• • • • • • • • •

It can be seen that, with these relationships, a vertical velocity profile can be constructed so long as the basic statistical description of the characteristics is known. In particular, it is usual to want to know what is the wind speed that will occur on average only once in a 50-year period. This wind speed is popularly known as the 'fifty-year wind'. This terminology is unfortunate, since it tends to lend a deterministic air to a random quantity. In

Table 5.2 Typical values for roughness length Zo and zero-plane displacement d

Terrain description

0.7 0.3 0.1

0.03 0.01 0.003 0.001

d 15 to 25 5 to 10 o to 2

fact, this is a 'probability 0.02' wind. In other words, there is a 0.02 annual probability of occurrence. During any 50-year period, there is a roughly 63 % chance of occurrence.