understand the natural and human history of cold.”

Also about Cold: chapters by month, July to June. How the Inuit prepared for winters, the architecture of igloos and transitions of those with temperature, and essential clothing details. Cold tolerance of humans, which varies by acclimation and race. How organisms survive winters buried in the snow/ice, where it is much warmer than above. How mammals hibernate for many months there and other organisms freeze and thaw successfully. Having lived in Anchorage, lots of stories about trips around Alaska.

Finishing Cold, the final Notes chapter reviews the others, mentioning key bits and other authors' books. There are a bunch of topics we may wish to read up on. Wikipedia pages for some: Beringia, Igloos, Arctic Climate (global warming near bottom), Iceman and Frostbite (pictures), Ice Ages, Permafrost, and Greenhouse Gases. Geological Society of America for a geologic time scale chart. World Atlas about the Inuit people.

Bill's books

The SA library has four of these, including the first one, which was published in 2024.

• A Sea Full of Turtles – the Search for Optimism in an Epoch of Extinction
• In Oceans Deep – Courage, Innovation and Adventure Beneath the Waves
• And Soon I Heard a Roaring Wind – a Natural History of Moving Air
• Heat – Adventures in the World's Fiery Places
• Cold – Adventures in the World's Frozen Places

Outings: The Call of the Wild

Visit the Alamo Sierra Club Outings page on Meetup for detailed information about all of our upcoming Sierra Club Outings.

The Alamo Sierran Newsletter

Richard Alles, Editor
Published by the Alamo Group of the Sierra Club, P.O. Box 6443, San Antonio, TX 78209, Website.
The Alamo Group is one of 13 regional groups within the Lone Star Chapter of the Sierra Club.

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