Because metal expands when heated, the iron structure of the Eiffel Tower can become up to about 15 centimetres taller on hot summer days, a reversible change caused by thermal expansion rather than any permanent alteration to the landmark.
Why it happens
Most construction metals expand when temperature rises due to increased atomic motion; the Eiffel Tower, built of wrought iron, lengthens slightly as parts warm in sunlight and ambient heat, increasing its overall height and causing the top to lean very slightly away from the sun on hot days.
How much it grows
The size of the change depends on the temperature difference and the linear thermal expansion coefficient of the iron; under strong summer heating the tower’s height can increase by several centimetres, with commonly cited maxima around 10–15 centimetres, a small but measurable amount for a structure of its scale.
Implications and design
Engineers account for thermal expansion when designing large structures through expansion joints, tolerances and flexible connections so that seasonal size changes do not harm integrity; Gustave Eiffel’s original designs anticipated movement and allowed the tower to flex safely with temperature and wind.
Everyday note
The tower’s seasonal growth is a neat example of everyday physics at work and a reminder that even apparently fixed landmarks subtly change with the environment while remaining structurally sound and iconic.