Pressure & altimetry

B. Under-read

Flying towards higher pressure, the altimeter sees more pressure and shows a lower height than you actually have - it under-reads, and you are higher than indicated. The dangerous case is the reverse: "High to low, look out below."

B. Isobars closely packed and strong winds

Steep gradient = pressure changes quickly over a short distance = isobars crowded together = strong wind.

B. QNH

QNH is the aerodrome pressure reduced to MSL using the ISA, so with QNH set the altimeter shows height above mean sea level - on the ground, the aerodrome elevation. With QFE set it would read zero.

B. Bad weather

Air converges into a low and rises. Rising air cools, reaches saturation and builds cloud and precipitation - hence bad weather. A high does the opposite: sinking air warms and dries.

A. QFE reduced to mean sea level using the ISA

QNH = aerodrome pressure brought down to MSL assuming ISA conditions. When the actual temperature is used instead, the result is QFF (the value on weather charts). 1013.25 hPa is the standard setting used for flight levels.

A. Trough

A trough is a line of lowest pressure running out from a low. Move away from it at right angles on either side and the pressure goes up. (Across a ridge, it falls.)

B. 1003.0 to 1003.9 hPa

QNH is rounded DOWN to the whole hectopascal, so 1003.0 up to 1003.9 all go out as Q1003. Rounding down keeps the altimeter error on the safe side.

A. Altimeter

The altimeter is a barometer calibrated in feet: it uses the standard relationship between pressure and height to turn the pressure it senses into a height.

A. Isobars

Isobars join equal pressure, isotherms equal temperature, isohyets equal rainfall, isallobars equal pressure tendency. Closely spaced isobars mean strong wind.

C. Pressure tendency

The correct answer is: Pressure tendency.

B. Decrease

Pressure falls as a cold front approaches (the altimeter reads higher) and rises sharply after it passes (the altimeter reading drops). A rise in pressure always makes a fixed-setting altimeter show less height.

A. Lower

The correct answer is: Lower.

C. Pressure datum set on its sub-scale

An altimeter is a barometer. It reads height above whatever pressure level you set: with QNH, height above MSL; with QFE, height above the aerodrome; with 1013.25, a flight level.

C. At mean sea level

QNH and QFF are both QFE corrected down to sea level. If the aerodrome is already at sea level there is nothing to correct, so all three are equal whatever the temperature.

A. 1022 hPa

At an aerodrome at MSL, QFE, QNH and QFF are all the same value, because no height correction is needed.

C. QNH

In a METAR, Q followed by four figures is the QNH in hectopascals. (A followed by four figures is the altimeter setting in inches, e.g. A2992.)

C. Less rapid

Warm air is less dense, so you must climb further to lose each hectopascal - pressure falls more slowly. That is why a pressure level is higher in warm air, and why an altimeter over-reads in cold air.

A. A high

Highs are stable: sinking air forms an inversion that traps haze, smoke and fog near the ground, and the light winds cannot clear it. Lows bring cloud and rain but usually cleaner air between showers.

A. Aneroid

The aircraft altimeter is an aneroid barometer: a sealed, partly evacuated capsule expands and contracts with pressure, and the movement drives the pointers.

C. 40 000 ft

Useful ISA pairs: 700 hPa ≈ 10,000 ft, 500 hPa ≈ 18,000 ft, 300 hPa ≈ 30,000 ft, 200 hPa ≈ 40,000 ft.

A. Barometer

A barometer measures pressure (a barograph records it continuously). An anemograph records wind and a ceilometer measures cloud base.

B. 5 000 ft

Standard pressure levels: 850 hPa is about 5000 ft, 700 hPa about 10,000 ft and 500 hPa about 18,000 ft.

B. Ascending and converging

At the surface air spirals in towards the low (convergence) and, having nowhere else to go, rises. That rising air is the source of the cloud and rain around a low.

B. Higher

QFE is the actual pressure at the aerodrome. QNH is that pressure reduced to sea level, and pressure increases as you go down, so QNH is higher - by about 1 hPa for every 27-30 ft of elevation.

B. Lower

A trough is an elongated extension of low pressure, so the pressure along its axis is lower than on either side.

C. The weight of the air above (gravity)

Pressure at any point is the weight of the column of air above it, pulled down by gravity. That is why it always falls as you climb - there is less air above you.

A. The change of pressure with horizontal distance

The horizontal pressure gradient is how quickly pressure changes across the ground, at right angles to the isobars. It drives the wind. Change with time is pressure tendency.

A. 1013.25 hPa

Flight levels are pressure altitudes: every aircraft sets the standard 1013.25 hPa so that all are measured from the same datum and vertical separation holds.

A. Clear skies with haze, mist or fog and poor visibility

Sinking air in a high is stable and dry above, with light winds. Smoke, dust and moisture get trapped under the subsidence inversion, so a winter high over land means clear skies but poor visibility, haze and fog.

C. 80 gpm

Contours on the 700 and 500 hPa charts are drawn every 40 geopotential metres; on the 300 and 200 hPa charts, every 80 gpm.

A. 660 ft

Setting 1013 instead of 993 is 20 hPa too high, and at about 27 ft per hPa the altimeter over-reads by about 540 ft. So the elevation is about 1200 - 540 = 660 ft.

B. 3006 ft

Lowering the sub-scale by 22 hPa lowers the reading by about 22 × 27 ≈ 594 ft: 3600 - 594 ≈ 3006 ft. Lower setting, lower reading.

A. Higher

The sub-scale is 4 hPa higher than QNH, and each hPa of excess setting adds about 27 ft to the reading, so the altimeter over-reads by roughly 110 ft - the aircraft is lower than the altimeter says.

A. FL340

Standard pressure levels: 500 hPa is about FL180, 300 hPa about FL300, 250 hPa about FL340 and 200 hPa about FL390.

B. 33 hPa

Near sea level pressure falls by about 1 hPa for every 27-30 ft, so 1000 ft of height is worth about 33 hPa.

A. The one over the warm air mass

Cold air is dense, so pressure falls quickly with height and the aircraft sits lower than its altimeter says. Over warm air the pressure levels are spread further apart, so the same indicated altitude is genuinely higher. Same reading, two different true altitudes.

B. QFF

A surface analysis chart is drawn in QFF - aerodrome pressure reduced to mean sea level using the actual conditions of the day. QNH is the equivalent reduced using ISA, which is why the two differ.

A. 1335 ft

The sub-scale is 5 hPa above QNH, and near the ground 1 hPa is worth about 27 ft, so the altimeter over-reads by roughly 135 ft: 1200 + 135 = about 1335 ft. Setting a higher sub-scale always gives a higher reading.

A. 540 ft

QNH minus QFE is the pressure difference between mean sea level and the aerodrome: 20 hPa. At about 27 ft per hPa that is 20 x 27 = 540 ft of elevation.

Our independent check found that more than one option can be argued for, as worded in the recall. The key gives the answer most references support.

A. Land areas strongly heated in summer

A thermal low is made by heating, not by fronts: the ground warms, the air above it expands and rises, and the surface pressure falls. That is the origin of the seasonal low over north-west India and Pakistan in summer, which is the engine that pulls in the south-west monsoon.

B. 900 ft

The correct answer is: 900 ft.

A. Higher temperature

Warm air is less dense, so the pressure levels spread apart and each one lies higher than the standard atmosphere says. The altimeter goes on reading the standard height for that pressure, so the aircraft is actually HIGHER than indicated - the altimeter under-reads. The dangerous case is the reverse: from high to low, look out below.

B. The approach of a cyclonic storm

Pressure tendency is a forecasting tool: a steep, sustained fall means a low is deepening or approaching, and the steeper the fall the stronger the system. Lines joining equal pressure change are isallobars, and the region of greatest fall is the isallobaric low.

B. Less

Pressure has a twice-daily (semi-diurnal) swing with maxima near 1000 and 2200 and minima near 0400 and 1600 local time. So 1600 sits at a minimum and 1000 at a maximum, and the afternoon value is the lower of the two. The swing is largest in the tropics.

C. 30 000 ft

Working the standard atmosphere down from 1013.25 hPa, the 300 hPa surface comes out at about 30 065 ft, so 30 000 ft is the figure to carry. It is a useful one to know because the 300 hPa chart is the standard upper-wind chart for jet cruising levels.

B. 33 hPa

Near sea level 1 hPa is worth about 30 ft, so 1000 ft costs roughly 1000/30 = 33 hPa. The relationship is not constant - higher up the air is less dense and 1 hPa covers far more height - which is why altimetry corrections are always quoted for low level.

B. Barometer

A barometer measures atmospheric pressure (the recording form is a barograph). An anemograph records wind speed and direction, and a hygrograph records humidity - the prefixes give them away: anemo- wind, hygro- moisture, baro- weight or pressure.

C. Col

A col is the slack saddle in the middle of that arrangement. The pressure gradient there is weak, so the winds are light and variable - which makes a col a favourite place for fog to form, and in summer for thunderstorms.

C. Tropics

The twice-daily pressure wave is largest near the equator, where it reaches about 3 hPa, and shrinks polewards where travelling weather systems swamp it. That is why the tropical maxima near 1000 and 2200, and minima near 0400 and 1600 local time, stand out so clearly.