Why Mobile Network Speed Slows Down in Extreme Heat: Atmospheric Waveguide and TDD‑LTE Explained

During an unusually hot summer in Hangzhou, a persistent subtropical high pressure system (the "副热带高压") settled over the region, bringing intense heat and dry air that dramatically altered the atmospheric structure.

This high pressure caused two abnormal weather phenomena: temperature inversion (where temperature increases with altitude) and inverse humidity (dry air aloft while near‑surface air remains moist). Both effects create an atmospheric waveguide that can trap electromagnetic waves.

When mobile signals travel through this waveguide, they experience super‑refraction : the signal bends more than normal, staying closer to the ground for longer distances. This results in the signal being reflected multiple times between the ground and the ionosphere, allowing it to propagate farther with lower loss.

Mobile networks using TDD‑LTE (Time Division Duplex) allocate a single frequency band for both uplink and downlink, alternating between "up" and "down" slots (UpPTS and DwPTS) and inserting a Guard Period (GP) to avoid interference. Under normal conditions the GP duration is set just longer than the round‑trip time to the farthest user (e.g., a 3 ms guard for a 10 km cell).

Because the atmospheric waveguide extends the effective propagation distance, the signal travel time can exceed the GP, causing the uplink and downlink slots to overlap and the network to become chaotic. This manifests as severe slowdown of mobile data, especially for 5G services that rely on TDD.

Older generations (2G/3G/4G) use FDD (Frequency Division Duplex) and are not affected by this phenomenon, which is why the slowdown is observed mainly on 5G networks.

In summary, extreme heat creates a subtropical high that induces atmospheric waveguide conditions, leading to super‑refraction of TDD‑LTE signals, longer propagation paths, and ultimately degraded mobile network performance during heatwaves.