Wind speed at a location is considered random. Suppose I measured wind speed for an entire day and it remained absolutely constant value 10 m/s(on that particular day. However, tomorrow, wind speed may be entirely different. So, is it this uncertainty that makes wind speed random or is the inability of the wind speed to express as a closed form expression of time that makes it random. I believe the later case is called 'arbitrary' but not the random. Kindly offer insight into what is 'Random' in Random vibration.
Dear Kumara,
That is a very interesting question. Based on my understanding, wind speed is said to be a random parameter because of its partial unpredictability. I will try to clarify this. Wind speed is subject to both variability and partial unpredictability. Variability refers to the natural variation with time as a result of whether conditions. For instance, during the course of a particular day, wind speeds are higher in late morning and evening hours than those measured in the afternoon (so called diurnal variation). For that matter, wind speeds also vary daily, seasonally, and even annually. All this is what is often called horizontal uncertainty, which is a natural variation along the time axis (which is supposedly horizontal). However, the horizontal uncertainty is not the main reason behind the random nature of wind speed. The main reason is the inability to tell (predict) the readings of your anemometer with a "good degree" of certainty, be it in the near or far future. This means wind speed at a particular time in the future is also subject to a vertical variation (which is often referred to vertical uncertainty). Let's say you want to predict the wind speed for the first hour of Day A in advance. But you can't do so without introducing a prediction error (which is what I called partial unpredictability at the beginning of my comment). You may say that the reading of your anemometer corresponding to hour 1 of Day A will be 10m/s with a probability p1. But there may also be other possible measurement outcomes (say 8.9, 9, 9.5, 10.001, 11, ...) m/s, each with a given probability (p2, p3, ...). This means your random parameter (wind speed) can be represented by a set of probable outcomes (possible values), each associated with a probability (p1, p2, p3, ...).
To sum up, the fact that you get different series of readings (wind speed measurements) for different days is due to natural variation. The weather condition for one day is not necessarily the same as that of another one. As a result, it is natural that your anemometer reads different values for different days. But you happened to have predicted values at hand for the wind speed series corresponding to each day, and these turns out to deviate from the measured (actual) values. This is because of the random nature of wind speed.
I hope this helps.
Kind regards,
Desta
Usually the average wind speed is higher than the mode (most probable wind speed. This is because there is a lower limit to wind speed (zero), but for all practical purposes no upper limit (the speed of light vastly exceeding any wind speed on Earth). So the wind speed probability distribution is skewed towards values above the mode. Due to friction with the ground, air flow near the ground is turbulent with many eddies, which contributes to fluctuations of wind speed (and wind direction) near the ground. At higher altitudes, winds are usually much more steady with respect to speed (and direction). Expected or average values of wind speeds about which fluctuations occur are determined by weather conditions. Some examples: Usually but not always winds are stronger in cyclones than in anticyclones, and in tropical cyclones than in extratropical cyclones. If the surface is warmer than the overlying air mass, convection mixes down faster-moving air from aloft, raising wind speeds near the ground. Such convection is strongest during the warmest part of sunny days.
Dear Kumara_Raja_Eedara,
Wind speed is always random. As explained by Fellow Researchers, it varies between time of the day , it is usually higher during night time (depends on geography) and between the elevations from earth surface and in different seasons of the year and more so the location whether it is on shore or off shore , whether it is close to equator or away. This is the reason why a wind mast is deployed and a Three year random data is taken from the data logger to arrive at a given average wind speed at a given mast height , though extrapolation is also done for a limited wind mast height .
It is always better to avoid extrapolation and stick to an average mast height of 80-100 m from ground as most of the wind turbines have nacelle at that height these days for an average yearly wind speed to about 5 m/sec.
Thanks
Subrata
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