Density Altitude
12-09-2009, 08:30 PM
#11
Gets Weekends Off
Joined APC: Jun 2009
Posts: 317
Originally Posted by gestrich1311
I read this forum but still am a little uncertain why the statement "Hot to cold look out below" is true. It seems contrary to what you would expect. I don't think the PHAK explains it very well or at least I am not getting it. There was an illustration previously posted from the PHAK above but I would like to make sure my interpretation of the illustration is correct.
In the mean time, before I post, I suggest reviewing those items and thinking about them heavily. If you don't figure it out that way then I will hopefully shed some light on it tomorrow for you. Night.
12-09-2009, 09:36 PM
#12
On Reserve
Joined APC: Dec 2009
Posts: 15
Just to clarify- I feel the PHAK did a very good job of explaining the interaction of pressure, temperature and humidy. It is specifically the explanation of "hot to cold, look out below" that I feel is lacking or at least my understanding.
12-10-2009, 03:49 AM
#14
Gets Weekends Off
Joined APC: Nov 2008
Posts: 826
Originally Posted by gestrich1311
I read this forum but still am a little uncertain why the statement "Hot to cold look out below" is true. It seems contrary to what you would expect. I don't think the PHAK explains it very well or at least I am not getting it. There was an illustration previously posted from the PHAK above but I would like to make sure my interpretation of the illustration is correct.
Is it because in colder air, the pressure decreases with altitude more abruptly (faster rate) resulting in a "tighter stack" of pressure change? Therefore when encountering colder air, you will descend to stay at the same indicated altitude? That makes sense to me but I am not sure if it is correct.
The PHAK's explanation is that this occurs because cold air is more dense but I am not sure that really explains it.
Is it because in colder air, the pressure decreases with altitude more abruptly (faster rate) resulting in a "tighter stack" of pressure change? Therefore when encountering colder air, you will descend to stay at the same indicated altitude? That makes sense to me but I am not sure if it is correct.
The PHAK's explanation is that this occurs because cold air is more dense but I am not sure that really explains it.
Picture your airplane is riding on top of a column of air that is enclosed on the sides, but can expand up and down (use flyanddive's graphic to picture this).
Temperature rises, the air, like any gas, expands, so the column rises. Temperature lowers, density increases, and the column of air shrinks.
The airplane is sitting on top of the column. So it moves higher with increases in temperature moves lower with decreases. Problem is the altimeter doesn't change unless you adjust it (for pressure anyway)
So, you're altimeter reads 2000'. The pressure setting is correct, but it's an unusually cold day. At any given altitude, You're riding lower than the standard =temperature= day that the altimeter is set for.
12-10-2009, 07:59 AM
#15
Gets Weekends Off
Joined APC: Jun 2009
Posts: 317
This will be long, I apologize. But I hope it helps clear things up for anyone reading it. I would love feedback as well, did it work, didn't it, why or why not? Thank you. (Note: The section on "temp effects on air" was prewritten, the rest I just made up today.)
Density Altitude: A term designed to confuse the living crap out of pilots IMO. We will get back to its definition later.
Temperature Effects on Air
An aircrafts performance, especially the engine, is based on the density of the air. If the air is more dense, the engine gets more air and can produce more power. This is true for the prop and the wings generating lift as well. But let's ignore that, just think about the effects on the engine to keep it simpler.
One clear way to know this is true is to consider what we use to increase an engines power, a supercharger or turbocharger. Each of these operates by forcing more air into the engine for more power. Simple.
Let's finally throw pressure into the mix here. Consider the water bottle from before and pressure becomes pretty clear. If we increase heat, the air in the bottle tries to spread out. If there is a wall keeping that air from spreading out, such is the case with the water bottle, then the pressure in the bottle increases. This is evident if you attempt to crush the bottle after heating, it will be difficult to do. The opposite occurs with a decrease in temperature.
Putting it all together, consider:
The Relationships:
Temperature Increase | Volume Increase | Density Decrease | Performance Decrease| Density Altitude Increase | Pressure Increase
Temperature Decrease | Volume Decrease | Density Increase | Performance Increase | Density Altitude Decrease | Pressure Decrease
Back to Density Altitude: Defined as the altitude the airplane will perform as if it was at. It is easier, IMO, to just understand the relationships and how we arrived at those relationships, described above, to more clearly understand density altitude.
One other way to think about it is density altitude varies inversely with actual density. So if actual density goes up the density altitude goes down. Remember this and all that is left is understanding what effects density.
Pressure Altitude: Defined as 29.92 in your altimeter, helpful right?
We just explored what happens to pressure as we go up versus down and how temperature effects that. Review the relationships if your confused. To help think about this consider swimming in a pool. When we swim to the bottom the pressure on our ears goes up drastically, right? The reason is we have more water resting on top of us so to speak.
The atmosphere operates much the same way, air, like water, has a weight too. It is merely significantly less. As we increase in altitude we have less air above us pushing down, vice versa when we decrease altitude.
Relationship: Quite simple here, if the pressure increases, say from increasing temperature, then the pressure altitude increases. If the pressure decreases, from decreasing temperature, then pressure altitude decreases. Apply this to the relationships above to complete the relationships for this.
Practical Thinking & the Mercury Thermometer: So what is an altimeter setting? It is a measure of pressure based on mercury. Remember how a mercury thermometer works? If the temperature increases the mercury expands, increasing its pressure, and rising up in the thermometer. The opposite occurs when you decrease temperature. So a standard day of 15 degrees C gives mercury a pressure of 29.92.
The Pressure Altimeter: The final piece of this huge puzzle is knowing how your altimeter works. It doesn't know your altitude. Instead it relates altitude to a pressure. When you set the pressure in the kollsman window you are more or less setting the temperature on your altimeters, "mercury thermometer." From there it will determine how high or low you are based on standards.
If you increase the number in the window the altitude indication will increase and vice versa if you decrease it. Just like the mercury rises and falls in a thermometer. So in essence, that altimeter thinks in pressure, thus it is a pressure altimeter.
For you high time guys that might read this in an attempt to clear up some noobie knowledge. Consider the altimeter is just like your FMS. If you don't set it right you might get shot down somewhere over in China.
Scenario: The first reply here, post #2, had a very nice visual picture for flight from high to low and low to high. I will post that again here:
High (Temperature/Pressure) to Low (Temperature/Pressure): Look at the big picture first, what happened? The pressure went down! What did we just say about the altimeter? It relates pressure to an altitude, it doesn't care what the actual altitude is, that was your job to set the starting point.
So in the example, the altimeter is always looking for a pressure of 24.92 to give an indication of 5,000 feet. (29.92 - 24.92 = 5.0 * 1000 = 5,000 feet) So your pressure altimeter concludes that as long as it is at 24.92, with the setting 29.92, it will always be at 5,000 feet.
Well since we lowered the pressure, the altimeter had to decrease the actual altitude to find its pressure of 24.92. The problem is, it still will show you at 5,000 feet because it thinks 24.92 is 5,000 feet, even though you are not. Hence the name, high to low look out below. You are lower than your altimeter is indicating, uh oh!
The reverse is true from low to high, I don't feel like typing it and I doubt you want to read it after all of this.
Calculating Density Altitude: I wanted to throw this in here because it is something that I was always hung up on. When you are trying to find pressure altitude the easiest way to do it is put 29.92 on the bottom and always subtract it. For instance:
29.00
-29.92
-.92 * 1000 = -920 feet. The pressure altitude is minus 920 feet.
30.92
-29.92
1.00 * 1000 = 1000 feet. The pressure altitude is plus 1000 feet.
Hopefully you will never have an issue answering one of those questions again.
Again, apologies for the crazy long post but I hope the information is helpful.
Density Altitude: A term designed to confuse the living crap out of pilots IMO. We will get back to its definition later.
Temperature Effects on Air
Density: Defined as an objects mass per unit volume and figured out by mass/volume. We must ask you to deal with some math here, as we were unable to figure out a simpler way to break this down other than by showing basic division.
Compare: When looking at the density of any object, such as air, a quantity of less dense air will rise above a quantity of denser air. This can be compared to less dense vinegar rising above denser oil.
Increase Heat: When heat is increased, particles begin to move faster and thus the object being heated expands.
Compare: When looking at the density of any object, such as air, a quantity of less dense air will rise above a quantity of denser air. This can be compared to less dense vinegar rising above denser oil.
Increase Heat: When heat is increased, particles begin to move faster and thus the object being heated expands.
Consider Yourself: How do you react when you are hot? Your want more space around you to try and cool off and likely won’t sit curled up in a ball, thus you expand your own body and stretch out.
Volume Increased: Volume is the space an object takes up. You just spread out, taking up more space, right? So, in the case of heating up an, objects volume will increase.
Density Decreases: Let us demonstrate this with simple math; take an object with mass 10 and volume 5, mass of 10 / volume of 5 = density of 2 and let us increase its volume to 10, mass of 10 / volume of 10 = density of 1. Simply put, you can see density has decreased from 2 to 1.
Volume Increased: Volume is the space an object takes up. You just spread out, taking up more space, right? So, in the case of heating up an, objects volume will increase.
Density Decreases: Let us demonstrate this with simple math; take an object with mass 10 and volume 5, mass of 10 / volume of 5 = density of 2 and let us increase its volume to 10, mass of 10 / volume of 10 = density of 1. Simply put, you can see density has decreased from 2 to 1.
Decrease Heat: As an object is cooled, the particles move slower and thus the object shrinks.
Consider Yourself: Well, when we are warm we spread out, so the opposite of course is true when we are cold, we bundle up and if possible, group together.
Volume Decreased: Before you were spread out from being hot and thus, took up more space, now you are curled up in a ball trying to stay warm and take up less space, or less volume.
Density Increases: Again we can use the same math as before, starting with a mass of 10 and a density of 10, mass of 10 / volume of 10 = density of 1 and decrease the volume from 10 to 5 again, mass of 10 / volume of 5 = density of 2. Again you can see a clear increase from 1 to 2.
Volume Decreased: Before you were spread out from being hot and thus, took up more space, now you are curled up in a ball trying to stay warm and take up less space, or less volume.
Density Increases: Again we can use the same math as before, starting with a mass of 10 and a density of 10, mass of 10 / volume of 10 = density of 1 and decrease the volume from 10 to 5 again, mass of 10 / volume of 5 = density of 2. Again you can see a clear increase from 1 to 2.
Demonstration: If you wish to see this in action, you can do so with an empty water bottle. Take the water bottle and run warm water around the sides, with the top off, the air inside will heat up. Do this for a minute and then put the cap on the water bottle. Now open your freezer and stick the bottle inside and watch the bottle as it shrinks. Conversely, take the top off and put it in the freezer, so the air inside cools down, and put the top on it. Now take this bottle of cold air and run it under hot water and feel the bottle expand in your hand.
Now that you have read through that you understand density. Pressure is nothing more than the final demonstration, we will get back to that.An aircrafts performance, especially the engine, is based on the density of the air. If the air is more dense, the engine gets more air and can produce more power. This is true for the prop and the wings generating lift as well. But let's ignore that, just think about the effects on the engine to keep it simpler.
One clear way to know this is true is to consider what we use to increase an engines power, a supercharger or turbocharger. Each of these operates by forcing more air into the engine for more power. Simple.
Let's finally throw pressure into the mix here. Consider the water bottle from before and pressure becomes pretty clear. If we increase heat, the air in the bottle tries to spread out. If there is a wall keeping that air from spreading out, such is the case with the water bottle, then the pressure in the bottle increases. This is evident if you attempt to crush the bottle after heating, it will be difficult to do. The opposite occurs with a decrease in temperature.
Putting it all together, consider:
The Relationships:
Temperature Increase | Volume Increase | Density Decrease | Performance Decrease| Density Altitude Increase | Pressure Increase
Temperature Decrease | Volume Decrease | Density Increase | Performance Increase | Density Altitude Decrease | Pressure Decrease
Back to Density Altitude: Defined as the altitude the airplane will perform as if it was at. It is easier, IMO, to just understand the relationships and how we arrived at those relationships, described above, to more clearly understand density altitude.
One other way to think about it is density altitude varies inversely with actual density. So if actual density goes up the density altitude goes down. Remember this and all that is left is understanding what effects density.
Pressure Altitude: Defined as 29.92 in your altimeter, helpful right?
We just explored what happens to pressure as we go up versus down and how temperature effects that. Review the relationships if your confused. To help think about this consider swimming in a pool. When we swim to the bottom the pressure on our ears goes up drastically, right? The reason is we have more water resting on top of us so to speak.
The atmosphere operates much the same way, air, like water, has a weight too. It is merely significantly less. As we increase in altitude we have less air above us pushing down, vice versa when we decrease altitude.
Relationship: Quite simple here, if the pressure increases, say from increasing temperature, then the pressure altitude increases. If the pressure decreases, from decreasing temperature, then pressure altitude decreases. Apply this to the relationships above to complete the relationships for this.
Practical Thinking & the Mercury Thermometer: So what is an altimeter setting? It is a measure of pressure based on mercury. Remember how a mercury thermometer works? If the temperature increases the mercury expands, increasing its pressure, and rising up in the thermometer. The opposite occurs when you decrease temperature. So a standard day of 15 degrees C gives mercury a pressure of 29.92.
The Pressure Altimeter: The final piece of this huge puzzle is knowing how your altimeter works. It doesn't know your altitude. Instead it relates altitude to a pressure. When you set the pressure in the kollsman window you are more or less setting the temperature on your altimeters, "mercury thermometer." From there it will determine how high or low you are based on standards.
If you increase the number in the window the altitude indication will increase and vice versa if you decrease it. Just like the mercury rises and falls in a thermometer. So in essence, that altimeter thinks in pressure, thus it is a pressure altimeter.
For you high time guys that might read this in an attempt to clear up some noobie knowledge. Consider the altimeter is just like your FMS. If you don't set it right you might get shot down somewhere over in China.
Scenario: The first reply here, post #2, had a very nice visual picture for flight from high to low and low to high. I will post that again here:
High (Temperature/Pressure) to Low (Temperature/Pressure): Look at the big picture first, what happened? The pressure went down! What did we just say about the altimeter? It relates pressure to an altitude, it doesn't care what the actual altitude is, that was your job to set the starting point.
So in the example, the altimeter is always looking for a pressure of 24.92 to give an indication of 5,000 feet. (29.92 - 24.92 = 5.0 * 1000 = 5,000 feet) So your pressure altimeter concludes that as long as it is at 24.92, with the setting 29.92, it will always be at 5,000 feet.
Well since we lowered the pressure, the altimeter had to decrease the actual altitude to find its pressure of 24.92. The problem is, it still will show you at 5,000 feet because it thinks 24.92 is 5,000 feet, even though you are not. Hence the name, high to low look out below. You are lower than your altimeter is indicating, uh oh!
The reverse is true from low to high, I don't feel like typing it and I doubt you want to read it after all of this.
Calculating Density Altitude: I wanted to throw this in here because it is something that I was always hung up on. When you are trying to find pressure altitude the easiest way to do it is put 29.92 on the bottom and always subtract it. For instance:
29.00
-29.92
-.92 * 1000 = -920 feet. The pressure altitude is minus 920 feet.
30.92
-29.92
1.00 * 1000 = 1000 feet. The pressure altitude is plus 1000 feet.
Hopefully you will never have an issue answering one of those questions again.
Again, apologies for the crazy long post but I hope the information is helpful.
12-10-2009, 10:20 AM
#16
Gets Weekends Off
Joined APC: Jun 2009
Posts: 317
Originally Posted by gestrich1311
Just to clarify- I feel the PHAK did a very good job of explaining the interaction of pressure, temperature and humidy. It is specifically the explanation of "hot to cold, look out below" that I feel is lacking or at least my understanding.
12-10-2009, 10:35 AM
#17
Gets Weekends Off
Joined APC: Oct 2009
Position: PT Inbound
Posts: 219
"High to Low = Look out Below!"
With regards, to pressure, I think most people have that concept down however, to understand the temp idea, you must think of the atmosphere where we fly.
When we fly at say 5,000ft, we are actually cruising along the top of a 5,000ft tall column of air. If std temp / pressure existed at the surface, then the ambient pressure at 5,000ft (top of the column) would be 24.92 (29.92 - 1hg/1000ft)).
So to recap, at the top of this column, we are at 24.92 HG and that pressure results in an 5,000ft alt indication on the altimeter.
OK, now this column of air is like any other substance on the planet Earth with regards to heat. If you heat it up, the column will expand and get higher and if you cool it off, it will contract and get shorter thus is where the "high to low look out below" memory aid helps.
If we are flying in a warmer than std area, our "5,00ft indicated alt" column of air is actually larger (taller) because it is warmer than std thus at the top of our column (where we indicate 5,000ft) we are actually higher than 5,000ft due to the temp being high and the column being taller.
The same concept, except reverse, applies to a colder than std column of air.
Finally, if we fly from the our warm air column (high than 5,000ft) to our cold air column, we will actually be descending IN ORDER TO MAINTAIN THE INDICATED 5,000 FT. Thus if you fly from "High to Low" Temp, the people below should "Look out" because you will be descending even though we will still be indicating 5,000ft the entire time.
Hope this helps!
With regards, to pressure, I think most people have that concept down however, to understand the temp idea, you must think of the atmosphere where we fly.
When we fly at say 5,000ft, we are actually cruising along the top of a 5,000ft tall column of air. If std temp / pressure existed at the surface, then the ambient pressure at 5,000ft (top of the column) would be 24.92 (29.92 - 1hg/1000ft)).
So to recap, at the top of this column, we are at 24.92 HG and that pressure results in an 5,000ft alt indication on the altimeter.
OK, now this column of air is like any other substance on the planet Earth with regards to heat. If you heat it up, the column will expand and get higher and if you cool it off, it will contract and get shorter thus is where the "high to low look out below" memory aid helps.
If we are flying in a warmer than std area, our "5,00ft indicated alt" column of air is actually larger (taller) because it is warmer than std thus at the top of our column (where we indicate 5,000ft) we are actually higher than 5,000ft due to the temp being high and the column being taller.
The same concept, except reverse, applies to a colder than std column of air.
Finally, if we fly from the our warm air column (high than 5,000ft) to our cold air column, we will actually be descending IN ORDER TO MAINTAIN THE INDICATED 5,000 FT. Thus if you fly from "High to Low" Temp, the people below should "Look out" because you will be descending even though we will still be indicating 5,000ft the entire time.
Hope this helps!
12-10-2009, 12:37 PM
#19
On Reserve
Joined APC: Dec 2009
Posts: 15
Originally Posted by shdw
Gah, just read this now. So just fast forward to the second half of that write up. Anyways, what does humidity have to do with any of this?
With respect to humidity -
higher relative humidity = lower density of air
Lower relative humidy = higher density of air
12-10-2009, 02:02 PM
#20
Gets Weekends Off
Joined APC: Jun 2009
Posts: 317
Originally Posted by gestrich1311
higher relative humidity = lower density of air
Lower relative humidy = higher density of air
Lower relative humidy = higher density of air
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