Sunlight causes steel skyscrapers to expand and grow several inches taller during the day.

Thermal expansion is a fundamental physical property where matter changes in shape, area, and volume in response to a change in temperature. In steel skyscrapers, the kinetic energy of the atoms increases as they absorb solar radiation, causing them to move more vigorously and take up more space. This physical shift is most notable in massive structures like the Willis Tower in Chicago or the Eiffel Tower in Paris.The Eiffel Tower is a famous example of this phenomenon. Measurements show that the iron structure can grow up to 6 inches (15 centimeters) taller during a hot summer day. Because the sun only hits one side of the tower at a time, the uneven heating causes the metal to expand lopsidedly, making the top of the tower tilt in a circular motion of up to 7 inches (18 centimeters).For modern skyscrapers like the 1,450-foot Willis Tower, engineers must account for both vertical growth and horizontal leaning. On a clear day, the side facing the sun expands more than the shaded side, causing the building to lean up to 3 inches (7.6 centimeters) away from the sun. To prevent structural failure, buildings are designed with expansion joints and 'curtain walls' that allow the exterior to move independently of the rigid interior frame.These calculations are vital for the integrity of elevators and plumbing systems. If a building grows 2 to 4 inches throughout the day, rigid pipes would snap and elevator cables would lose proper tension. Modern engineering standards, such as those from the American Institute of Steel Construction (AISC), provide specific coefficients of expansion that architects must use to ensure skyscrapers remain safe despite these constant daily movements.