How Altimeters Work

[FalcoColumbarius]

I was surfing the internet looking for answers. I wanted to better understand my altimeter and what more I could use it for and this is what I found:

How Altimeters Work

An altimeter does not actually measure altitude directly, but rather just atmospheric pressure. So an altimeter is actually a barometer created for a specific purpose. The altimeter uses the changes in atmospheric pressure to determine the changes in altitude. Why does pressure change with altitude? Simply put, the pressure at any given point in the atmosphere around the earth is a result of the weight of the atmosphere above it (pulled down by gravity). For this reason, the higher in the atmosphere you are, the less atmosphere you have above, and the less pressure exerted on you.

Measuring altitude with barometric pressure is not free from difficulties, however. Atmospheric pressure can also change with changes in weather patterns. On an uneventful weather day it is not too unusual for an air pressure change of 1 mbar, caused by temperature changes alone. This 1 mbar change in pressure could result in a skewed altitude reading of up to 26 feet (8 meters). But on an afternoon full of weather changes, like an approaching cold front, air pressure could change by as much as 5 mbar, or more. This could result in a skewed altitude reading of up to 130 feet (40 meters), or more.

Typically, when bad weather is approaching, the pressure will be falling. And the altimeter thinks this decrease in pressure is due to an increase in altitude, so it will read higher than you really are. And the opposite is the case when the weather conditions are improving – the altimeter will read lower than you really are.

In order to compensate for weather changes, an altimeter must be calibrated using a known altitude or a known pressure value. A known altitude can typically be taken from a specific landmark on a topographic map. But if you do not have a known altitude to use, you will need to use a known pressure value. This is typically done with a barometric pressure value. "Barometric Pressure" is the current air pressure at sea level, for a specific location. Barometric pressure is measured several times per day, and can usually be obtained from flight service stations or aviation weather reports.

With all of the changes that affect an altimeter, your elevation can easily be skewed by a few hundred feet in as little as one day’s time. You will want to calibrate your unit at least once per day, but for improved accuracy (especially in changing weather conditions), you may want to calibrate it every couple of hours at a known elevation.

The cool thing about this is you can anticipate the weather by keeping tabs on your altitude (by periodically checking your actual altitude on your topographical map). Going into the mountains where the weather changes often one can use the altimeter as a barometer by comparing with the map.
Falco.

[mararmeisto]

Given the potential for skewing the altitude reading based on weather-related atmospheric changes, this is probably why the gauges on the dash reads in 100s-of-meters (? I think it was before I removed mine). That way, a couple of psi either way won't really register on the dash pod.

Another way to think of change in atmospheric pressure comparing sea level to that up in the mountains: it's the exact same as diving into water - the deeper one goes, the greater the pressure. Of course, the only way to go on land is upwards. And it's air, instead of water.

Good job, Falco. Now, can you tell us how to put the fluid back into the wobble ball?

[jessef]

After going from hot to cold to hot to cold climates (south of the border and north), to me it's just a static display. It doesn't read accurately enough to warrant being on there. I'm tossing in something useful. Not sure what yet.

I played around with it for quite a while to try and get it to stick with accurate readings but my last two where all over the place.

[konadog]

I like the altimeter, or at least the idea of it... I had one on a wrist-watch years ago and loved it - it was very accurate as long as I kept at resetting it when at known elevations. It measured 5 meter increments, not the 100 meters of the one in the deli which is too coarse a measure to be much more than a novelty for me - it is a cute novelty though

[Jonny5ive]

Most Altimeters have to be calibrated frequently.

For us true west coasters it's as easy as heading to the beach and zeroing it.

Otherwise you need to know the true Altitude of where the start of your assent/descent is then you can have a fairly accurate understanding of the climb or drop.

[FalcoColumbarius]

As I know the altitude of my garage I set it at night and I set it in the morning. When I set it in the morning I look to see which way it has changed, thereby getting a feel for the weather any given day. When I go on a sortie I set it with my Garmin and check it from time to time when I cross an elevation line (which conveniently is marked in the hundreds), this way I have a basic ballpark understanding of how high I am at a glance and also am monitoring the weather ~ so I know what to expect weather wise. Between reading this and the elevation lines I can estimate my potential situation as well as my current situation. For instance:

• There is a two foot deep creek that I am about to ford and I know I have to come back this way. I check with my GPS and I see that I am surrounded by very steep mountains with a lot of creeks; my GPS says that I am at 1,137 metres ~ yet my altimeter says that I am at a little under 1,300 metres ~ and seeing as all those other creeks feed into the one that I am about to ford ~ my altimeter has basically told me that when I get back to this two foot deep creek..... it might be a six foot deep torrent.

The wobbly ball (as Mararmeisto puts it) I think warrants the pod in the first place. It tells you what you can get away with, especially when you are on those tricky deactivated FSRs that are switching back & forth on the side of a mountain. I don't have the inside/outside temperature gauge, which I think is a little decadent. I do have the window..... I just stick my hand outside (knowing that the driver behind me is not going to see it an think that I am signalling, unless of course s/he's another Delica driver in which case they will understand).

I like the pod.

Falco.

[FalcoColumbarius]

From the same site:


Predicting The Weather With Your Altimeter

As discussed in How Altimeters Work , the air pressure changes measured by your altimeter can be either the result of weather changes or altitude changes. When you are at a constant elevation is easy to realize that the recognized changes in pressure are totally related to the changes in the weather conditions, and not in fact altitude changes. When your altitude is changing it is a little more difficult to isolate the pressure changes due to weather conditions, but it can be done if you have known altitudes to work with. In any case, these pressure changes due to changing weather conditions can be used to forecast the weather for the coming hours and days.

In it's crudest sense, falling pressure typically leads bad weather, and rising pressure indicates improving weather. And remember, if you are looking at an altimeter, falling pressure is equal to an increase in altitude. So if your altimeter says your altitude is going up, and you know you're not, bad weather is approaching. But, here are a few general statements from the National Weather Service that can provide even more assistance with your basic weather predictions...

1) When the wind sets in from points between south and southeast and the barometer falls steadily, a storm is approaching from the west or northwest, and its center will pass near or north of the observer within 12 to 24 hours, with wind shifting to the northwest by way of south and southwest

2) When the wind sets in from points between east and northeast and the barometer falls steadily, a storm is approaching from the south or southwest, and its center will pass near or to the south of the observer within 12 to 24 hours, with winds shifting to northwest by way of north. The rapidity of the storm's approach and its intensity will be indicated by the rate and amount of the fall in the barometer.

3) As a rule, winds from the east quadrants and falling barometric pressure indicate foul weather, and winds shifting to the west quadrants indicate clearing and fair weather, but again there are exceptions and in some parts of the country these rules do not apply.

Falco.

[storm80y]

So, when are we going to get some bloody snow? ;)

[Golf Cart]

Id rather look at this to tell me what the weather is doing

[marsgal42]

I've never worried too much about the accuracy of Gumdrop's altimeter. It's generally within +/- 100 meters, and that's fine for me. I've seen what it says at places like Coquihalla Summit, Begbie Summit and Kootenay Pass. It will do.

GPS receivers can tell you your altitude, but the results can be whimsical. What they are doing is working out your location in an (x, y, z) coordinate system centred on the Earth, then relating that to a model of the Earth (the WGS-84 geoid, if you want to bore people at parties) to determine your latitude and longitude. They can then determine how far you are away from the average Earth at your latitude, and determine your altitude. Since this is a very small difference between two large numbers, the result isn't always very accurate.

While I don't do weather forecasting with them, anybody can receive signals from the low-orbit weather satellites. At one time people used mechanical scanning machines, now you use a computer with a sound card, plus a scanner that can tune the 136-138 MHz satellite band. The pictures are really neat.

...laura

[mararmeisto]

From the same site:


Predicting The Weather With Your Altimeter

As discussed in How Altimeters Work , the air pressure changes measured by your altimeter can be either the result of weather changes or altitude changes. When you are at a constant elevation is easy to realize that the recognized changes in pressure are totally related to the changes in the weather conditions, and not in fact altitude changes. When your altitude is changing it is a little more difficult to isolate the pressure changes due to weather conditions, but it can be done if you have known altitudes to work with. In any case, these pressure changes due to changing weather conditions can be used to forecast the weather for the coming hours and days.

In it's crudest sense, falling pressure typically leads bad weather, and rising pressure indicates improving weather. And remember, if you are looking at an altimeter, falling pressure is equal to an increase in altitude. So if your altimeter says your altitude is going up, and you know you're not, bad weather is approaching. But, here are a few general statements from the National Weather Service that can provide even more assistance with your basic weather predictions...

1) When the wind sets in from points between south and southeast and the barometer falls steadily, a storm is approaching from the west or northwest, and its center will pass near or north of the observer within 12 to 24 hours, with wind shifting to the northwest by way of south and southwest

2) When the wind sets in from points between east and northeast and the barometer falls steadily, a storm is approaching from the south or southwest, and its center will pass near or to the south of the observer within 12 to 24 hours, with winds shifting to northwest by way of north. The rapidity of the storm's approach and its intensity will be indicated by the rate and amount of the fall in the barometer.

3) As a rule, winds from the east quadrants and falling barometric pressure indicate foul weather, and winds shifting to the west quadrants indicate clearing and fair weather, but again there are exceptions and in some parts of the country these rules do not apply.

Falco.

Now is all of that just in the Lower Mainland (considering all the mountains which surround that area), or is it generally true for the northern hemisphere? I ask, because weather patterns go the other way south of the equator...

[FalcoColumbarius]

Perhaps the weather patterns differ.... maybe they are opposite, like whirl pools ~ but I would think that pressures would remain the same ~ enough to tell you that a storm is coming or good weather is coming. However, I think that as far as Canada is concerned it is a good formula for reading the weather.

Here is a question: Why are the high altitude lines in a different colour? Is it to do with engine performance? Do I need to wear a G suite at the higher altitudes? Maybe there is a compartment in the ceiling that we have yet to discover that drops down an oxygen masque when you get to a certain altitude....

Falco.

[Fishtank]

Falco, I think bringing up the subject of weather awareness is an interesting side-topic and certainly has merit since alot of us spend time in more 'weather critical' areas. For the most part the weather info you posted is quite accurate. I just want to drop $.02 to help clarify a few things;

From the same site:
Predicting The Weather With Your Altimeter
When your altitude is changing it is a little more difficult to isolate the pressure changes due to weather conditions, but it can be done if you have known altitudes to work with. In any case, these pressure changes due to changing weather conditions can be used to forecast the weather for the coming hours and days.

This is true only IF, and I stress IF, you have an accurate altimeter. The Delica Altimeter is not sensitive enough to detect these changes accurately especially if the vehicle is climbing or descending any amount.

In it's crudest sense, falling pressure typically leads bad weather, and rising pressure indicates improving weather. And remember, if you are looking at an altimeter, falling pressure is equal to an increase in altitude. So if your altimeter says your altitude is going up, and you know you're not, bad weather is approaching.

Once again, you need an accurate altimeter for this, which we don't have.

1) When the wind sets in from points between south and southeast and the barometer falls steadily, a storm is approaching from the west or northwest, and its center will pass near or north of the observer within 12 to 24 hours, with wind shifting to the northwest by way of south and southwest

2) When the wind sets in from points between east and northeast and the barometer falls steadily, a storm is approaching from the south or southwest, and its center will pass near or to the south of the observer within 12 to 24 hours, with winds shifting to northwest by way of north. The rapidity of the storm's approach and its intensity will be indicated by the rate and amount of the fall in the barometer.

A simple rule to keep # 1 & 2 straight and locate the Low pressure centre (storm) is this; Put the wind at your back, and the low is to your left.

3) As a rule, winds from the east quadrants and falling barometric pressure indicate foul weather, and winds shifting to the west quadrants indicate clearing and fair weather, but again there are exceptions and in some parts of the country these rules do not apply.

In regards to # 3, when winds change direction the wind is either veering or backing. When the wind Veers it changes direction clockwise, and when it backs it changes counter-clockwise. Another simple rule regarding wind changes is the following; If the wind is veering, the skies will be clearing. If the winds are backing, best get packing.

Being a simple guy, I always try to simplify things to make them easier to remember, these simplifications are just that. There are many other factors involved in weather that can cause variations.

Now is all of that just in the Lower Mainland (considering all the mountains which surround that area), or is it generally true for the northern hemisphere? I ask, because weather patterns go the other way south of the equator...

Mararmeisto, the info Falco posted applies for the Northern Hemisphere, and you are correct in that the Low and High pressure systems rotate in the opposite direction due to something called the Coriolis Effect. Which is too much for me to explain here, so check out this link to Wikipedia http://en.wikipedia.org/wiki/Coriolis_effect.

[mararmeisto]

Now is all of that just in the Lower Mainland (considering all the mountains which surround that area), or is it generally true for the northern hemisphere? I ask, because weather patterns go the other way south of the equator...

Mararmeisto, the info Falco posted applies for the Northern Hemisphere, and you are correct in that the Low and High pressure systems rotate in the opposite direction due to something called the Coriolis Effect. Which is too much for me to explain here, so check out this link to Wikipedia http://en.wikipedia.org/wiki/Coriolis_effect.

Quick and dirty summation: there is an image nearly at the bottom of the page which shows the rotation of weather systems in the northern and southern hemisphere (clockwise and anti-clockwise, respectively). The simple explanation is because the Earth rotates anti-clockwise on its axis (to the east, as it were), the air masses are 'dragged' in the same direction. The greatest amount of 'drag' is at the equator (where rotational speeds are the highest) and least at the poles, therefore the air masses start to rotate away from the equator: upwards into the northern hemisphere, and downwards into the southern hemisphere. This sets up the rotation as previously described.

N.B. The circles in the graphic are 'inertial circles', and we all know that inertial forces act in opposition to momentum.

Here's another thought: we in the northern hemisphere consider 'up' to be towards the North Pole. Do Australians and New Zealanders consider 'up' to be towards the South Pole? And if not, why not?

[FalcoColumbarius]

You are right Fishtank, they are not as accurate as some altimeters out there. I live at about 100 meters. I set my altimeter at 100 meters in the morning, and I set it at 100 meters in the evening when I shut her down, if I need to. Last night when I came home my altimeter was at 150ish, today it is raining. My altimeter didn't tell me the nature of this bad weather or the name of the storm ~ but it told me that it was coming and they usually wear name tags, anyway. I have a use for it and as I mentioned earlier, when I go on sorties I use it against an accurate ordinance survey map (Topo Canada) that I have in my computer.

By the way, I think your two cents are worth at least a dime, I have been out in the toolies and seen the weather changing enough to know we ought to "get packing", that is a good and simple maxim, and as you say simplicity is where it is at.

Falco.