How Reliable Are Wind Meter Readings?
By Eric Mihelbergel (Written November 2019)
How reliable are wind meter readings? This is actually a very complicated question. Wind meter readings can vary based on elevation, surface texture, temperature, time of day, accuracy of calibration, local land features, and other factors. We often look at wind meter readings to decide if it's worth driving to the beach, which beach to select, and what size sail/kite to rig. Let's examine some of the biggest factors that affect the reliability of wind meter readings.
Elevation - Generally, wind blows harder as elevation increases. In fact, elevation is such a sensitive factor that windsurfing sail manufacturers design sails with twist in the sail because the wind blows that much faster at the top of the sail than at the bottom due to friction between the wind and the water's surface (click here for an article "Know Your Sail"). A different angle of attack is required at the top of the sail so that the top of the sail does not stall. With this sensitivity to elevation in mind consider the location of most commercial or government wind meters. Typically we find wind meters at heights of 20-40 feet above the surface of the water. It makes sense that the wind we experience at the water's surface will be less than the wind at the height of the wind meter. But how much less? We will see in a moment.
Surface Texture - It's not as simple as just taking into account the height of a wind meter. We must also consider the texture of the surface that the wind is blowing across. Wind blowing across a field of corn will experience more friction than wind blowing across the surface of a large parking lot. Similarly, wind blowing across a choppy wave-filled lake (high wind speeds) will experience more friction than wind blowing across a smooth lake (low wind speeds). Scientists use a formula that relates the speed of wind at two different elevations and includes differences in surface texture. Here is the formula:
V2 = V1(H2/H1)^n H1 = height of wind meter
H2 = height of the windsurfer
V1 = wind speed at the height of the wind meter
V2 = wind speed at the height of the windsurfer
n = wind shear exponent that corresponds to the texture of the surface
Values of n are: water n = 0.11
low grass n = 0.15
high grass n = 0.18
tall crops n = 0.20
V2 = V1(H2/H1)^n H1 = height of wind meter
H2 = height of the windsurfer
V1 = wind speed at the height of the wind meter
V2 = wind speed at the height of the windsurfer
n = wind shear exponent that corresponds to the texture of the surface
Values of n are: water n = 0.11
low grass n = 0.15
high grass n = 0.18
tall crops n = 0.20
So, if we take an example, let's say you're windsurfing at Olcott on Lake Ontario and the wind meter on the end of that pier is showing 30mph (V1), and that meter is located at a height of 40feet (H1) above the water's surface, and your head is located at 6feet (H2) above the water, and the exponent factor for water is 0.11.....then, the wind speed where your head is located (near the middle of the sail) is approximately 24.3mph. This could vary, however, if there are a lot of waves because your height compared to the height of the wind meter will be changing as you reach the trough and crest of each wave. Also, the n factor will likely be greater if there is more chop on the water. So, this is not a perfect calculation, but it does have some value for the windsurfer or kitesurfer.
As we might expect based on this discussion, there will be different implications for windsurfers and kitesurfers. A kite typically flies at line lengths of around 20-28 meters. This may be approximately the height of some commercial and government wind meters. So a kitesurfer may be able to better rely on wind meter readings without conversion.
Temperature - Air temperature and water temperature could affect this situation as well, particularly in the case of 'bubbling' that we are familiar with on the Great Lakes. The 'bubble' is caused by a temperature inversion. If the water temperature is say 60 degrees then the air temperature directly at the water's surface will also be close to 60 degrees. And, on a hot day, if the air temperature at 200 feet is 75 degrees, then the wind will prefer to travel at the higher elevation where the air is less dense. The height of commercial and government wind meters are often around 20-40feet above the surface of the water. The temperature difference 20-40feet above the water's surface may not be drastically different than the temperature right at the water's surface, but it could be enough to cause a difference in wind speed. This could be especially true at a wind meter location like the Dunkirk Lighthouse on Lake Erie where the meter is located on top of a tall poll that is on top of a high cliff. Air temperature could be much different at that increased height. I do not know of any scientific formula that relates differences in wind speed to differences in temperature during a 'bubble' event, but it is worth mentioning the phenomenon here so that we can consider it in our windsurfing/kitesurfing approach.
A reverse type of phenomenon can occur when land cools very quickly overnight directly next to a lake. I have witnessed this in the Finger Lakes Region. These lakes are very long and narrow and lie on a north/south orientation. We usually ride here when the wind is coming from the south. If the land cools very quickly overnight, the cooler air over the land can force the wind to blow over the warmer lake. The wind meter on land near the side of the lake might be showing very little wind, but it may be blowing quite hard in the middle of the lake. An interesting phenomenon that would be difficult to quantify with any formula. However, it is worth understanding how this works in case you look at the wind meter readings from your kitchen table at home and try to decide if it is worth the drive to the lake. You may want to call someone that is already at the lake who can report on whether or not the wind is good in the middle of the lake.
Distance From Shore - We all have sailed at places where it doesn't feel like it's blowing much when we are standing on shore (with onshore wind conditions), but often the farther you get from shore the more wind you find. This is especially true at locations where there may be a hill or tall trees along the shoreline. Most wind meters are located on land. I have not seen any scientific formulas to quantify a difference in wind speed on shore compared to away from shore. However, if such a formula existed, the formula would need to account for land features on the shore as these land features would almost certainly affect wind speed. For purposes of this discussion let's just keep it simple and recognize that we might find more wind if we get a little farther away from shore. Keep a log detailing your experiences at specific locations so that you may easily reference the log when returning to those locations. This will allow you to rig more accurately each time.
Differences in Meter Readings - Keep in mind that not all meters are created equal. Well, maybe the 'meters' are created equal, but the data interpretation is not equal. Different data processing systems will average the wind speeds in different ways when reporting the data to us. One meter may use a 10 minute average, while another may use an 8 minute, 5 minute, 2 minute, or 1 minute average. Longer averages give a more comprehensive interpretation of the general wind behavior, but shorter averages that are reported more frequently give a better representation of the up's and down's of the wind. There is not a lot of consistency between meters, and it makes a difference in the published numbers that we see. Because of this, I almost always rig my sails based on the maximum wind gusts that are reported, not averages. This video link explains more if you are interested. Once you get to know the meters in your area you will be able to interpret the data so that it is more useful to you.
In conclusion, let us recognize that the mechanics of the actual wind meter must be working properly, and the meter must be calibrated properly in order for the above discussion to have value. Again I will mention that keeping a detailed log can be a great help. In my experience, a sailor really needs to get to know each wind meter very personally in order to get the full benefits from that meter. This can really only happen if you sail at or near a particular meter frequently and keep a detailed accurate log. Then the meter can have real benefit to your individual sessions and your sailing career. Have fun!
