Topic: Anemometer Height - what is best for me?

[KA8MAV]

Okay, I have everything set up and working great.  However, I'm wondering if I need my anemometer higher than I have it.  My backyard is completely unobstructed.  I put the station online so people at our lake can see current weather conditions and especially the wind speed at/near the surface of the lake.  I currently have everything at 5ft above ground level including the anemometer.  There is really nothing to the east, west, or north (my house is south of the station but about 80 feet away) except a lake. 

Do I really need the anemometer at 33 ft.?

[jwyman]

By offical standards it's usually that high, but if you can't do it, try to find the highset unobstructed spot you can...


Jim

[KA8MAV]

Is 33 ft. the official standard even if what I'm wanting to measure is the wind speed on the surface of the Lake?  Like I said, even at 5 ft above ground level, it's unobstructed.  We have a lot of water skiers that don't want to get the boat out if it's windy on the main section of the lake which is where my station is located.  This is the "official" weather station for Lake Holiday.  What's important is the condition *on* the water... not 33 ft. above it.  Well, we do have some folks that can almost get that much air!    

[capeweather]

See page 52

http://home.comcast.net/~dshelms/CWOP_Guide.pdf

[George Richardson]

FWIW.

I have pondered this thread and looked at your site. For most of what you are claiming, the 5' height is perfect; there is one problem. I don't see how you can claim to be a NWS site without at least attempting to conform to the 33' anemometer elevation standard. So, my take would be drop that NWS tag on your site and let her fly.

George

[blackjack52]

Since he's obviously structured with NWS, wouldn't it be easier to just put up another anemometer?  Then you can have one at 5 feet and another at 33.  Provided VWS will handle two anemometer.

[MesquiteWx]

Its not going to do you much good with it only 5' on the inland unless you could mount it to a buoy out on the main lake somewhere. Davis makes a wireless anemometer transmitter. The best option for you is at least 30' if you are going to have it on land so you can get a more accurate reading if you are going to mount it inland. 5' you will have a wind shadow from your house. The winds on the lake will always be stronger then they are over land so unless you can get it on the lake or higher up to compensate then your readings are going to be 10-15 mph off.

[KA8MAV]

It *does* make sense to get it up to "official" height.  I've taken measurements with my certified Kestrel 4000 and there is *no* difference between 5' and >30' at this location (there really are no obstructions).  But I do want to be "official" as there really isn't a decent station in this area.  The next closest PWS is 20 miles to the East of me.  There isn't an "official" station anywhere close.  So, I'll get the anemometer up where it's supposed to be this weekend (or next week at the latest).  I can't get a tower in my back yard, but I can get a 20' pole which would put it right at 33' above "lake level".  Close enough?  The wind up there should be pretty close to what it is at the surface of the water anyway... right?

Btw, I've done a lot of measurements with GPS using a program that logs over several hours and substracts out any errors due to satellite drift, etc. and looks at the average for an exact measurement.  My GPS readings for Lat/Long and Alt are more accurate that what I'm finding on the net (using the map from CWOP).  Which is correct?  Do I use the lat/long that map the "pointer" correctly or what I know is accurate?

Thanks for all the suggestions.

[MesquiteWx]

I've taken measurements with my certified Kestrel 4000 and there is *no* difference between 5' and >30'

Have you taken measurements over the main water body compared to land? How many days of data do you have to support the readings? That all has to be taken into consideration as well when you do yout siting. If you were testing this with little or no wind on land your readings are probably going to be the same. Its when you start getting sustained winds about 10-15 mph with 20-30mph gust is where you will see the difference between 5' and 30' both land and on water. The wind on the water will always be a little more then it will on land even on a calm day. So your station could be showing clam to 2-3 mph winds and on the lake it could be 10 mph with gusts. 

The most ideal situation is if you could get a weather buoy or maybe a dock. How far away is your house from where you have it? Their is a lot of things you have to take into consideration if you want to get the most accurate readings off a lake.

Btw, I've done a lot of measurements with GPS using a program that logs over several hours and substracts out any errors due to satellite drift, etc. and looks at the average for an exact measurement.  My GPS readings for Lat/Long and Alt are more accurate that what I'm finding on the net (using the map from CWOP).  Which is correct?  Do I use the lat/long that map the "pointer" correctly or what I know is accurate?

Thanks for all the suggestions.

What program are you using I would be interested in something like that? Using GPS to get your Lat/Long should be fine but I would take couple of readings just to be safe. I have read and I have done testing that using a GPS to get your Alt is not a good idea as you will get false readings. It is best just to use the Topo Maps online. I really couldn't see how a program go take out the error of the Alt readings even with drift since it would have to know what is correct and what is not. If it could do that then it should just be able to give you the correct Alt without the need for the GPS since it would be the correct Alt to correct the error off of, does that make sense? Google Maps and then some USGA?? Topo should do you just fine. They are pretty accurate. I have a Garmin GPSMAP 60csx which at the time I bought it a year ago it was the best hand held on the market. I tested the Alt just by laying it on the ground under my station and the numbers never stabilized on one set numbers. I also have a Garmin eTrex Vista that the FBI and Military use and  compared tried that one as well and just kept getting bounces. Never got a solid reading. So be careful when you use a GPS to get the Alt because it probably won't be correct.

[jwyman]

KA8MAV,
  I found that using a county-supplied topo map with elevation seems to be a better place for elevation. Even CWOP says I am off a little compared to my reading. But it's close enough within their range.


Jim

[George Richardson]

Yeah, CWOP says the elevation I gave them isn't exactly accurate either. They say my site is actually 2 feet under water. NOT!

George

[MesquiteWx]

Yeah, CWOP says the elevation I gave them isn't exactly accurate either. They say my site is actually 2 feet under water. NOT!

George

I think this is more what the are probably referring to, not that your elevation is actually under water.

http://en.wikipedia.org/wiki/Sea_level

[George Richardson]

Actually, I live on a lake controlled by the U.S. Army Corps of Engineers. The lake level at full pond is 795'; the top of dam is 802'. It was very easy to determine that my ISS is mounted at 804'. The elevation was reported correctly by me but was corrected to 791' by CWOP (4' not the 2' I misstated earlier, under water) The point I was trying to make was that CWOP is not a standard, but rather a guide.

FWIW

George

[MesquiteWx]

I get what you are saying, you are basing it against the lake though where CWOP isn't. I think CWOP validates against the NED Contiguous U. S. 1/3E arc second elevation data which is based on sea level. So while the top of the lake at full pond is 795' that is 795' above Sea Level. So with terrain changes it is very possible for your station to so show that is is lower then the lake. Your elevation is being validated against sea level and the terrain changes which create these elevations. So while it sounds funny that it is below your lake level it is actually validated against sea level so the terrain changes 2-4' lower then the bank can make this possible. Now if you lived at the top of a hill then I would be asking questions, but if you live on the damn side then it is entirely possible for your elevation to be lower then full pond. Below is a couple of examples. Doesn't take much change in the terrain.

[KA8MAV]

Okay, I've got it up in the air now.  I'll have to get a more accurate measurement but the anemometer is now exactly 18' above ground level (completely unobstructed) and 23-25' above the current lake level.  The ISS is 5' above ground level.

Better? 

Once I get a chance to replace the pole with a taller one (so much for flying a flag - but my weather station is beautifully landscaped!)... I'll get it to exactly 33' above ground level. 

Would it be best if it were 33' above ground level or lake level since it's close to the water?  Which data is going to be more accurate and statistically relevant?

Regarding GPS... here's the program I use:
Note: This software is freeware!

VisualGPS
http://www.visualgps.net/VisualGPS

Here is what they state about the methods of averaging:

"Statistical Data - VisualGPS has the ability to average position and altitude displaying the data. VisualGPS uses two methods of averaging, standard running average (sum of the data / samples) and a curve fitting or a Least Squares best fit method. These averages are both displayed graphically in the Survey frame and in text in the Statistics frame.

The standard deviation shows how much the position is moving around. Graphically, the standard deviation is displayed in the Survey Frame and the Elevation Frame. For a normal curve, 68.27% (one sigma) of the cases are included within the areas (the thick circles in the Survey Frame and between the two thick black lines in the Elevation Frame) on the graph. Using the standard deviation is a good way to see how the effects of SA  (Selective Availability) has on position. Smaller numbers of standard deviations indicates that SA may be turned down."

Although SA is turned off, it still gives an accurate reading over time to subtract out the drift from the satellites.  I think it gives a very accurate "average" using a lot of samples for Lat/Lon and Altitude.  Give it a try and see what you think.

[MesquiteWx]

Do you have any pictures of your siting? Do you have a lake front home? You should be good with it higher.

I will check out that program, like you said it takes the average which sounds about right. How close is it with the software compared to USGS topo?

[NorthNJwx]

Do you have any pictures of your siting? Do you have a lake front home? You should be good with it higher.

Agreed; the height difference should definatley be beneficial. After some time has passed, it would be great to know how much of a difference you notice in wind speed after the height raise. Even with great exposure like that, I bet there's gonna be a fairly significant difference.

[KA8MAV]

Actually, I only had the anemometer at about 6' for a couple of days before I raised it.  Looking back at the stats, I didn't see much of a difference at all raising it... although I'm sure over time I'd see a difference.  Anyway, it's up in the air and working great.  It's at 18' right now as I'm having a lot of trouble finding a way of getting it up to 33' (I can't put a tower in the middle of my back yard next to the lake).

So, considering where it's at (if you've looked at the sat photos of my siting it's where the flagpole is about 20-30' from my dock) if I mount it on top of the house there are actually more obstructions.  So, this is the Lake Holiday Weather Station.  I'm sure I can make some improvements to the siting (and I'll try to get some photos this weekend - weather and time permitting) but for now, it's the best I can do without obstructing it.

One problem I already see is... it's mounted on the top of a flag pole that has already been struck by lightning in the past.  It melted the eagle right off of it.  I'd love to get a PVC solution but I'm still trying to work out the details of how to get it sturdy enough (maybe start with a 16' 4x4 post (4' in the ground) with one or two 10' sections of PVC up in the air (and made so it could tilt down for adjustments, etc.).  Guy wires are out of the question.  Any suggestions on the best way to get PVC sturdy enough?

By the way, has anyone noticed the easiest way to get the anemometer pointed due north (not magnetic) is to simply point it towards the North Star (Polaris) on a clear night?  It is within a half degree of the pole which is probably closer than most can measure with a compass.  Of course it's constantly changing due to precession... but the annual motion is only about 50.3 seconds of arc per year or 1 degree every 71.6 years.  I used a compass and corrected for the magnetic deviation and it's right on.  So, for those of you who want an easier way to make sure you're pointing the right direction, just wait for a clear night and point it at the "pole star".  Works great!