Some other runways appear to go up and down at different points. Answer: No, runways are not flat. They are crowned to help drain water off the sides during rain, and often one end of a runway is higher or lower than the other. When preparing takeoff performance calculations, pilots include the slope of the runway.

Airport runways are purposely designed with a slight "crown" or slope rather than being perfectly flat primarily for water drainage. If a runway were perfectly level, rainwater would pool on the surface, creating a massive risk for hydroplaning (where a plane's tires lose contact with the pavement). To prevent this, runways are typically built with a 1% to 1.5% transverse slope from the center toward the edges, allowing water to shed off the sides quickly. Additionally, many runways have a longitudinal slope (a "hump" or a "dip") because it is prohibitively expensive to move the millions of tons of earth required to flatten natural terrain for a two-mile stretch of pavement. The FAA and ICAO have strict limits on these gradients—usually no more than a 1.5% to 2% grade—to ensure that pilots can still safely see the far end of the runway. Furthermore, the "unflat" look is often exaggerated by telephoto lenses used in aviation photography, making gentle slopes look like steep hills.