Winds are generally most favorable the first hours after sunrise and the last hours before sunset.  The sun's uneven heating of the earth's surface causes strong, variable winds.  In the morning, it takes a few hours for the sun to heat the earth's surface enough to generate the thermal activity that creates wind.  In the evening, the sun's intensity has diminished enough to reduce winds to acceptable flight levels.  Ideal winds are 3 to 6 mph.