Hot air balloons primarily fly shortly after sunrise because that is when the atmosphere is most stable. In the morning, the ground is cool, which prevents the formation of "thermals"—rising columns of warm air created as the sun heats the earth. These thermals create turbulence and unpredictable vertical currents that can make a balloon difficult or dangerous to control. Additionally, morning winds are usually at their lightest; most commercial balloon flights require wind speeds of less than 10 mph for a safe launch and landing. As the sun rises higher, the air becomes more active and gusty, which is why most flights are concluded by 9:00 or 10:00 AM. A secondary flight window sometimes occurs in the early evening as the ground begins to cool down again, but the morning remains the "gold standard" for calm, scenic ballooning.