The Eiffel Tower, originally known as the Tour de 300 mètres, was designed to be a 300-metre marvel. However, during summer heatwaves, this iconic structure actually grows taller due to thermal expansion.
A Lightweight Iron Structure
Built for the 1889 World's Fair to celebrate the centenary of the French Revolution, the Eiffel Tower was constructed using puddled iron. This material, chosen by Gustave Eiffel for its strength and lightness, allows the tower to withstand high stress while remaining relatively light. In fact, the tower weighs 7,300 tonnes, nearly equal to the weight of the air inside it, which is about 6,300 tonnes.
The tower is a gigantic triangular lattice, similar to the Garabit Viaduct and the Forth Bridge. All these structures expand when temperatures rise, but the Eiffel Tower primarily experiences vertical growth and shrinkage—a phenomenon known as thermal expansion.
Materials That Grow and Shrink
Most solids expand when heated and contract when cooled because increased temperature causes atoms to vibrate more, increasing the average distance between them. Different materials expand at different rates: ceramics expand less than metals, and metals expand less than polymers. The puddled iron and steel components of the Eiffel Tower have a coefficient of expansion around 12x10⁻⁶ per degree Celsius, meaning a one-metre bar expands by just 12 microns per degree—less than the thickness of a human hair.
Noticeable Effects on Large Structures
While the expansion per metre is tiny, the Eiffel Tower's height of 300 metres and the wide temperature range in Paris—from below -20°C in winter to over 40°C in summer, with direct sunlight heating metal to 60-70°C—make the effect significant. A simple calculation shows that a 300-metre iron bar would expand by 36 cm over a 100°C temperature change.
However, the actual tower, made of over 18,000 riveted iron pieces, behaves differently. The sun heats one side more than the other, causing a slight lean away from the sun. Experts estimate the Eiffel Tower grows between 12 and 15 centimetres from the coldest winter days to the hottest summer days. Thus, beyond being a landmark and communications tower, the Eiffel Tower also functions as a giant thermometer.
