Your Experience May Vary: How Microclimates Shape Heat Stress

Individual experience may vary

Check the weather app on your phone and you’ll get a single temperature value. But how often do we think about where these values come from? After all, most smartphones don’t have a built-in temperature sensor. Unless you are standing directly next to a weather station, the temperature given for your location comes from a statistical interpolation- essentially, an estimate based on your phone’s position and the actual, observed readings from weather stations that can be miles away.

Usually, this statistical interpolation is pretty accurate, good enough for us to go about our daily lives with an understanding of what local weather to expect. However, in some locations, the heat stress you experience can vary significantly from what your phone tells you. In fact, surface temperatures within the same city can vary by an astounding 27^oF depending on where you are!  

In this article, we dive into the science of microclimates and why microclimates matter. Surroundings like pavement, buildings, trees, and bodies of water all contribute to the thermal stress we experience at our immediate location, with implications for our individual heat stress experience.

Same Place, Different Temperature

If your weather app tells you it’s 86^oF, you might be willing to play a weekend match of tennis or pickleball, a pickup game of soccer or baseball, or get in a quick late-morning run on the track. You’d probably encourage your kids to get outside and play at your local park. Sure, 86^oF is a warm, but you’d bring water, and 86^oF isn’t exactly extreme heat.

But what if the temperature was 98^oF? That’s hot- hot enough that you’d probably rethink your outdoor plans. After all, temperatures approaching 100^oF aren’t just uncomfortable; combined with sustained physical effort, they can be dangerous.

Those temperatures- 86^oF from the weather station and values approaching 100^oF- can exist in the same city at the exact same time. A study by Arizona State University measuring temperatures at various athletic fields, like tennis courts, stadiums, and tracks, found these locations to be, on average, nearly 10^oF warmer than the temperature reported from four nearby city weather stations.  

Such temperature differences between regional weather stations and athletic fields are also greatest earlier in the day, corresponding to the time when we might reasonably presume it’s better to get outside before the day warms up too much. However, athletic fields, especially those with artificial turf or manmade surfaces, warm up much faster than many surrounding areas. The same can be true to concrete worksites and blacktop delivery routes.

Temperature differences even exist within the same building. A research effort at the Davis Wade Stadum on Mississippi State University’s campus measured temperatures at 50 locations on the field, the concourse, and in seating areas and compared them to a nearby weather station. The average temperature within the stadium was 7-9^oF warmer than the corresponding value reported from the weather station. Within the stadium, seating areas in the sun were (unsurprisingly) warmer, but the difference was significant- seating areas on the sunny side were found to be up to 10^oF hotter than on the shaded side.

These findings show that a single large manmade structure like a stadium can be much hotter than surrounding areas, and that even within a single structure, microclimates create difference conditions.

Microclimate differences aren’t limited to city blocks and stadiums. The U.S. Army is aware that heat stress can differ between individual training sites on the same installation. An open motorpool or exposed rifle range can be significantly hotter than a woodland training site. The Army’s heat stress manual encourages units to measure the wet bulb globe index (the military’s preferred thermal index) “in close proximity to training” to capture the heat stress actually experienced by soldiers, with the value provided by installation public health serving only as a “general guide”. In fact, during warm periods, units often employ on-site measurement devices as a risk mitigation measure.

Difference in heat stress between two locations at U.S. Army installation Ft Benning, GA. Any “X” not falling directly on the diagonal line represent different temperature between sites. “Xs” outside the shaded boxes represent heat stress category differences, implying different work-rest cycles and risk mitigation protocol are likely applied at each site. Source: TB MED 507.

Understanding Why: Urban Geography & Temperature

More generally, urban city centers are almost always warmer than the surrounding countryside. This phenomenon is known as the urban heat island effect. Covered in concrete, steel, and asphalt, cities and other built environments absorb more of the sun’s radiation during the day and hold on to excess heat energy through the night. As city surfaces warm, they transfer heat to the surrounding air through radiation and convection, raising local air temperatures.

Image: The variation of land surface and air temperature along a generic rural-city-rural transect. Source: U.S. EPA.

Factor #1: The concrete jungle

The urban heat island effect has been widely studied across the globe, with similar repeat findings: dense urban centers are consistently warmer than their surroundings, with variability even on a neighborhood-by-neighborhood scale. Such neighborhood-scale variations are rarely captured by interpolated weather station data, which represent broader regional conditions.

The most obvious example of this phenomenon is asphalt and dark colored pavement ubiquitous in cities. As anyone who’s parked in the back of a crowded Walmart parking lot in the summer knowns instinctively, such surfaces get hot in the summer sun, making the walk to the building uncomfortable!

This is because dark urban surfaces like asphalt have a low albedo, meaning they reflect as little as just 3% of the sun’s solar radiation. Instead, these dark surfaces absorb nearly all the solar radiation, and about of energy that can be substantial. At solar noon on a clear day, the sun provides ~1,000 watts of power for every square meter- a tremendous amount that, when absorbed, heats the surface up.[1] As solar radiation is absorbed, much of the energy is re-radiated, increasing the radiative heat load on anyone standing nearby, while around 50% of the absorbed solar energy is released through convection, warming air above the surface.

This heat load can be dangerous. In the summer, emergency rooms in places like Pheonix and Las Vegas regularly treat burn victims who fall onto hot asphalt surfaces. Exposed to the summer sun, concrete or asphalt temperatures can be 60^oF hotter than the surrounding air temperature, exceeding 150^oF– hot enough for a sidewalk egg frying competition in one Arizona town.

Los Angeles street temperatures composite by NASA. Measured by satellite, street temperatures in this single city range from about 95^oF to over 120^oF – all within the same city. 

Factor #2: Grey Buildings & Green Trees

A second contributor to microclimate variation within a city has to do with buildings and, importantly, what they’ve replace: vegetation. We instinctively understand that a shady forest floor or urban park is likely to be cooler than an open parking lot, even if both locations receive the same amount of sun.  

Tree cover, and vegetation in general, reduces air temperature because of evapotranspiration, the movement of water from soil and plants into the atmosphere. Plants lose water- a process called transpiration- when they breath in CO₂. This water draws in heat as it changes phase from liquid to vapor, the same process that, for sweating humans, cools us as sweat evaporates off our skin. A similar process occurs on damp soil; unlike bare asphalt or concrete, much of the solar energy striking soil goes into evaporating soil moisture instead of increasing the surrounding temperature.

Why care about this wonky aspect of plant physiology? Because areas with lots of trees, like a shady neighborhood or park, can be significantly cooler than other parts of a city. Solar energy that evaporates water is called “latent heat”, while solar energy that increases temperature is called “sensible heat”. In one widely cited study, researchers found only ~4% of incoming solar energy went to latent heat in urban areas, with the vast majority converted to sensible heat. In comparison, more than half of all solar radiation might be converted to latent heat in vegetated areas. This can make green spaces in cities more than 7^oF cooler than surrounding, built up areas.

Factor #3: Man-made Heat Sources

A final variable making some urban areas hotter than others is, well, us. Cities are hubs of industry and commerce, run on electricity. Nothing the uses power to do work (including the human body) is 100% efficient. Everything consuming power- from industrial machinery to office server stacks - generates waste heat. Concentrated in buildings and cities, this waste heat can measurably add to the area’s heat burden.

Buildings themselves are the greatest contributor of direct human-caused warming, adding about 65% of the total excess heat from human sources. Somewhat ironically, air conditioning is often the largest single contributor to building-related waste heat release. Running any AC system has a net warming effect; in addition to heat generated from inefficient mechanical processes, cooling the inside of a building is done by moving heat outside. During heatwaves, everyone runs the AC,  compounding the already hot outdoor temperature with more hot AC exhaust. In one study of Los Angeles, more than 86% of the total waste heat from buildings during heatwaves could be attributed to AC exhaust.

Vehicle traffic in cities also contribute to local temperature rise, found to account for 4-13% of local warming in one study (other studies put this amount higher, up to 30%). Converting gas into motion involves combustion- literally burning gas- generating hot tailpipe emissions. On city streets with particularly unfavorable geometry (such as narrow roads flanked by wind-blocking tall buildings), one study found local street temperature increases beyond 10^oF attributed to “vehicular heat flow”.

A final contributing factor to urban warming is humans themselves. We all constantly burn food to fuel our daily lives and, just like a vehicle or piece of machinery, give off significant waste heat in the process. Put enough people together and the combined effect of everyone’s metabolic waste heat can be significant. One study found people’s metabolism in New York City contribute between 4-8% of the city’s total manmade heat burden. In the densely populated megacity of Mumbai, the same study concluded that, at times, the waste heat generated by people can rival or exceed contributions from buildings and vehicles!

Conclusion

The world is rapidly urbanizing. Already, more than half the world’s population lives in cities. By 2030, the world is expected to have more than 43 megacities, dense urban clusters with more than 10 million people in each.

Cities are where most of us work and live, but heat stress on city streets can be significant. Cities are almost always warmer than the surrounding countryside- sometimes significantly more so. Even within the same city, temperatures can be very different depending on surroundings; treelined streets or locations near city parks, often in wealthier areas, can remain relatively cool while industrial areas and expanses of asphalt and concrete quickly heat up. Temperature differences exist even at small scales within the same stadium, factory compound, and city block.

Knowing your surroundings and the effect they have on the immediate area's heat stress is an important part of being prepared to thrive on hot city streets. Another way is with the Qore Performance IceAge ecosystem, designed to give you an edge in even the hottest conditions.

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About the author: Dr. Erik Patton holds a PhD from Duke University where he conducted research on the challenges rising temperatures pose for military training. An Army veteran, Erik has served in a variety of extreme climates ranging from deserts in the U.S. Southwest and Middle East (120^oF) to Arctic conditions in central Alaska (-42^oF).