Physics
How Thermal Expansion Changes the Height of the Eiffel Tower
Quick fact
The Eiffel Tower's height changes by about 15 centimeters (6 inches) between the coldest winter and hottest summer days, because the iron expands when heated and contracts when cooled.
Why this is interesting
The Eiffel Tower stands over 300 meters tall in winter, yet in summer it grows by up to 15 centimeters—taller than a pencil lying on its side. How can a solid iron structure simply stretch with the seasons?
Read the full explanation
Understanding How Thermal Expansion Changes the Height of the Eiffel Tower
Imagine the tower as a stack of countless tiny iron cubes, each connected to its neighbors. When the sun warms the metal or the air temperature rises, the atoms in the iron jiggle more vigorously, pushing each other slightly farther apart. This microscopic increase in spacing accumulates over the entire height of the tower, producing a noticeable change of several centimeters. Because the tower is made of iron, which has a known coefficient of linear expansion (about 12×10⁻⁶ per degree Celsius), the change in height can be predicted by multiplying the original height by the temperature change and that coefficient. In summer, the entire structure warms evenly, so it grows uniformly without bending. In winter, it cools and contracts back to its 'normal' size.
A deeper explanation
The underlying mechanism is thermal expansion: as temperature rises, the average kinetic energy of atoms increases, causing them to vibrate with greater amplitude. In a solid, the atomic bonds are like stiff springs; higher vibrations move the atoms to slightly larger equilibrium separations, increasing the material's dimensions. For linear expansion, the change in length ΔL is given by ΔL = α·L₀·ΔT, where α is the coefficient of linear expansion, L₀ is the original length, and ΔT is the temperature change. For the Eiffel Tower, L₀ ≈ 300 m, α ≈ 12×10⁻⁶/°C, and a temperature swing of 30°C (e.g., from -5°C to 25°C) yields ΔL = 12×10⁻⁶ × 300 × 30 = 0.108 m ≈ 10.8 cm. The observed variation is even larger, around 15 cm, because the tower's height includes latticework that also expands, and the recorded difference may involve greater temperature extremes. Importantly, because the whole tower is uniformly heated, the expansion is free to occur without internal stress. This is why the Eiffel Tower doesn't crack—unlike constrained structures such as railway tracks, which buckle if gaps aren't provided. The same physical principle forces engineers to design expansion joints in bridges, pipelines, and long buildings to accommodate thermal size changes.