Driver170, I know it's confusing, and many syllabus do not help and have huge nonsense written on them... like some aviation books describing the origin of the lift force...

 

Well, there's that old saying - "from high to low, watch out bellow"  -  Just think that it has two equally valid applications, pressure side ( A ) and temperature side ( B  )  

 

 

A ) If you fly from a high pressure area to a low pressure area, irrespective of the temperatures, you'll be lower in terms of height from terrain than your altimeter tells you are when using a same altimeter setting ( that's why the controllers give you area settings, and transition altitudes and levels are used too... ).

 

B ) If you fly from a region of higher temperature to one of lower temperature, you will also be lower than you think when the sea level pressure is the same or lower on the colder region.

 

Basically, what you have to memorize is that travelling from high pressure areas to low pressure areas maintaining the altimeter settings can put you in danger because in terms of actual separation from terrain you will be lower than you think!

 

And... if you're travelling from a higher temp region to a lower temp region, even if the sea level altimeter setting is the same on both regions, because the troposphere "shrinks" in the colder regions, all of the pressure levels will suffer a deflection down, and so you will actually be lower than what your altimeter tells you are in terms of separation from terrain. 

 

You might also find THIS an interesting read...

I think you are confusing a region with a general barometric pressure with pressure increasing as one descends.

 

If you measure the pressure at sea level and then climb to 18,000 feet MSL and measure it again the pressure reading will be roughly half of the reading taken at sea level.

 

The reading one takes at sea level will vary from hour to hour as the properties of the mass of air over that point on the ground change.

 

Once you get past the theory, what is far more important is what JComm posted above in the application of this knowledge.

 

When flying VFR the risk is not likely that one will fly into terrain in daytime, but you may be off altitude and in the path of other IFR or VFR traffic if you fail to adjust your altimeter. You also need to consider the performance impact on your aircraft especially if landing in hot locations at lower pressures and at higher altitudes. Think high density altitude.

 

When flying IFR failure to properly account for pressure changes can be the difference between life and death in IMC.

One example of where changes in true altitude from temperature variations causes problems is chicago o'hare. On one of the ILS approaches, if one follows the glideslope down, one can be potentially violated for an altitude deviation for crossing a fix below the minimum altitude. On a warm day, if you follow the glideslope, you will indicate a lower altitude at each fix since your path is refrenced to the ground while the atmosphere is expanding higher above you. So if you end up crossing a fix on glideslope, but indicating 6900 when the chart has a minimum of 7000 for that fix, atc can peg you with a violation. They like to do that at chicago due to Midway traffic that cross underneath Ohare arrivals. If you show 6900 on glideslope when there is a southwest 737 beneath you at 6000, then a loss of separation will have occured and the controller will not be the one who takes the hit for it when they can blame you for indicating 100 feet below what the chart states as the minimum.

Thanks for the response guys. I already know about from high to low watch out below and how temp affects altitude readings.

 

But my issue is - in a low pressure system is the air cold or hot?

 

Kevinau wouldn't that be impossible as you will have the local setting set into your barometer from ATC?

No in low pressure the air is neither cold nor hot. You can have the same low pressure in the coldest days of winter and in the hottest days of summer.

 

The pressure of the air mass is not the same as the relative pressure change as you climb and descend within that same air mass.

 

Under ideal conditions, as the air pressure decreases as you climb, the temperature will drop and vice versa. This is true as you move up or down in a column of air with the same general properties.

 

As you fly across the country you will encounter different air masses with vastly different properties including their humidity, pressure and temperature. The temperature of that new air mass does not dictate whether or not it will be higher or lower pressure.

 

What KevinAU is saying that one might be following the glideslope during the approach without noting their altitude is lower than required over a specfic point on the ground on a day with a high density altitude. The altimeter does not tell you how high you are above the ground even after it is adjusted for the current barometric pressure setting. That is only the case on a standard day.

 

Gets an example I found on skyvector.

 

KAVP is reading 29.84 at 9C.

KTMB 29.88 at 29C

 

If you check a prog chart you will find the pressure over Florida is closer to standard and the New York area is a little lower. The Temps and air masses are very different as NYC is getting hit with a noreaster right now with cold wet air.

Exactly,

 

don't try to establish a cause an effect relationship between temp and press.

 

You can think of the Azores Anticyclone, usually associated with higher temps, but also the Siberian Anticyclone, with rather cold air...

 

You can have thermal depressions, with the air that sinks in it's borders getting compressed and even hotter or cut-off lows with rather cold air on them compared to the air surrounding them...

 

It depends on where you are, what epoch of the year, the air masses involved, etc...

So if its winter and you have low pressure the air is rising because its unstable and the DALR is warmer than the ELR?

JComm, I would say it is okay to make that connection between temp and pressure but only in the vertical and in the local area.

 

That will matter to understand the theory and application in determining cloud base heights, identifying temp inversions, deriving true airspeed and altitude.

 

I'm not saying you need this to fly the airplane, but being able to form a mental picture using the raw data is of great value for safety and planning IMHO.

True Oracle, but when we add to the "equation" advection and subsidence.... then it is even easier to understand it... I just didn't want to add even more "variables" :-)

 

Driver, it depends on the mechanism that created the depression. Again you're trying to see it just in terms of temperature gradients, which in the relation between DALR and ELR matter in terms of convection... but cold air can simply be advected ( ported "horizontally" ), or subside from the upper levels of the troposphere. A good example are the cut-off lows, just to give you an example that involves exactly the opposite - colder air... 

 

Now, colder air being brought, by advection, or subsidence, will create instability... and then you can think of lapse rates, convection, convective cloud formation and associated precipitation, etc...

 

On a frontal system you have a mix of air masses of differing temperatures... In such a type of system, there is a depression system associated, and you will be able to notice the pressure, wind direction and temperature variations as the various areas of the frontal system cross a given point.

 

It depends on the origin of the high or low pressure area.

I mean't to say the DALR is greater than the ELR

Correct on DALR AND ELR.

 

No, air does not rise because of lower pressure. Air rises because it is less dense than the air around it. That happens when it is heated or if it is forced up by mechanical effect. How does air get hotter? The answer is from the heating effect of the sun. A cold front passing through will force warmer air upward. This warmer air bring pushed up into cooler air above will keep rising until it is stable. If the DALR<ELR the column of air is unstable, if the other way around then it will stop rising once the air density has equalized.

Well the DALR 1.5c every 1000ft and the ELR is 2c every 1000ft

Hehe jcomm, I keep thinking, "Should I bring those up?"

We're bouncing around a little now Driver170. Not sure if you're clear on pressure and temp dropping as you go up and increasing as you go down in a column of air?

 

ELR is not a constant. The value at a given moment in time for a given parcel of air in a given column of air is what determines if the air is stable or not.

I thought air rises in a low pressure system? Its warm and it rises then cools and drops creating high pressure system?

 

I understand pressure drops every 30mb for 1000ft and if its cold the pressure drops quicker with alt gain than if the air is warm.