There can be significant changes in wind speed/direction even at much lower levels. Many is the time I have taken off in a light (5-7kt) breeze, only to find at 1000ft it's 15-20kt. This could easily increase to 30kt or more at 2-3000ft, which makes a hell of a difference to me as my microlight only cruises at 45kt!! Near the East coast we often get an "Inversion" where a cold easterly wind suddenly gives way to a much warmer westerly flow, some times at as little as 600ft.