A simple consideration - based on an analytically solvable model of the momentum balance - is presented, which calculates the reduction in wind speed at hub height in an indefinitely large wind park as function of surface roughness, atmospheric thermal stability, and the mean distance between the wind turbines in this wind park. Weakest reduction occurs for a wind park erected on a rough land surface during unstable thermal stratification (minus 2%). Highest reduction occurs for an offshore wind park over a very smooth sea surface during stable thermal conditions (minus 45%). This model can be used to find the optimum distance between wind turbines in wind parks. Likewise this model calculates the wake length of entire wind parks as function of surface roughness and thermal stability. For offshore wind parks wake lengths between 10 and 30 km are found, for onshore parks this length is much less. This additional information can be used to find the optimum distance between adjacent wind parks.