Every runner knows the feeling of lacing up on a crisp winter morning or a scorching summer afternoon, but few consider that the very materials underfoot are quietly rebelling against the weather. Cushioning technologies may feel luxurious in a climate-controlled store, yet their performance shifts dramatically when mercury drops or spikes. Air, Boost, React, and modern supercritical foams each respond to temperature in distinct ways, and understanding these reactions reveals why no single cushioning solution is universally perfect.

Nike’s Air units rely on sealed pockets of gas, usually nitrogen or air, trapped within a flexible thermoplastic urethane casing. As temperatures fall, the gas contracts, reducing internal pressure. A colder Air unit feels firmer, less forgiving, because the lower pressure allows the casing to compress less easily with each footstrike. Conversely, heat expands the gas, raising pressure and making the shoe feel springier, almost bouncy. However, this thermal volatility has limits. The casing itself becomes stiffer in extreme cold, so even at reduced pressure the overall ride loses its plushness. On a frozen morning, an Air Max may feel like a plank with a whisper of give; on blacktop baked to 50°C, it becomes a trampoline. Runners in seasonal climates must recalibrate their expectations week to week.

Adidas’ Boost, introduced as a revolution in energy return, is made of expanded thermoplastic polyurethane, or eTPU. These foam pellets are closed-cell and famously resilient, but they are not immune to temperature. The glass transition temperature of TPU sits well below freezing, which means Boost does not suddenly shatter or become plastic. Yet cold does stiffen the polymer chains, raising durometer and reducing the squish factor. The energy return percentage, while still impressive, dips noticeably. At 0°C, Boost feels denser, less marshmallowy, though it retains a trampoline-like snap that many runners appreciate. In extreme heat, the pellets soften, leading to a more dramatic sink-in sensation that can sacrifice stability and increase foot fatigue over long distances. Boost’s advantage is its broad acceptable range, but it still follows the fundamental rule that polymers fear the cold.

React, Nike’s proprietary foam blend of ethylene-vinyl acetate and olefin-based elastomers, offers a more balanced chemical phalanx. It was engineered to resist breakdown and provide consistent responsiveness, and that engineering extends partially to temperature. React’s rubbery elasticity remains fairly stable from 10°C to 35°C, but below that threshold, the foam noticeably loses compliance. The structure becomes glassy, akin to a stiff hiking boot insert. In sub-zero races, runners often report that React shoes feel flat, lacking the lively rebound that reviewers praise in spring. Above 40°C, react begins to soften more dramatically, producing a lazy, energy-absorbing ride rather than a propulsive one. Compared to Air’s gas physics, React’s solid-state matrix reacts slower to temperature swings but still cannot escape the thermodynamic dance.

Supercritical foams, such as Peba-based versions found in premium racers, represent the newest frontier. Nitrogen-infused and processed in a high-pressure chamber, these foams boast tiny, evenly distributed bubbles. Their polymer structure is designed to be highly elastic, and remarkably, that elasticity is maintained across a wider temperature window than traditional EVA or TPU. At −5°C, a Peba midsole still returns around 80% of its energy, while Boost might drop to 65% and React to 60%. In heat, Peba foams resist excessive softening because their cells are internally pressurized, providing a natural buffer. This is why elite marathoners choose these shoes for warm-weather races, knowing the platform will not turn to mush.

The true lesson is that cushioning is not a static specification. It is a living response to environmental stress. Air offers theatrical changes, Boost provides robustness with a cold stiffness, React trades some range for a familiar feel, and supercritical foams quietly dominate through versatility. Runners must match their shoe’s personality to their local climate, or accept that comfort is a moving target. Whether training in Minnesota winters or Arizona summers, the midsole is never truly neutral—it is always whispering the temperature.