The Eiffel Tower Is Taller in Summer

The Eiffel Tower is not one height. It is a range. On a hot summer afternoon it stands several centimetres taller than on a freezing winter morning, and nobody has to do anything to make that happen. Nothing is added, nothing is adjusted, and the visitors riding the lifts have no idea that the building beneath them has quietly stretched.

The size of the change is surprisingly large. In an article for The Conversation, Federico de Isidro Gordejuela of Universidad CEU San Pablo writes that the tower grows by between 12 and 15 centimetres when its size on cold winter days is compared with the hottest days of summer. His explanation works with a temperature range from winter lows below minus 20 degrees Celsius to summer highs of around 40 degrees.

The tower is made of iron, and a great deal of it. According to the tower’s official website, its metal frame weighs 7,300 tonnes and is made of 18,038 iron parts joined with 2.5 million rivets. It was built as the 300-metre tower, and with its antennas it now stands about 330 metres tall. The Conversation article notes that Gustave Eiffel chose puddled iron for its ability to withstand high levels of stress.

Why does warm iron get bigger? When the temperature rises, the article explains, there is greater agitation in the atoms, which leads to an increase in the average distance between them. For puddled iron the thermal expansion coefficient is roughly 12 millionths per degree Celsius. That is tiny for any single beam. But multiply a very small fraction by a structure 300 metres tall and a swing of dozens of degrees, and the result is about the width of a hand.

It also leans, which is the better part. The sun always shines on one side of the tower, so that face warms and expands more than the others. De Isidro Gordejuela describes the result as a slight curve, as if the tower were leaning away from the sun. The bend is slight, far too small to see from the street, but it is the same physics as the summer growth, just unevenly shared out. As the sun moves across the sky during the day, the side it heats changes, and the lean follows.

None of this is a flaw. Large metal structures are designed to move, and this one has been doing it every day since 1889. A tower that could not stretch, lean or sway would be far more worrying than one that quietly grows every summer and settles back every winter. The Eiffel Tower is simply doing, on a grand scale and in full view of Paris, what every warm piece of metal does.