I'm building my first mobile mesonet for the upcoming hurricane season and I could really use some pointers for the equipment to use. I've seen pictures of people's meso's, but if someone could point me in the right direction of what instruments I need, that would be awesome! Thanks everyone.
Building a Mobile Mesonet
- Thread starter john.sibley
- Start date
Joey Ketcham
I think before you go spending all the money and time into building one, you need to ask yourself what purpose your mobile mesonet will serve. What do you plan to do with the data collected from it? How will having this better you as a storm chaser?
If you are going to do scientific research, then you will need to spend a couple thousands of dollars on the equipment alone. If you are planning to use consumer brand equipment, forget it. It's not 100% accurate and it'll break.
In my bloody opinion, mobile mesonets serve no other purpose than to draw attention to the storm chasers who uses it. It's a way for them to scream out "HEY, LOOK AT ME.. I STORM CHASE". It's true. This of course doesn't include people associated with Vortex 2 or those involved in legit scientific research. I've been chasing for 13 years and not once have I ever felt that having a mobile mesonet would make me a better chaser.
It's a waste of time and money, just my .02 cents.
If you are going to do scientific research, then you will need to spend a couple thousands of dollars on the equipment alone. If you are planning to use consumer brand equipment, forget it. It's not 100% accurate and it'll break.
In my bloody opinion, mobile mesonets serve no other purpose than to draw attention to the storm chasers who uses it. It's a way for them to scream out "HEY, LOOK AT ME.. I STORM CHASE". It's true. This of course doesn't include people associated with Vortex 2 or those involved in legit scientific research. I've been chasing for 13 years and not once have I ever felt that having a mobile mesonet would make me a better chaser.
It's a waste of time and money, just my .02 cents.
Well, instead of preaching my own agenda and disregarding the original poster's intent I can offer some insight. What's definitely not a waste of time and money is knowing the wind speed when you are chasing hurricanes (or supercells for that matter). I use an Inspeed Vortex anemometer. it works quite well and the accuracy is acceptable for personal use (about 2% I believe). The sensors are definitely cheap enough for entry level use. The plastic cups are fragile, however. Although they can withstand winds well over 100 mph, if any debris hits them, you're probably going to damage them. They also have a serial port connector for interfacing with your laptop as well as software to display and log the data and various mounting options. Its a decent start for an entry level weather station.
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Joey Ketcham
Ok, so even if you buy an Inspeed anemometer, tossing it on your vehicle without any research of the aerodynamics of your vehicle and mounting it so that the air flow around your vehicle doesn't effect the data isn't going to give you much. You can't just toss equipment on your vehicle and call it good. You need to study the aerodynamics of your vehicle, how the winds flow around it and mount your equipment in a position so that it's not effected by the winds flowing against and around the vehicle. If the OP isn't willing to take the time to research this and mount the equipment properly, then what does he expect to do with the inaccurate data collected? How will this help?
Ask any of the engineers who designed and built the mesonets for Vortex, they spent a lot of time and money researching how the aerodynamics of a vehicle effected the data collected by meteorological instruments. They invested a lot of time and money in making sure that the equipment was mounted just right so that the data collected would be accurate. Is the OP willing to invest the time and money for that? If the answer is no, then what is the point of having a mesonet if the data you get is inccurate?
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Can I add my 2 cents worth?
For Hurricane chasing some form of mobile weather recording is an advantage to the chaser. But I agree may be off little real use scientifically. I normally like to record peak wind speed and pressure. Surface Temp and Dewpoint is a secondary consideration.
I use an RM young Anomometer head coupled to a Wind tracker display. I have written my own custom USB enabled data logger to record wind speeds to my laptop. (Note that you need the 0-2.5volt output wind tracker, as the common 0.5V output Wind Tracker peaks output at 100mph)
For Pressure I use a Davis Vantage Pro Console. This I have set the pressure to be accurate at sea level + 20 feet. Depending on your actual height above sea level, the pressure reading will fluctuate if you are mobile and going up and down hills.
Temp and Dew point is recorded from the Davis external ISS sensor which I have cut down to remove the rain bucket etc.
The advantage of my system is that is VERY small as it all has to be packed into a flight case (I live in the UK)
If I was to re do this – I would pick an Ultra sonic Anemometer head – just because they are smaller.
For Hurricane chasing some form of mobile weather recording is an advantage to the chaser. But I agree may be off little real use scientifically. I normally like to record peak wind speed and pressure. Surface Temp and Dewpoint is a secondary consideration.
I use an RM young Anomometer head coupled to a Wind tracker display. I have written my own custom USB enabled data logger to record wind speeds to my laptop. (Note that you need the 0-2.5volt output wind tracker, as the common 0.5V output Wind Tracker peaks output at 100mph)
For Pressure I use a Davis Vantage Pro Console. This I have set the pressure to be accurate at sea level + 20 feet. Depending on your actual height above sea level, the pressure reading will fluctuate if you are mobile and going up and down hills.
Temp and Dew point is recorded from the Davis external ISS sensor which I have cut down to remove the rain bucket etc.
The advantage of my system is that is VERY small as it all has to be packed into a flight case (I live in the UK)
If I was to re do this – I would pick an Ultra sonic Anemometer head – just because they are smaller.
Thanks for the feedback. I've had the Inspeed Anemometer for two years now and it has held up really well to two trees that have fallen on my vehicle. For $89, it has been worth it. I guess my question is if I'm going to be doing research, should I drop the big bucks on a V2 setup or stay at a consumer level with basic equipment.
If you are doing research, then yes you will have to get scientific grade instruments, which aren't cheap. Simply buying a high end anemometer is not going to result in an accurate measurement, however. Joey is right, you'll need to calibrate it and take into account the turbulence your vehicle creates. I can't recommend a research worthy setup, as the one I've created is for my own personal use and for reporting severe wind events back to the NWS.
What instruments you get completely depends on the research you're hoping to do. The project manager would want specific payloads with specific response parameters. If this is something personal, just stick with the basic stuff you're doing now.
Keep in mind research grade instruments need to be routinely calibrated too... can't just slap them on your vehicle and call it good for the life of the instrument.
Instrumentation
I am planning on using some National Instruments instrumentation and a LabVIEW program I wrote. I'm still picking out the sensors that I'd want to use. My purposes are strictly for my own fun- answering the question "Can I develop and deploy a set of instruments in this type of (moving)environment?"
I am probably just going to take temperature data this year (and maybe dew point? it's in the works), as it's easy to interface a simple J or K thermocouple, and less easy to develop a decent routine for calculating wind speed and direction in a moving vehicle.
I've thought about getting two anemometers, mounting them at right angles to each other and doing the vector math to figure out wind speed and direction, though they will still go nuts as you drive.
I am planning on using some National Instruments instrumentation and a LabVIEW program I wrote. I'm still picking out the sensors that I'd want to use. My purposes are strictly for my own fun- answering the question "Can I develop and deploy a set of instruments in this type of (moving)environment?"
I am probably just going to take temperature data this year (and maybe dew point? it's in the works), as it's easy to interface a simple J or K thermocouple, and less easy to develop a decent routine for calculating wind speed and direction in a moving vehicle.
I've thought about getting two anemometers, mounting them at right angles to each other and doing the vector math to figure out wind speed and direction, though they will still go nuts as you drive.
Seth, I think that dual anemometers/vanes would be redundant since you need vehicle direction and speed from another independent source, i.e. a GPS. Given that, computing true wind is a rather simple vector calculation.
Google USBTenki, for a sensor I'm experimenting with, FWIW.
Google USBTenki, for a sensor I'm experimenting with, FWIW.
Tyler was talking about integrating chaser mobile mesonet with Spotternet or another type of network for online data collection on a large scale. Perhaps you should talk with him about what he requires and how far along that project is.
I have a Davis Vue and so far I like it quite a bit. Temperature, wind, dew point, barometer, rainfall (rate and total) is all measured by it, and it was pretty cheap. And no...I'm not out there to say "HEY LOOK AT ME!!! I'M A STORM CHASER!". I want to be ACCURATE when I submit a report and not estimate all the time. I'm sure the NWS like a measured report rather than an estimated, although both are valuable
Jason Foster
I've used the Davis Monitor II (even trying to sell it) and currently using a Davis Vantage Pro. I know somewhere else I've posted pictures of it, but here is an old blog with photos of my unit. Link to blog.
Like some have mentioned, getting it up above the vehicle as much as possible is important. My doesn't scale high like a V2 mesonet but it's a good 36 inches or more above the roof....enough to make it fairly accurate. So many other things can interfere with the reading that have nothing to do with the station itself (like buildings, trees, etc.).
However, one thing I learned from Chris Collura's "WeatherLab III" is to have at least the anemometer removable from the station. This is exactly how my station is design. I will mention however that if you are chasing lots of cold weather (like I did though these past storms in DC), a polycarb type cutting board will break with all the ice build up, snow, and high wind load from the resistance of the station itself. Stick with PVC, steel or aluminum.
Who have you researched for the various mesonet designs? I'm always looking for ideas myself.
Add: Here is a photo of the station on top of the car in the first DC Mega snow storm:
Like some have mentioned, getting it up above the vehicle as much as possible is important. My doesn't scale high like a V2 mesonet but it's a good 36 inches or more above the roof....enough to make it fairly accurate. So many other things can interfere with the reading that have nothing to do with the station itself (like buildings, trees, etc.).
However, one thing I learned from Chris Collura's "WeatherLab III" is to have at least the anemometer removable from the station. This is exactly how my station is design. I will mention however that if you are chasing lots of cold weather (like I did though these past storms in DC), a polycarb type cutting board will break with all the ice build up, snow, and high wind load from the resistance of the station itself. Stick with PVC, steel or aluminum.
Who have you researched for the various mesonet designs? I'm always looking for ideas myself.
Add: Here is a photo of the station on top of the car in the first DC Mega snow storm:
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I caught a great presentation last year by one of the guys who worked on the V2 mesonets -- the one thing that really stuck in my mind was that if you want accurate wind measurements, you have to get your instrumentation waaaay up above your car's roof in order to get it out of the car's slipstream. A lot of the setups I see from chasers aren't that terribly high off the roof of the car. That doesn't make them useless, of course, but if your goal is to get accurate measurements, you've got to have it way up there. I think the NOAA probes were 11 foot off the ground and the CSWR stuff was 15 feet.
They all seemed rather fond of Young anemometers for wind; I suspect Young is the gold standard when it comes to accuracy and performance.
They all seemed rather fond of Young anemometers for wind; I suspect Young is the gold standard when it comes to accuracy and performance.
Ryan, for future reference I was the one who gave that presentation during V2. I will also be giving another one this year as I have developed a new instrument to solve the issues I discussed last year.
As for this thread, really opening a can of worms here. Tyler Allision started a similar thread which can be found here: http://www.stormtrack.org/forum/showthread.php?t=22304. We exchanged several emails on the topic. I could spend PAGES talking about this type of issue, but I'll try and keep it short for now.
My experience on mobile instrumentation is far more than most. I built all of the mobile mesonets that are being used for the VORTEX 2 project. I designed and built the computer equipment and mounting inside the vehicle, and was significantly involved with the racks themselves. I have also been doing a large amount of research on the mobile mesonets. What I have found is disturbing, at least to me.
There is a TREMENDOUS amount of research that must be done to ensure even the slightest bit of accuracy in mobile measurements. An interesting example is the "J" tube that almost every researcher uses for housing temperature an RH sensors while on a mobile vehicle. You can read up on the design of the "J" tube here, http://ams.allenpress.com/archive/1520-0426/13/5/pdf/i1520-0426-13-5-921.pdf, which was for the original VORTEX project over 2 decades ago. Almost everyone accepted this shield to be accurate without question. For my CAPSTONE project last year, I found and documented several serious errors that can occur in certain situations. I see a lot of chasers that have a cup anemometers mounted 3 inches off their car roof and call it accurate data. I wouldn't trust data in those cases for anything. There are a lot of considerations that must be made to obtain even reasonable data. Take a temperature sensor for example. JUST BECAUSE IT HAS SPECIFICATIONS FOR AN 18 SEC TIME CONSTANT DOES NOT MEAN THAT IS WHAT YOU ARE GOING TO GET. NSSL uses a temperature sensor in the "J" tube that has a manufacturer specified time constant of 18 seconds. However, when placed in the "J" tube that time constant rises to almost 1 min, 30 secs (one of the many findings of my CAPSTONE). This means that it take a minute and a half to reach 63% of a step change in temperature. Just because it says it is accurate to 0.2 degrees C does not mean anything in the real world. I takes the "J" tube on average over 10 minutes to reach the final value, and that is assuming that there are NO other changes occuring.
Everything influences your measurements: speed, heading, height, time of day, rain vs no rain, ect. There is nothing remotely simple amount measurements, let alone measurements on a moving platform. If you just want to look cool, fine. If you just want to know the fact that the temperature changed, go for it. But if you are trying to get even remotely accurate data, especially for use in research, be prepared to spend a lot of money and time.
As for this thread, really opening a can of worms here. Tyler Allision started a similar thread which can be found here: http://www.stormtrack.org/forum/showthread.php?t=22304. We exchanged several emails on the topic. I could spend PAGES talking about this type of issue, but I'll try and keep it short for now.
My experience on mobile instrumentation is far more than most. I built all of the mobile mesonets that are being used for the VORTEX 2 project. I designed and built the computer equipment and mounting inside the vehicle, and was significantly involved with the racks themselves. I have also been doing a large amount of research on the mobile mesonets. What I have found is disturbing, at least to me.
There is a TREMENDOUS amount of research that must be done to ensure even the slightest bit of accuracy in mobile measurements. An interesting example is the "J" tube that almost every researcher uses for housing temperature an RH sensors while on a mobile vehicle. You can read up on the design of the "J" tube here, http://ams.allenpress.com/archive/1520-0426/13/5/pdf/i1520-0426-13-5-921.pdf, which was for the original VORTEX project over 2 decades ago. Almost everyone accepted this shield to be accurate without question. For my CAPSTONE project last year, I found and documented several serious errors that can occur in certain situations. I see a lot of chasers that have a cup anemometers mounted 3 inches off their car roof and call it accurate data. I wouldn't trust data in those cases for anything. There are a lot of considerations that must be made to obtain even reasonable data. Take a temperature sensor for example. JUST BECAUSE IT HAS SPECIFICATIONS FOR AN 18 SEC TIME CONSTANT DOES NOT MEAN THAT IS WHAT YOU ARE GOING TO GET. NSSL uses a temperature sensor in the "J" tube that has a manufacturer specified time constant of 18 seconds. However, when placed in the "J" tube that time constant rises to almost 1 min, 30 secs (one of the many findings of my CAPSTONE). This means that it take a minute and a half to reach 63% of a step change in temperature. Just because it says it is accurate to 0.2 degrees C does not mean anything in the real world. I takes the "J" tube on average over 10 minutes to reach the final value, and that is assuming that there are NO other changes occuring.
Everything influences your measurements: speed, heading, height, time of day, rain vs no rain, ect. There is nothing remotely simple amount measurements, let alone measurements on a moving platform. If you just want to look cool, fine. If you just want to know the fact that the temperature changed, go for it. But if you are trying to get even remotely accurate data, especially for use in research, be prepared to spend a lot of money and time.
Jason Foster
That kind of matters on what you mean as "accurate". What variations have you monitored in your research (not having checked it out YET for myself). while I obviously have one, I'm not likely to submit much obs from the station because I accept a 2 or 3 degree difference. However, driving across a dryline or measuring Hurricane winds, I found it's accurate enough! I think it is important to make obs while stopped as much as possible, which for this thread talked about tropical chases, is the case 90% of the time.
Just like Ryan noted too, RM Young would be required for true scientific data...trusting the best consumer market station (Davis) really isn't good enough for true comparison or inclusion with any V2 data for example.
I will say though, having check with various other stations and equipment, the Davis station I've show in the previous post has been fairly close to official stations. It would be of course ideal to compare to Mesonets similar to or directly with what is used on V2.
Yes, my sense of "accurate" may be different than others. But something to remember is what you are using the data for. Driving down the road and sitting still both have their respective problem situations. While most of your data (in a general sense) would be close (2-3 C, ~5 mph, +/- 2-3 degrees wind direction, ect) to an "official site", remember that those sites may also have issues. A simple example would be during a driving rain (like in a hurricane). In that scenario, there are several temperature shields that end up allowing the temperature sensor to become coated in rain, which results in the measurement of the rain temperature, not the air temperature. There can be very large errors (>5 C) in these situations.
Again it comes back to what you want to do with it. If you want to know that something changed (or is changing) but you're not too concerned with the exact magnitude of the change, then you're probably ok with the less expensive equipment. But if you are trying to give reliable, "accurate" data to forecasters, or determine the width of a boundary, then you are going to have to spend more time and money developing and testing the design.
Again it comes back to what you want to do with it. If you want to know that something changed (or is changing) but you're not too concerned with the exact magnitude of the change, then you're probably ok with the less expensive equipment. But if you are trying to give reliable, "accurate" data to forecasters, or determine the width of a boundary, then you are going to have to spend more time and money developing and testing the design.
But won't the speed and direction will be influenced by the vehicle moving? I mean, if you are headed north at 60 mph, and the wind is 20mph from the west, you will likely end up with 60mph North for your wind? I'm not sure I fully understand the way to do this properly.
Maybe it helps to visualize it, as here is the vector math David was talking about:
Your car is the big orange blob moving towards the top of the screen. In the example you gave, the car is going to feel winds at 63.25 mph from 341.57 degrees relative the front of the vehicle (the purple line). If we draw a line on a graph of length 63.25 and this angle, we can extend a line down from the top point representing our vehicles speed and direction of travel. We know the car was moving 60 at the time and its moving forward so the line goes straight down at a length of 60 (the green line). You then simply draw a third line to complete the triangle and that's the actual wind corrected for the car's movement (the blue line).
You simply add the wind created by the car moving forward at 60: <0, 60> and the surface winds <20, 0> and you wind up with a 2D vector of <20, 60>. The length of this vector is the wind speed = sqrt((20*20) + (60*60)) = 63.25. I think its a little easier to look at the picture though.
David Goines
Seth, just subtract the vehicle's speed from the anemometer's "y-component" to get a resultant wind speed vector with the motion of the vehicle being in the positive y direction.
"<[Anemometer's X] , [Anemometer's Y]-[vehicle speed]>
And use pythagorean's theorem to find the actual wind speed.
IF the wind speed has a positive y-component, Arctan([wind speed x-component] / [wind speed y-component]) should give you an angle clockwise from the car's forward motion.
IF it has a negative y-component, add 180 degrees to the angle.
With a direction now in relation to the vehicle, take ([wind direction in relation of the car]-360 +[compass direction]) to get the direction of the wind's origin clockwise from the the Earth's north.
Example: If a car is heading east (90 degrees) and is reading a wind direction of 315 degrees (front left of the driver). 315-360+90=45degrees (NORTH EAST wind)
Edit: Yeah Skip beat me to it but the picture really helps. But like said before, a wind direction in relation to a car is nothing unless it's integrated with a GPS or compass... and to me that's the tricky part.
"<[Anemometer's X] , [Anemometer's Y]-[vehicle speed]>
And use pythagorean's theorem to find the actual wind speed.
IF the wind speed has a positive y-component, Arctan([wind speed x-component] / [wind speed y-component]) should give you an angle clockwise from the car's forward motion.
IF it has a negative y-component, add 180 degrees to the angle.
With a direction now in relation to the vehicle, take ([wind direction in relation of the car]-360 +[compass direction]) to get the direction of the wind's origin clockwise from the the Earth's north.
Example: If a car is heading east (90 degrees) and is reading a wind direction of 315 degrees (front left of the driver). 315-360+90=45degrees (NORTH EAST wind)
Edit: Yeah Skip beat me to it but the picture really helps. But like said before, a wind direction in relation to a car is nothing unless it's integrated with a GPS or compass... and to me that's the tricky part.
I plan on publishing what Sean and I discussed as I think we pretty much documented all the major issues. I just need to find the time.
-Tyler
Could I please have a translation in English for a non-physics major whose math is a bit dodgy.......I'm trying to find a table or easy rule to work out both wind speed and direction from the Vantage Vue that has just sprouted on the roof of the car....
"just subtract the vehicle's speed from the anemometer's "y-component" to get a resultant wind speed vector with the motion of the vehicle being in the positive y direction.
"<[Anemometer's X] , [Anemometer's Y]-[vehicle speed]>
And use pythagorean's theorem to find the actual wind speed.
IF the wind speed has a positive y-component, Arctan([wind speed x-component] / [wind speed y-component]) should give you an angle clockwise from the car's forward motion.
IF it has a negative y-component, add 180 degrees to the angle.
With a direction now in relation to the vehicle, take ([wind direction in relation of the car]-360 +[compass direction]) to get the direction of the wind's origin clockwise from the the Earth's north.
Example: If a car is heading east (90 degrees) and is reading a wind direction of 315 degrees (front left of the driver). 315-360+90=45degrees (NORTH EAST wind)"
Thanks heaps in advance!!
"just subtract the vehicle's speed from the anemometer's "y-component" to get a resultant wind speed vector with the motion of the vehicle being in the positive y direction.
"<[Anemometer's X] , [Anemometer's Y]-[vehicle speed]>
And use pythagorean's theorem to find the actual wind speed.
IF the wind speed has a positive y-component, Arctan([wind speed x-component] / [wind speed y-component]) should give you an angle clockwise from the car's forward motion.
IF it has a negative y-component, add 180 degrees to the angle.
With a direction now in relation to the vehicle, take ([wind direction in relation of the car]-360 +[compass direction]) to get the direction of the wind's origin clockwise from the the Earth's north.
Example: If a car is heading east (90 degrees) and is reading a wind direction of 315 degrees (front left of the driver). 315-360+90=45degrees (NORTH EAST wind)"
Thanks heaps in advance!!
What Skip was talking about in the post above would only really work under ideal conditions.
Jane: I would not even worry about the Pythagorean theorem as vector calculations are easier to apply in this use case. I spent a good bit of time researching this topic on my own and found some good resources online. Interestingly enough I found the answer within the marine and boating community. I can't recall what site I got the information from but I saved the calculations in a document and will list them below.
Vehicle:
Speed and direction derived via GPS while in motion, otherwise digital compass is supplemented in while stationary.
SOG: Speed Over Ground
COG: Course Over Ground
Anemometer:
This is the wind relative to the vehicle.
AWS: Apparent Wind Speed
AWD: Apparent Wind Direction
True Wind:
The derived estimate wind speed/direction as though you were stationary while in motion.
TWS: True Wind Speed
TWD: True Wind Direction
Finding the vector components of the apparent wind:
a = (AWS)cos(AWD)
b = (AWS)sin(AWD)
Finding the vector components of vehicle speed/direction:
c = (SOG)cos(COG)
d = (SOG)sin(COG)
Now to subtract the vehicle motion from the apparent wind observation:
u = c - a
v = d - b
An alternative way to find the true wind components is to do everything in one step. For the purposes of this post, I've shown the long and below show the short way.
The alternative does everything in one go, and is probably the preferred option given it's more efficient:
u = (SOG)cos(COG) - (AWS)cos(AWD)
v = (SOG)sin(COG) - (AWS)sin(AWD)
With true wind now derived as u and v components, all that is left to do is calculate wind speed and direction.
True wind speed is pretty straight forward:
TWS = SQRT((u*u)+(v*v))
Find the square root of the added combination of each squared component.
True wind direction calculation is a little more involved however. Because of how the cartesian plain works, each quadrant requires its own separate formula (I'm not an expert on mathematics so I couldn't tell you why this is, it just is, I'm sure a google search could yield some results).
I put together and labeled the corresponding quadrants with the component values. The output for true wind direction is denoted as theta θ. In a computer program or logger software the output can automatically be supplemented.
QUADRANT I
u > 0
v > 0
θ = arctan(v/u)
QUADRANT II
u < 0
v > 0
θ = 180 + arctan(v/u)
QUADRANT III
u < 0
v < 0
θ = 180 + arctan(v/u)
QUADRANT IV
u > 0
v < 0
θ = 360 + arctan(v/u)
So now determining true wind direction is as straight forward as:
TWD = θ
I hope this clears up your confusion and helps anyone looking for information on this topic.
More information on wind component vectors can be found here.
Cheers
Jane: I would not even worry about the Pythagorean theorem as vector calculations are easier to apply in this use case. I spent a good bit of time researching this topic on my own and found some good resources online. Interestingly enough I found the answer within the marine and boating community. I can't recall what site I got the information from but I saved the calculations in a document and will list them below.
Vehicle:
Speed and direction derived via GPS while in motion, otherwise digital compass is supplemented in while stationary.
SOG: Speed Over Ground
COG: Course Over Ground
Anemometer:
This is the wind relative to the vehicle.
AWS: Apparent Wind Speed
AWD: Apparent Wind Direction
True Wind:
The derived estimate wind speed/direction as though you were stationary while in motion.
TWS: True Wind Speed
TWD: True Wind Direction
Finding the vector components of the apparent wind:
a = (AWS)cos(AWD)
b = (AWS)sin(AWD)
Finding the vector components of vehicle speed/direction:
c = (SOG)cos(COG)
d = (SOG)sin(COG)
Now to subtract the vehicle motion from the apparent wind observation:
u = c - a
v = d - b
An alternative way to find the true wind components is to do everything in one step. For the purposes of this post, I've shown the long and below show the short way.
The alternative does everything in one go, and is probably the preferred option given it's more efficient:
u = (SOG)cos(COG) - (AWS)cos(AWD)
v = (SOG)sin(COG) - (AWS)sin(AWD)
With true wind now derived as u and v components, all that is left to do is calculate wind speed and direction.
True wind speed is pretty straight forward:
TWS = SQRT((u*u)+(v*v))
Find the square root of the added combination of each squared component.
True wind direction calculation is a little more involved however. Because of how the cartesian plain works, each quadrant requires its own separate formula (I'm not an expert on mathematics so I couldn't tell you why this is, it just is, I'm sure a google search could yield some results).
I put together and labeled the corresponding quadrants with the component values. The output for true wind direction is denoted as theta θ. In a computer program or logger software the output can automatically be supplemented.
QUADRANT I
u > 0
v > 0
θ = arctan(v/u)
QUADRANT II
u < 0
v > 0
θ = 180 + arctan(v/u)
QUADRANT III
u < 0
v < 0
θ = 180 + arctan(v/u)
QUADRANT IV
u > 0
v < 0
θ = 360 + arctan(v/u)
So now determining true wind direction is as straight forward as:
TWD = θ
I hope this clears up your confusion and helps anyone looking for information on this topic.
More information on wind component vectors can be found here.
Cheers
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