Pressure, Density and the ISA - ATPL 050 — Meteorology

Temperature had the first two lessons; pressure and density get this one. Pressure is the quantity the whole of practical meteorology is drawn in (every surface chart is a map of it), and density is the quantity aircraft actually fly on. The lesson ends by freezing both into the standard atmosphere, the ISA, whose numbers the first lesson already used and the altimetry lesson will spend.

Pressure, and how it is measured

Atmospheric pressure is the force exerted by the weight of the column of air above a unit of area: stand anywhere, and the whole depth of the atmosphere presses down on you. Aviation measures it in hectopascals (hPa), with inches of mercury surviving in American practice, and the instruments are the two classic barometers:

Mercury barometer — the air column balances a column of mercury whose height reads the pressure.
Aneroid barometer — a partly evacuated capsule that is squeezed as pressure rises and relaxes as it falls, driving a needle: the mechanism inside every altimeter.

The shapes on the chart

On a surface weather chart, pressure is drawn with isobars: lines joining points of equal pressure, normally at 2 or 4 hPa intervals. Where the isobars close into rings, they name the chart's main characters:

High — a region of pressure higher than its surroundings, closed isobars around a centre marked H.
Low — the opposite: closed isobars around a centre of lower pressure, marked L.
Trough — an elongated extension of low pressure reaching out from a low, the isobars kinking along its axis.
Ridge — an elongated extension of high pressure reaching out from a high, the isobars bulging along it.
Col — the saddle of nearly uniform pressure between two highs and two lows, where the chart goes quiet.

Pressure and density with height

Pressure falls with height for the simplest of reasons: climb, and there is less atmosphere left above you to weigh down. The fall is not linear: the lower air, compressed by everything above it, is densest, so each foot of climb near the surface removes more weight than a foot higher up. Near mean sea level (MSL) the exam value for this barometric lapse rate is 1 hPa for every 30 ft (9 m) of height, and the figure grows as the air thins — to roughly 50 ft per hPa by 18,000 ft, and ever larger above. The anchor percentages are exam property too: pressure is about 50 per cent of its MSL value at 18,000 ft, while density is about 50 per cent at 22,000 ft and 25 per cent at 40,000 ft (in ISA conditions).

Why does density lag behind pressure on the way up? Because of the relationship between pressure, temperature and density: density rises with pressure and falls with temperature. Climbing, the falling pressure thins the air, but the falling temperature simultaneously packs it tighter, so density falls with height more slowly than pressure does, which is exactly why its 50-per-cent level sits 4,000 ft above pressure's.

QFF and the surface chart

A station in the mountains measures a much lower pressure than one at the coast, without any weather being responsible: height alone explains it. To make stations comparable, the measured station pressure (QFE, the pressure at aerodrome or station level) is reduced to mean sea level: QFF is the station pressure reduced to MSL using the actual temperature conditions at the station — as if the atmosphere continued, as it really is, down to the sea. Surface weather charts are drawn in QFF: every isobar of the previous section joins equal QFF values, so the chart shows weather, not topography. (The altimeter's own MSL reduction, QNH, uses ISA assumptions instead; the next lesson separates the two.)

Columns of warm and cold

The cross-section explains how surface systems and upper-air systems are related. A cold column is a short, dense column: its isobaric surfaces huddle near the ground. A warm column stands tall, carrying its surfaces high. Fly at a constant height across the frame and the pressure rises toward the warm side: aloft, warm columns wear highs and cold columns wear lows, whatever the surface chart beneath them shows. A surface high can therefore weaken with height (a cold, shallow winter high) or strengthen (a warm, deep subtropical high): the temperature of the column decides which, and the pressure-systems module returns to both.

The International Standard Atmosphere

Real pressure, temperature and density change hourly, yet altimeters must be built, performance calculated and aircraft compared against something fixed. That is the use of standardised values: the ISA (International Standard Atmosphere) is the agreed reference atmosphere, and its main values are pure exam recall:

MSL pressure 1,013.25 hPa — the standard altimeter setting.
MSL temperature +15 °C — the starting point of every ISA temperature sum.
Lapse rate 0.65 °C per 100 m (2 °C per 1,000 ft) — up to the tropopause, isothermal above it to FL 650 (flight level 650).
Tropopause at 11 km (36,090 ft), −56.5 °C — the corner in the profile the first lesson drew.

What the exam asks

The chart vocabulary and ISA values are recall; the barometric lapse rate and the percentage anchors feed calculations, and the warm-column/cold-column logic is the understanding question that returns in every later module. The overview, following the learning objectives:

Define atmospheric pressure, its units and the two barometers that measure it

Identify isobars, highs, lows, troughs, ridges and cols on a surface chart

Explain the pressure variation with height, the 30 ft per hPa rule and its growth, and the 50%/50%/25% anchors

Describe the pressure-temperature-density relationship and density's vertical variation

Define QFF, explain the reduction from QFE, and state its use on surface charts

Illustrate how surface and upper-air pressure systems are related through column temperature

Explain the use of the ISA and list its main values