One Last Question on Berkeley Sensor

[WNDZRFR]

Mike,

When you make the forecast for HLS do you make it for what the sensor is going to read that day?

Or -

Do you make it for what the actual wind is going to be that day?

We have all adjusted are mindsets to the new sensor readings but aren't clear when it comes to the forecast numbers.

Thanks again

[WNDZRFR]

After another half dozen or so days of sailing at HLS I can substantiate that the IWINDSURF forecast is for what the sensor will be reading during the day, not for the actual wind.

Every one of those days sailing I've been able to rig a full sail size smaller than the forecast and what the sensor is reading.

Take today for example - forecast avg. 18 knots (20.7mph) was reading avg. 22-24 mph. While on the water it was really blowing a constant 25-27 before it jumped up another couple mph later. Berkeley was the windiest site in the Bay today yet only the regulars showed up. In the past the parking lot would have been swarming with sailors rigging from all over the bay.

Shred Long And Prosper

[dllee]

I think it's almost exactly correct.
Sat, arrived at 3, looking 5.0 for me.
Took a short walk, came back and rigged 4.5. Gusts to 27. Perfect 4.5 and 100 liter board conditions.
Sailed till past 5:30, thinking of trying out a wave board, didn't, was slightly OP'd near the end, but fine
Winds 20-28, I should be on 4.5. I was.

[WNDZRFR]

I sailed earlier than you from 2:30-3:30 when the average was 21-22 (which is a 5.9 for me all day long) but yesterday I was sailing a 5.3 powered which I only bring out in 25-27mph avg. 93L, 175lbs.

I don't think you are as near as heavy as I am which is why you can sail such small sails.

Other sailors were saying they were over powered on small gear for the 21-22 reading and that it had to be 25+!

[kevinkan]

it seems that the sensor now is reasonably accurate and much better than before. the graph from Saturday shows average in the mid 20s w/ gusts to the high 20s and even 30 for the time I was out (3:30 to 5:30) It felt about that on the water when I was out... fully powered on 4.4/86 @ 190lbs

[windfind]

Hi WNDZRFR ,

The short answer is that I don’t forecast for the winds at the sensor at Berkeley nor do I forecast for the winds in the main sailing/kiting area.

The only sites I do forecast for the winds around the sensor is at 3rd. and Sherman Island where the sensor is located near the kiting/windsurfing action and even at those locales I usually modify the forecast by making comments in the bullets and also below a sites table like “wind toward Access” or “weak and unreliable inside”

The reason I don’t forecast for the sensor is that at most Bay Area sites the wind is weaker at the sensor than in main sailing/kiting area. This is because the wind hitting land based sensors, even if they are right at the edge of the water, is weakened and redirected by friction from trees, buildings etc. (For more detail see note at the bottom of this post.)

Let me give an example of how tricky it is to pick the zone for which we are forecasting for at a specific site: On days at Crissy where the marine layer clouds are nearby it is common for experienced sailors to slog out 1-2000 feet or more to the wind line and then have a great day towards the north tower in low/mid 20's wind. All the while less skilled or smarter people sit on the beach due to the faint winds inside. So if on that type of day I forecast low/mid 20’s for Crissy those who wasted an afternoon sitting on the beach would call it a crappy forecast and the Anita Rock sensor would back them up. But if I forecast low teens those who were ripping in low to mid 20’s wind would say I was crazy. The same scenario often plays out a 3rd., Berkeley and many other sites.

So my compromise is to try to forecast the winds in the "just outside zone" that the mythical average kiter or windsurfer is likely to be able to reach on the average day.

Hopefully this will mean that most people will be able to reach the forecast winds, get on a plane and power further outside. My feeling is that most people are not going to complain if they find winds stronger than forecast but they are sure to whine if they can’t get on a plane at all.

It also means that my forecast will be wrong from the perspective of those lacking the skill or gear to get to the forecast winds.

It also means that people having the skill and gear will conclude that my forecasts are usually too low since they find stronger wind outside than I forecast.

To make things worse that "just outside zone” varies with different wind patterns. For example at Crissy in a overall NW wind pattern the "just outside zone” is inside Anita Rock with blowing sand on the beach and stronger wind than forecast mid bay. While in a foggy WSW pattern my "just outside zone” is well past Anita Rock. I try to narrow down zone I am forecasting for by text hints. For example the forecast text may say “winds start inside” or “Forecast is for channel”. And on the day WNDZRFR noted in the posts above that my forecast average was for 18 knots while he found 25-27 knots I mentioned twice in my text that “much stronger wind than forecast is near Berkeley”.

To conclude… the basic function of the forecast is to tell you if there is a chance of getting out on the water given your sail/kite size preference. And each person has to learn how to interpret the forecast/sensor reading in different conditions. Using our sensors is not like using the Pilots sensor below.

Mike Godsey
iwindsurf.com/ikitesurf.com
Weatheflow.com


Geektails: Sensor data is not simple to interpret. The worse case is a sensor like the Berkeley unit which is on top of a building. Such a location creates 2 issues. 1. The strength of wind is proportional to the elevation so a such a sensor will tend to read high. That is not too much of an issue since people will learn to correct for that error. 2. The vertical walls of the building create turbulence and eddies. When the wind is relatively weak the turbulent zone is relatively low over the roof so it may be below our sensor mast. The stronger the wind the greater the height of the turbulent zone over the roof and once this slower moving turbulent zone reaches the sensor the sensor begins to show lower average wind. Worse the impact of the turbulent zone changes with the wind angle eg. wind at right angles to the walls of the building creates a higher turbulence zone while wind hitting a corner of the building has much less turbulence. That is the reason why sensors sometimes seem to have a near perfect reading and other times seem way off. One way of helping users see this effect is to for us to post the 1 minute average of the sensor readings. But when we tried that most people complained saying the graphs were too busy and hard to interpret.

[windsrf]

Excellent summary Mike - thanks!

[WNDZRFR]

Thanks for getting back to me on the forecasting of the sensors/sites Mike.

I think we all wish for the forecasting to be an exact science so we will know in advance if we will be sailing that day and in what conditions but with all the variables you mention this is just not possible.

I appreciate how you explained that the wind force and direction can and will affect the sensors accuracy compared to the actual wind being experienced at that time.

To that end, after iwindsurf readings get me to a site I use a Weatherflow Anemometer (which I purchased a couple weeks ago) at the waters edge for a more accurate reading. My sessions seem to work out best if I rig by the high reading on the anemometer as compared to the average reading on the sites sensor. You would think after so many decades of sailing that I would immediately know what to rig by seeing but because I have become so dependent on the readings it just isn't the case. It's the same as drivers who are reliant on their GPS systems in their vehicles telling them where to go. Technology will be the death of us all I say!

Thanks again for taking the time to explain the process of forecasting wind!

[windward1]

Hi Mike,

Is iWindsurf selling those stones to go along with their new weather stations?

W1

[windfind]

Hi WNDZRFR,

Sorry for the delay responding to your question. I actually composed a long reply the day after your query and hit submit but it was never published and I neglected to verify. When you take your reading with your new WF sensor be sure to select share so your report goes to everyone.

Mike Godsey
iwindsurf.com/ikitesurf.com
Weatheflow.com