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# Pressure Temperature relationships

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# Pressure Temperature relationships

11th May 2019, 06:05

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Pressure Temperature relationships

Hello everyone, I am going to be writing my CPL exam soon and had a quick meteorology question, it may sound silly so I apologize in advance. There are many temperature and pressure relationships (Northern Hemisphere, although it probably wont make any difference but though of adding it either way). For example:

1) As height increases, Pressure always decreases, due to the weight of the atmosphere decreasing but the Temperature varies from layer to layer.

2) When True Altitude is higher than Indicated we can see that this happens when both Pressure and Temperature increases

3) Then comes Pressure Systems, High Pressure Systems usually means Cold Temperatures and Low Pressure Systems usually has High Temperatures

I may be missing out something obvious but I would really appreciate some help to get me back on track. Thank you!
11th May 2019, 17:18

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Not exactly sure what the question is but I'll just deal with your statements.
1. This is correct.

2. There are really two parts of this. On the surface, it is true that True Altitude depends on correction of both temperature and pressure, but in slightly different ways. In the following examples, we will assume that Indicated Altitude is the same as Calibrated Altitude. Remember, too that, in aviation, we're really not concerned with what we might hit above us. We're always concerned with how far we are above stuff. So, I've framed my answers in that context.

a.) With respect to Pressure, at a given temperature, pressure decreases relatively logarithmically with altitude from the point where the pressure is taken. Altimeters adjust for the logarithmic decrease. So, if you increase your altitude directly above the point where the pressure is taken and your altimeter is correctly set before you start climbing, your True Altitude will be the same as your Indicated Altitude throughout the climb. There may be a minor error due to local pressure but it is relatively small. However, if you fly to an area of higher pressure to lower pressure without adjusting your altimeter, your True Altitude will be lower than your Indicated Altitude. From a safety perspective, it's better to think of this as "From High to Low, look out below!"

b) From the perspective of temperature, it's more like, "If the temp is low, look out below". At very low temps, your True altitude may be significantly lower than your Indicated Altitude It doesn't really matter if you MOVE from a warmer temperature to a colder one. What really matters is the local temperature at the station making the altimeter setting measurement.

Because the column of air between you and the ground is denser in cold air, it makes up a greater proportion of the entire column of air above the station. In a standard atmosphere, when you're 2500 ft above the reporting station you have, for lack of a better word, 2500 gronks of air between you and the station. But, in cold air, 2500 gronks fit into a much smaller space. That smaller space translates into a lower altitude. So, in this case, it doesn't matter if you move from a warmer to a colder area.... if it's cold, your True Altitude is lower than your Indicated Altitude.

One important piece of this is that the temperature factor depends a lot more on your height above the reporting station because it's only the gronks between you and the ground station that matter. The ICAO Pans Ops calculation is 4 ft per 1000'ft (above the station) per degree below ISA. At 200 ft and - 15c, the error is 4*(200/1000) * 30... 24 ft. But at 5000 ft, the error is 4 * (5000/1000) * 30 ... 600 ft.

Let's look at a scenario where you wanted to fly above the CN Tower (2050 ASL, 1815 AGL) at 3100 ft to clear it by 1000 ft. The temperature is -20c. At your indicated altitude of 3100 ft, your "indicated height" AGL is 2850 ft. But using the calculation, your True Altitude is 400 ft lower than your Indicated Altitude and you would only be at 2700 ft, clearing the tower by only 650 ft.

So, to reiterate, if you are flying from high pressure to low pressure without adjusting your altimeter, your True Altitude becomes lower than Indicated. If you are simply flying anywhere in a cold area, your True Altitude is lower than Indicated.

3. Again, mostly true.

High pressure systems rotate clockwise in the Northern Hemisphere. That means that they tend to have northerly winds ahead of them, lowering temperatures. In addition, high pressure generally means clearer skies, meaning more radiation cooling at night. These two factors tend to give cooler temperatures. Sunny skies during the day offset this to some extent. After the passage of the high pressure system, the return flow around the High will warm things up. Sometimes a lot.
Low pressure systems rotate counter clockwise providing a leading warmer southerly flow, especially when accompanied with a warm front. They tend to have more overcast conditions so less cooling at night. But they often follow with a cold front dropping temperatures after the passage of the Low.
So, to be more correct, temperature AHEAD OF and during High Pressure Systems tends to be colder and temperature AHEAD OF and during Low Pressure Systems tends to be warmer. Once these systems pass, the reverse flow will have the opposite effect.

Hope this helps.
13th May 2019, 03:52

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Thank you SO MUCH! That really cleared things up for me. Any tips for my CPL Exam?
14th May 2019, 01:08

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Sorry... it's been 42 years since I wrote mine. No idea what's on there nowadays.

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