Friction between the atmosphere and the Earth's surface causes a 20% reduction in the wind at the surface of the Earth.[10] Surface roughness also leads to significant variation of wind speeds. Over land, winds maximize at hill or mountain crests, while sheltering leads to lower wind speeds in valleys and lee slopes.[11] Compared to over water, maximum sustained winds over land average 8% lower.[12] More specifically, over a city or rough terrain, the wind gradient effect could cause a reduction of 40% to 50% of the geostrophic wind speed aloft; while over open water or ice, the reduction is between 10% and 30%.[8][13][14]
Bill is going on vacation this week, and is visiting two local sites. One, is Deep Valley, and the other is Mount Tall. He will be going to Deep Valley on Monday and Tuesday, and then visit Mount Tall on Thursday and Friday. He has decided that on his trip he wants to fly his kite at some point, but also wants to have a picnic on the other days. He would prefer an area of high wind for flying his kite, and an area of low winds to have his picnic so his food doesn't blow asunder.
Will there be more or less wind where Bill goes on Thursday than Tuesday?