Shouldn't the equator consider a low pressure area?

 

FF,

You've got blobs of air moving around the country. On any given day MSP or DEN may have a higher or lower pressure. But to know what is going on, the NWS has a protocol to bring every thing down to sea level and then we mere pilots can use that info to miss the mountains.

R1830 - I have a couple thousand hours behind you. Yeah, it's shocking but we are not the center of the universe.

 

The atmosphere is much thicker at the equator (extends to a higher alt).

 

Who still sets a Kholsman window!? Even my standby altimeter has a digital display.

 

Huh!? I couldn't imagine this, can you explain?

 

Due to its rotation, the whole earth is fatter at the equator, with a diameter about 27 miles greater than at the poles.

 

True, but a quick google search about "atmospheric circulation" explains the Low pressure at the equator due to the sun heating

 

The atmosphere is thicker (extends to a higher altitude) because a warm air mass is less dense than a cold air mass. A warm air mass will expand when heat is added. Since the air at the equator is usually hotter than its surroundings, the air at the equator expends vertically. This extra vertical expansion is why the atmosphere extends to a higher altitude at the equator.

In addition to a higher topping atmosphere, when the air at the equator is heated (from the hot surface), the air rises causing uplift. This rising air causes an area of low pressure to form at the surface as the air rises. This uplift causes air to be sucked in at the surface towards the equator. Air from the north and air from the south "converge" on the equator where this area of low pressure forms. This area of convergence near the equator is called the "InterTropical Convergence Zone" (ITCZ).

The Earth is, in fact, larger at the equator than it is at the poles due to its rotation. Although assumed to be a perfect sphere, the Earth is actual an oblate spheroid. While this is true, this is not the reason why the atmosphere extends to a higher altitude at the equator.

 

Since pressure changes with elevation, in order to compare relative high and low air pressure areas, they all must account for this elevation-related pressure change. Otherwise, there would be a permanent low pressure area over all the mountains (where the measuring station is at relatively high elevation) and a permanent high pressure in all the valleys (where the measuring station is at relatively low elevation) and no relative comparisons would be able to be made.

The way this is accounted for is by "reducing" all measured atmospheric pressure readings down to sea level. This way, since all pressure readings will be based on the SAME altitude(ie. sea level), all altitude related pressure anomalies will be removed and a pure air mass pressure comparison can be made. This how high and low pressure air masses are identified and quantified.

The way this is accomplished is by using an average assumed atmospheric pressure change for a given altitude. This assumed value (which is the same assumption aviation altimeters work with) is that pressure will decrease 1 inch of Hg for every 1,000ft of altitude above sea level.

An example is on a standard (29.92) day, the actual measured pressure at Denver (assuming 5,000 ft elevation) would be 24.92 because, assuming 1 Hg / 1,000ft pressure decrease, the pressure would be 5 inches of Hg lower than what it would be at sea level due to the 5,000ft elevation above sea level.

So, when you read an altimeter setting in Denver, you are actually reading the measured pressure at Denver REDUCED (using the 1 Hg/1,000ft assumption) to sea level (zero altitude). This reduction done to altimeter settings is what allows us to always know our height above SEA LEVEL (not just the local area elevation).

In the OP' original question, QNH is the altimeter setting reduced to sea level and the QFE is the ACTUAL measured pressure at the field (no sea level reducion = includes altitude-related pressure decreases).

So, on a standard day in Denver (5,000 ft field elevation:

QNH = 29.92 Hg (reduced to sea level)
- Altimeter would read 5,000 ft when set to QNH = Altimeter would read altitude above sea level

QFE = 24.92 Hg (NOT reduced to sea level = ambient pressure)
- Altimeter would read zero ft when set to QFE = Altimeter would read altitude above ground level

 

Fly_Boy_Knight thanks for the great explanation!

still not sure why at ADDIS ABEBA airport (7700ft elevation) a QNH would always be high as 1028 hPa.. I will convince myself it is a high pressure area as galaxy flyer stated (yet cant figure how a high pressure can occur at equator).

Again really thankful for such great explanation