The Eiffel Tower Grows Taller Every Summer

The Eiffel Tower is one of the most recognisable structures in the world. It stands 330 metres tall — at least most of the time. During summer, it quietly grows a little taller. During winter, it shrinks back down. The tower is not collapsing or being renovated; it is simply behaving exactly as the laws of physics say it should.

When materials are heated, their molecules move faster and take up more space. This causes the material to expand slightly in all directions. When cooled, the process reverses — the molecules slow down and pack more tightly together, causing the material to contract. This principle, known as thermal expansion, applies to virtually all solid materials, though the degree of expansion varies depending on the material.

Iron and steel are particularly responsive to temperature changes. For every degree Celsius of temperature increase, a one-metre length of iron will grow by approximately 0.000012 metres — a tiny amount for a short piece of metal, but significant when multiplied across a structure that is hundreds of metres tall.

The Eiffel Tower is made primarily of wrought iron. On a hot summer day in Paris, when surface temperatures can reach 40 degrees Celsius or more, the iron expands noticeably. Compared to a cold winter day, the tower can be up to 15 centimetres taller in summer. The most commonly cited figure — based on the difference between average summer and winter temperatures — is approximately 6 centimetres.

The expansion also causes the tower to lean slightly away from the sun. As the sun heats one side more than the other, that side expands a little more, pushing the top of the structure away from the heat source by as much as 18 centimetres.

The engineers who built the Eiffel Tower in 1889 understood thermal expansion well. Gustave Eiffel's team designed the structure so that the joints and connections between the iron sections could flex and move, rather than being rigidly locked in place. A structure that cannot accommodate expansion would develop dangerous internal stresses over time and eventually crack or fail.

This same principle shapes the design of bridges, railway tracks, and long concrete roads around the world. You may have noticed gaps at regular intervals in railway rails or on bridge decks. Those are expansion joints — deliberately built spaces that give the material room to expand without buckling under pressure.

The Eiffel Tower was never meant to be permanent. Gustave Eiffel built it for the 1889 World's Fair in Paris, and it was originally scheduled to be dismantled after 20 years. That it still stands today is partly due to its usefulness as a radio transmission tower — and partly because the people of Paris grew rather fond of it.

More than 130 years later, the tower continues to expand and contract with the seasons, just as its builders intended. It is, in a quiet way, still alive.