What pressure is, and how it is measured
Oxford opens this chapter by quoting the Handbook of Aviation Meteorology: the study of atmospheric pressure may be said to form the foundations of the science of meteorology. That is not flattery. Wind, cloud, altimetry and every chart you will ever be handed are built on what pressure is doing.
Atmospheric pressure is the force per unit area exerted by the atmosphere on any surface in contact with it. The more useful way to picture it is Joshi's: the pressure at any level is the weight of the column of air of unit cross section standing above it, all the way to the top of the atmosphere.
That second picture does the work. Climb, and there is less air left above you, so the column weighs less, so pressure falls with height. Everything on the next page follows from that one sentence.
Static and dynamic pressure
When air is at rest its molecules move randomly in every direction and press equally in every direction. That is static pressure, also called barometric pressure, and it is what this chapter is about. When the air is moving, an extra pressure appears in the direction opposing the flow. That is dynamic pressure, or wind pressure, and it belongs to Principles of Flight rather than here.
The units, and how they relate
The standard unit of force is the newton. Average sea level pressure is 101 325 newtons per square metre, which is 101 325 pascals. Because the older system used millibars, and 1 hPa is exactly 1 mb, aviation writes it as 1013.25 hectopascals and nothing has to be relearned. Joshi adds that 100 000 N/m² is one bar, and a millibar is a thousandth of it.
The other family of units comes from the height of a mercury column in a barometer. These three are the same pressure:
The instruments
Mercury barometer
The basic instrument, and the accurate one. Atmospheric pressure is balanced against the height of a column of mercury, and that height can be read off in any of the units above. It is accurate but bulky and fragile, which is why it lives in a met office and not in an aeroplane.
Aneroid barometer
The compact one. It uses partially evacuated capsules that expand and contract as the pressure outside them changes, with a system of levers turning that movement into a pointer on a scale. It is less accurate than mercury, and far easier to handle, so it is what gets used in practice.
This matters more than it looks. An altimeter is an aneroid barometer with its scale marked in feet instead of hectopascals, plus a subscale you can wind to a chosen pressure. Chapter 9 is entirely about that subscale.
Barograph
Replace the scale with a pen writing on a slowly rotating paper drum and the barometer becomes a barograph, a continuous record of pressure against time. Joshi notes it can be a daily or a weekly drum. What the forecaster reads off it is pressure tendency, the rise and fall of pressure over a period, which is one of the more useful short range forecasting tools there is.
The atmospheric pressure at any level is the weight of the column of air of unit cross-section extending vertically to the top of the atmosphere.IC Joshi, Aviation Meteorology, chapter 2
If pressure is considered as the weight of a column of air of unit cross-sectional area above a surface, then it can be seen that the pressure at the upper surface will be less than that at the lower surface.Oxford ATPL Vol 9, 2.2
How fast pressure falls with height
Pressure falls with height, but not at a steady rate. It falls quickly near the ground where the air is dense, and more and more slowly as you climb. Turn that around and you get the form aviation actually uses: the height you have to climb to lose 1 hPa gets bigger the higher you are.
Joshi gives the fall as a percentage, which makes the shape obvious: about 4 per cent per unit through the first 600 m, about 3 per cent up to 1.5 km, about 2.5 per cent up to 3 km. By 6 km the pressure is half its sea level value, and by 100 km it is negligible enough to be treated as vacuum.
The formula worth memorising
Both books quote the same relation for how much height one hectopascal is worth:
At 1000 hPa and 300 K: 96 × 300 ÷ 1000 = 28.8 ft per hPa.
Read the formula rather than just memorising it. Pressure is on the bottom, so as you climb and P falls, H grows. Temperature is on the top, so warmer air gives more feet per hectopascal and colder air gives fewer.
The pressure levels, and the heights they sit at
Both books ask this from the pressure side as well as the altimetry side, so it is worth having in both directions. In the standard atmosphere:
| Pressure | Height | Flight level |
|---|---|---|
| 1013 hPa | Mean sea level | — |
| 850 hPa | 5 000 ft | FL50 |
| 700 hPa | 10 000 ft | FL100 |
| 500 hPa | 18 000 ft | FL180 |
| 400 hPa | 24 000 ft | FL240 |
| 300 hPa | 30 000 ft | FL300 |
| 200 hPa | 39 000 ft | FL390 |
Read the middle two rows together and the shape of the atmosphere falls out: pressure roughly halves every 18 000 ft. Half of 1013 is about 500, and that is FL180. Half again is 250, near FL340.
The three numbers to carry into the exam
In ISA conditions, one hectopascal is worth roughly:
| Level | Feet per 1 hPa | Where it turns up |
|---|---|---|
| Mean sea level | 27 ft | Joshi's figure. Oxford uses 30 ft for mental arithmetic |
| 2 000 ft | 30 ft | The everyday altimetry number |
| 20 000 ft | 50 ft | Mid level charts |
| 40 000 ft | 100 ft | Cruise, and why levels are far apart in pressure terms up there |
Warm columns and cold columns
This is the single most useful idea in the chapter, and it comes back in altimetry, in upper winds and in every thickness chart you will ever read. Cold air is denser than warm air, so pressure falls faster with height over a cold column than over a warm one.
Follow the consequence carefully, because the two halves sound contradictory until you see them on a diagram.
- A given pressure surface, say 850 hPa, sits at a higher altitude over the warm column than over the cold one, because pressure is falling more slowly there.
- Therefore at any fixed height, the pressure is higher over warm air and lower over cold air.
Isobars, contours and thickness
Isobar
An isobar is a line joining places of equal pressure. On a surface analysis chart the isobars are drawn from corrected mean sea level pressure, which is QFF, and you will meet that on page 5.
Contour
Upper air charts turn the idea round. Instead of showing pressure at a fixed height, they show the height of a fixed pressure. The line joining places of equal height is a contour, and contours can be read exactly like isobars: low height means low pressure, high height means high pressure.
Joshi gives the spacings, and DGCA has asked for them. Contours are drawn every 40 gpm on the 700 hPa and 500 hPa charts, and every 80 gpm on the 300 hPa and 200 hPa charts. A geopotential metre, gpm, is gravitational potential energy per unit mass, where 1 gpm is 9.8 joules per kilogram. Contours are labelled in geopotential decametres, so 5280 gpm is printed as 528.
Thickness
The height interval between two pressure levels is the thickness of that layer, and it is a thermometer in disguise. Low thickness means the layer is cold, high thickness means it is warm. That is why the isopleths of thickness coincide with the isotherms of the layer's mean temperature.
Pressure gradient
The horizontal rate of change of pressure, measured perpendicular to the isobars and directed from high to low. Close isobars mean a steep gradient, widely spaced isobars mean a weak or flat one. Hold on to this, because chapter 10 shows that the pressure gradient is what actually drives the wind.
Isobaric Levels are at Lower Height over Cold Column than over Warm Column. Hence at higher Levels Low Over Cold Column and High over Warm Column.IC Joshi, Aviation Meteorology, Figure 2.2
Warm air will cause pressure to fall slowly with height, whereas cold air will cause pressure to fall rapidly with height. Therefore we would expect the pressure at any given height to be higher over warm air and lower over cold air.Oxford ATPL Vol 9, 2.4
The daily rhythm, and pressure tendency
Pressure at a station does not sit still even when the weather does. It rises and falls twice a day on a fixed schedule, and knowing that schedule is what stops you reading a routine daily swing as an approaching depression.
The curve is bimodal, two peaks and two troughs in 24 hours:
| Local time | What pressure does |
|---|---|
| 0400 | Minimum |
| 1000 | Maximum, the primary one |
| 1600 | Minimum |
| 2200 | Maximum, the secondary one |
Why the peaks are not where you expect
You would reason that the afternoon is hottest, so surface density is lowest, so pressure should bottom out in the afternoon, and that just after sunrise, when it is coldest, pressure should peak. The 1600 minimum fits that, but the timings are about three hours out of phase with temperature, and the second pair of peaks has no thermal explanation at all.
Both books give the same answer, and both admit it is not fully satisfying. The variation is probably a natural oscillation of the atmosphere with a period of about 12 hours, sustained by the 24 hour temperature cycle. Joshi adds the mechanical picture: the air is continuous, so a high on one side of the globe implies a low on the other, and as the earth rotates beneath that pattern a station sees two maxima and two minima a day.
Pressure tendency and isallobars
Pressure tendency is the change of pressure with time. In India it is worked out over the past 24 hours, and in higher latitudes over the past three hours.
Plot it and you get another family of lines. Isallobars join places of equal pressure change. They are read for movement rather than for state:
- The region of greatest fall enclosed by isallobars is an isallobaric low. Surface lows are likely to move toward it and intensify.
- The region of greatest rise is an isallobaric high, and it indicates a weakening pressure system.
QFE, QNH, QFF and QNE
A raw barometer reading is not yet a usable pressure. Before it is broadcast it gets corrected, then reduced to a common level so that two aerodromes at different elevations can be compared at all. Which level you reduce it to, and which temperature you use to get there, is what separates these four codes.
The corrections applied first
Joshi lists three, and they are worth knowing because they explain why a met office reading is trusted and a cheap barometer is not:
- Index correction, for the instrument's own error.
- Gravity correction, because gravity differs with latitude and a mercury column weighs accordingly.
- Temperature correction, because the mercury and the scale both expand.
The four settings
QFE
The pressure measured at the aerodrome reference point. Joshi notes the reference point is the highest point on the runway. With QFE set, an altimeter reads zero on the aerodrome, which is why it is also called the zero setting.
QNH
QFE converted to mean sea level using the ISA temperature and the ISA pressure lapse rate. Because it assumes the standard atmosphere, the correction depends only on the aerodrome's height above sea level and not at all on how hot or cold the day actually is. With QNH set, an altimeter reads aerodrome elevation on the ground. It is always a whole number and always rounded down.
Joshi adds regional QNH, which DGCA asks about: the forecast lowest pressure expected in an altimeter setting region, issued hourly and valid for the hour. Being the lowest expected value, using it errs on the safe side and guarantees terrain clearance.
QFF
QFE converted to mean sea level using the actual temperature, assuming isothermal conditions between the aerodrome and sea level. This is the true mean sea level pressure, so it is what the forecaster plots on synoptic charts and what the isobars on an analysis chart are drawn from.
QNE
The altitude an altimeter indicates with 1013.25 hPa set. Joshi notes it is normally used at high altitude airfields, where a QNH setting could fall outside the subscale range.
QNH against QFF
Since QNH uses the standard temperature and QFF uses the real one, the two only agree when the day is standard. Working out which is larger is a favourite question, and there is a clean way to do it.
Converting between QNH and QFE
Joshi asks this directly, and it is one subtraction once you see it. The gap between QNH and QFE is the aerodrome elevation, expressed in pressure.
160 ÷ 8 = 20 hPa of elevation. The aerodrome is above sea level so it must read lower than sea level pressure: 1005 − 20 = 985 hPa.
The same idea in feet, using 1 hPa as 30 ft: an aerodrome with QFE 950 and QNH 1000 has an elevation of (1000 − 950) × 30 = 1500 ft.
The range of values you should recognise
The lowest recorded in the North Atlantic is 882 hPa, in the eye of Hurricane Wilma in 2005. If a question offers you a sea level pressure outside roughly 950 to 1050, it is testing whether you know the normal range.
QNH is always a whole number without any decimal places and is always rounded down. When on the aerodrome with QNH set the altimeter will read aerodrome elevation.Oxford ATPL Vol 9, 2.7
QFF. It is the barometric pressure of an aerodrome reduced to msl, assuming the temperature of the place to be the temperature of the column of air extending up to the msl. This value is used for plotting on Synoptic charts and drawing isobars.IC Joshi, Aviation Meteorology, chapter 2