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Why can the Eiffel Tower grow taller in summer?
Heat makes the metal expand, adding up to about 15 cm.
Metals expand when heated because their atoms vibrate more and settle slightly farther apart. The tower is built from puddled iron, and for iron the linear expansion is on the order of 12 millionths of its length per degree Celsius. Applied to a structure of roughly 300 meters of iron, a temperature swing of tens of degrees produces a change of several centimeters in height, which is why a figure of up to about 15 cm is often quoted for the difference between a cold winter day and a hot summer day. The exact number depends on how hot the metal itself gets, and sunlit iron can be considerably warmer than the air.
Sunlight adds a second effect. The tower's official site explains that the sun heats only one of the four faces at a time, so that side lengthens more than the others and the top leans slightly away from the sun. On clear days the summit is described as tracing a small circle about 15 cm across as the sun moves around the structure. This is a different motion from the seasonal change in height, and the two figures are easy to confuse.
The official site stresses that these shifts do not affect strength and cannot be noticed by visitors. The lattice design has gaps that allow movement, and the wrought iron tolerates repeated expansion and contraction. The popular claim that the tower grows by a precise 15 cm each summer is best treated as an approximate upper figure rather than a fixed measurement, since the actual change varies with weather.