Pressure & temperature

I'm confused about what determines the local altimeter setting. My
understanding is that there is a barometer in the tower that measures
the weight of a column air extending up into the atmosphere. What
factors affect its weight? Is it purely temperature variations caused
by uneven heating by the Sun or is there something else?

In a similar vein, I understand that winds are caused by pressure
discrepancies which lead to winds moving from high to low pressure
areas. Again, are the pressure differences caused by temperature only?

Thanks.

On Oct 20, 8:57 pm, Terence Wilson wrote:
I'm confused about what determines the local altimeter setting. My
understanding is that there is a barometer in the tower that measures
the weight of a column air extending up into the atmosphere. What
factors affect its weight? Is it purely temperature variations caused
by uneven heating by the Sun or is there something else?

In a similar vein, I understand that winds are caused by pressure
discrepancies which lead to winds moving from high to low pressure
areas. Again, are the pressure differences caused by temperature only?

Thanks.

Uneven solar heating is the primary cause of all weather on earth.
There could be other sources, such as fluctuations in the earth's
magnetic field and gravity of the moon, but solar heating is the
primary source.

Andrew Sarangan wrote:
On Oct 20, 8:57 pm, Terence Wilson wrote:

I'm confused about what determines the local altimeter setting. My
understanding is that there is a barometer in the tower that measures
the weight of a column air extending up into the atmosphere. What
factors affect its weight? Is it purely temperature variations caused
by uneven heating by the Sun or is there something else?

In a similar vein, I understand that winds are caused by pressure
discrepancies which lead to winds moving from high to low pressure
areas. Again, are the pressure differences caused by temperature only?

Thanks.


Uneven solar heating is the primary cause of all weather on earth.
There could be other sources, such as fluctuations in the earth's
magnetic field and gravity of the moon, but solar heating is the
primary source.




What about varying ocean temperatures?

On Oct 21, 4:51 am, Bee wrote:
Andrew Sarangan wrote:
On Oct 20, 8:57 pm, Terence Wilson wrote:

I'm confused about what determines the local altimeter setting. My
understanding is that there is a barometer in the tower that measures
the weight of a column air extending up into the atmosphere. What
factors affect its weight? Is it purely temperature variations caused
by uneven heating by the Sun or is there something else?

In a similar vein, I understand that winds are caused by pressure
discrepancies which lead to winds moving from high to low pressure
areas. Again, are the pressure differences caused by temperature only?

Thanks.

Uneven solar heating is the primary cause of all weather on earth.
There could be other sources, such as fluctuations in the earth's
magnetic field and gravity of the moon, but solar heating is the
primary source.

What about varying ocean temperatures?- Hide quoted text -


Oceans are heated by the sun. This is why oceans near the equator are
warmer than at higher lattitudes. However, there could bea minor
effect from underwater volcanoes which are not directly related to the
sun, at least not since the earth separated from the earth billions of
years ago.

On Sun, 21 Oct 2007 20:06:30 -0400, "An Aviator"
wrote:

Current temperature is not used to convert
pressure to altimeter settings.

Can you please elucidate? Isn't current temperature implicitly used if
we use a barometer to measure local pressure and then extrapolate the
pressure at sea level (to be used in the Kollsman window)?

On Oct 21, 7:17 pm, Terence Wilson wrote:
On Sun, 21 Oct 2007 20:06:30 -0400, "An Aviator"

wrote:
Current temperature is not used to convert
pressure to altimeter settings.

Can you please elucidate? Isn't current temperature implicitly used if
we use a barometer to measure local pressure and then extrapolate the
pressure at sea level (to be used in the Kollsman window)?

The altimeter will read correctly at the place where the baro setting
was determined.

Above that, all bets are off! All those factors affect the lapse
rate.

And it's surprising how far it can be off.

Bill Hale

"Terence Wilson" wrote in message
...
On Sun, 21 Oct 2007 20:06:30 -0400, "An Aviator"
wrote:

Current temperature is not used to convert
pressure to altimeter settings.

Can you please elucidate? Isn't current temperature implicitly used if
we use a barometer to measure local pressure and then extrapolate the
pressure at sea level (to be used in the Kollsman window)?

The altimeter is calibrated using the ISA and the temperature under ISA is
15C.

Setting the altimeter to read airfield elevation in no way is a real
representation of what the sealevel pressure is precisely because no
adjustment is made for temperature.

Have a look at the cold weather adjustments needed for instruments
approaches. As the temperature deviates from ISA whilst the altimeter could
be reading 200' you will actually be well under and enough to bust a
checkride. For example, an ISA deviation of -15C is an correction of 12'.
approximately 4ft/1000ft for each C of difference. In this case, -15C ISA
is only a temperature of 0C or 32F.

S Green wrote:


Have a look at the cold weather adjustments needed for instruments
approaches. As the temperature deviates from ISA whilst the altimeter could
be reading 200' you will actually be well under and enough to bust a
checkride. For example, an ISA deviation of -15C is an correction of 12'.
approximately 4ft/1000ft for each C of difference. In this case, -15C ISA
is only a temperature of 0C or 32F.



That isn't done for instrument approaches in the U.S. except for VNAV
final segments on RNP SAAAR IAPs.

"Bee" wrote in message
...
S Green wrote:


Have a look at the cold weather adjustments needed for instruments
approaches. As the temperature deviates from ISA whilst the altimeter
could be reading 200' you will actually be well under and enough to bust
a checkride. For example, an ISA deviation of -15C is an correction of
12'. approximately 4ft/1000ft for each C of difference. In this
case, -15C ISA is only a temperature of 0C or 32F.

That isn't done for instrument approaches in the U.S. except for VNAV
final segments on RNP SAAAR IAPs.

In Europe its the easiest way to fail an IR checkride by failing to
compensate for the ISA deviation in the winter and being deemed to have
busted the limits

akjcbkJA wrote:

"Bee" wrote in message
...

S Green wrote:


Have a look at the cold weather adjustments needed for instruments
approaches. As the temperature deviates from ISA whilst the altimeter
could be reading 200' you will actually be well under and enough to
bust a checkride. For example, an ISA deviation of -15C is an
correction of 12'. approximately 4ft/1000ft for each C of
difference. In this case, -15C ISA is only a temperature of 0C or 32F.


That isn't done for instrument approaches in the U.S. except for VNAV
final segments on RNP SAAAR IAPs.


In Europe its the easiest way to fail an IR checkride by failing to
compensate for the ISA deviation in the winter and being deemed to have
busted the limits

Our FAA isn't that smart.