Atmospheric Pressure and Altimetry

What is the pressure the altimeter feels?

The weight of the sky

Five cards, as the teacher builds it. Tap Next.

1. A column of air Air has mass, and gravity gives it weight (W = m × g). The pressure at any level is the weight of the column of air above one unit of area, all the way to the top of the atmosphere. Climb, and there is less air above you, so the pressure falls.
2. Still air and moving air Still air pushes equally in every direction: static (barometric) pressure. This is what the altimeter senses. Moving air adds a push against whatever faces it: dynamic pressure. Hold your hand out of a car window to feel it; the airspeed indicator uses it.
3. hPa, mm and inches ICAO uses the hectopascal: 1 hPa = 1 mb = 100 N/m². The standard sea-level pressure is 1013.25 hPa = 760 mm of mercury = 29.92 inches. To turn hPa into inches, multiply by 0.02953.
4. Fast near the ground, slow up high Air can be squeezed, so gravity packs most of it near the ground. Pressure falls quickly at first and then more and more slowly with height: about 4% in the first 600 m, and half is gone by about 6 km (18,000 ft). By 100 km there is almost none. That is why there is no single lapse rate for pressure: near sea level 1 hPa is about 27 ft, at 20,000 ft about 50 ft, at 40,000 ft about 100 ft.
5. High to low When the pressure at the surface differs from place to place, the air flows from high to low pressure. That is wind. Pressure also changes with time: it has a small twice-daily rhythm, and it rises and falls as weather systems pass.

Memory hook: Pressure = the weight of the air above you. Less air above, less pressure.

Takeaway: Pressure is force per unit area. At any level it is the weight of the column of air above, so it always falls with height, fast near the ground and slower higher up. 1013.25 hPa = 760 mm = 29.92 in.

Why does pressure fall faster near the ground?

Pressure and height

The curve is the ISA. Tap Next.

1. The weight of the column Pressure at any level is the weight of the column of air above one unit of area, right up to the top of the atmosphere. Climb, and there is less air above you, so less pressure.
2. It falls at a decreasing rate Air can be squeezed, so most of it is packed low down. Pressure drops fast near the ground and more and more slowly higher up. At 100 km it is practically a vacuum.
3. Half by 5.5-6 km By about 5.5-6 km (18 000-20 000 ft) the pressure is half its sea-level value: the 500 hPa level.
4. Feet per hectopascal Because the air thins, each hPa of pressure takes up more height as you climb: about 27 ft at MSL, 30 ft at 2000 ft, 50 ft at 20 000 ft and 100 ft at 40 000 ft. Formula: height for 1 hPa ≈ 96 T / p ft (T in kelvin, p in hPa). At 1000 hPa and 300 K: 96 × 300 / 1000 ≈ 29 ft. Warmer than ISA: more feet per hPa. Colder than ISA: fewer.
5. Why your ears pop A fast descent, or a drive up or down a mountain, changes the pressure quickly; the ears pop as the pressure inside them catches up.

Memory hook: 27 · 30 · 50 · 100 ft per hPa.

Takeaway: Pressure falls with height at a decreasing rate: fast near the ground, slowly high up. Half is gone by about 5.5 km (500 hPa). So 1 hPa is 27 ft at sea level but 100 ft at 40 000 ft.

How is pressure actually measured?

Measuring pressure

Four instruments, one idea.

1. Mercury barometer The weight of the air pushes mercury up a glass tube with a vacuum at the top. At standard sea level pressure the column stands 760 mm (29.92 in) high. It is the most accurate barometer.
2. Aneroid barometer A thin sealed metal capsule, with most of the air taken out, squeezes when pressure rises and swells when it falls; levers move a needle. Less accurate than mercury, but small and easy to use, so it is the one used widely.
3. The altimeter is an aneroid An altimeter is an aneroid barometer whose dial is marked in feet of height instead of hPa. Its subscale lets the pilot set the pressure to measure from.
4. The barograph A barograph is an aneroid with a pen that draws the pressure on a slowly turning drum: daily or weekly charts.
5. From reading to report A barometer reading is corrected for instrument error (index correction), for gravity (it differs with latitude) and for temperature. The pressure is then reduced to a common level, mean sea level, so stations can be compared. Units: 1013.25 hPa = 760 mm = 29.92 in. hPa × 0.02953 = inches. 1 hPa = 1 mb = 100 N/m².

Takeaway: The mercury barometer is the accurate standard; the aneroid is handier and is the heart of the altimeter; the barograph draws pressure against time.

Which flight level goes with which pressure?

Standard levels

The ISA pressure altitude of each standard chart level. Tap a bar.

• 850 hPa 5000 About 5,000 ft (FL50).
• 700 hPa 10000 About 10,000 ft (FL100).
• 500 hPa 18000 About 18,000 ft (FL180): half the atmosphere is below you. The WD chart.
• 400 hPa 24000 About 24,000 ft (FL240).
• 300 hPa 30000 About 30,000 ft (FL300).
• 200 hPa 38000 About 38,000 ft (FL380), often rounded to 40,000 ft: jet-stream level.
• 100 hPa 53000 About 53,000 ft (FL530).

Memory hook: 850-5, 700-10, 500-18, 400-24, 300-30, 200-38 (about 40), 100-53.

Takeaway: 850 hPa ≈ FL50, 700 ≈ FL100, 500 ≈ FL180, 400 ≈ FL240, 300 ≈ FL300, 200 ≈ FL380-400, 100 ≈ FL530.

Why does the barometer rise and fall twice a day?

The daily rhythm

Local time. Tap a marker.

• Minimum The early-morning low.
• Main maximum The primary high of the day.
• Minimum The afternoon low, deeper than the morning one in the tropics.
• Second maximum The secondary high. The cycle is a natural 12-hour oscillation of the atmosphere, lagging the temperature by about 3 hours. Largest in the tropics (3-5 hPa), smallest at the poles.

Takeaway: Two highs (about 1000 and 2200 h) and two lows (about 0400 and 1600 h) every day: the semi-diurnal pressure wave, 3-5 hPa in the tropics and almost nothing near the poles.

Why does pressure fall faster in cold air?

Warm and cold air

The teacher's three columns, card by card. Tap Next.

1. Three columns, six 'molecules' each Picture three columns with the same amount of air: normal (ISA), hot and cold. Same weight of air, so the same pressure at the bottom: call it 6 units.
2. Pressure at FL100 Count the molecules above the FL100 line: ISA 2, hot 4, cold 0. So at FL100 the pressure is highest over the hot column and lowest over the cold one. The fall from the surface: ISA 4, hot only 2, cold all 6. Pressure falls fastest with height in cold air.
3. Isobaric surfaces dip over cold air A pressure level, say 850 hPa, is lower over a cold column and higher over a warm one. So at a fixed height there is low pressure over cold air and high pressure over warm air. On upper-air charts these heights are drawn as contours (lines of equal height of a pressure level), every 40 gpm on the 700 and 500 hPa charts and every 80 gpm on 300 and 200 hPa.
4. 96 T / p The height for 1 hPa of pressure change is about 96 × T / p feet, with T in kelvin. At 1000 hPa and 300 K that is about 29 ft. Warmer than ISA, each hPa covers more height; colder than ISA, less.

Memory hook: Cold air: molecules huddle low, pressure falls fast. Warm air: spread out, pressure falls slowly.

Takeaway: Every column has the same air in it, so the same surface pressure. In cold air the air is packed low, so pressure falls fast with height and any pressure level sits lower; in warm air it is spread up, so pressure falls slowly and the level sits higher. Aloft: low over cold air, high over warm air.

Where is the 850 hPa level on a cold day?

Isobars dip over cold air

Three columns with the same weight of air. Tap Next.

1. Same weight, same pressure at the bottom Three columns hold the same amount of air, so they press equally on the ground: 1000 hPa under each.
2. Cold air is squeezed shorter Cold air is denser, so the same air takes less height. Pressure falls faster with height: the 850 hPa level sits low over the cold column and high over the warm one.
3. Every level does the same The 700 hPa level, and every level above, is lower over cold air too. The difference grows with height.
4. At one height: low over cold, high over warm Draw a line at 5000 ft. Over the cold column it is above the 850 hPa surface (lower pressure); over the warm one it is below it (higher pressure). So aloft, lows sit over cold air and highs over warm air.
5. What the altimeter does An altimeter holding a constant reading follows a pressure level. Fly from warm air into cold air and that level sinks: the aircraft sinks with it while the altimeter shows no change. It over-reads. Warm to cold, or high to low: over-reads. Look out below.

Memory hook: Cold column: levels sink. Warm column: levels rise.

Takeaway: Cold air is denser, so pressure falls faster with height in it: every pressure level sits lower over cold air. At a fixed height, pressure is low over cold air and high over warm air.

Can you see a pressure level sag over cold air?

Pressure levels in 3D

A 3D model. Warm air on the left, cold air on the right. Drag to turn it.

• Warm and cold air side by side Two blocks of air sit side by side: warm on the left, cold on the right. At the ground both have the same pressure. Cold air is denser, so the same weight of air is packed into a shallower layer. Climbing through it, the pressure falls faster than in the warm air.
• One pressure level Follow one pressure level, 700 hPa. It sits high over the warm air and sags low over the cold air.
• Fly it at a constant reading An altimeter measures pressure, so holding a constant reading means flying along this surface. Flying into the cold air, the aircraft goes down with the surface, and the altimeter does not show it.

Takeaway: Over cold air, every pressure level sits lower. An aircraft that holds the same altimeter reading follows the level down as it flies into the cold air.

How many feet is one hectopascal?

Feet per hPa

In ISA. Tap a card for the detail.

What do the lines on an upper-air chart mean?

Contours and thickness

Upper-air charts show the height of a pressure level, not the pressure at a height.

1. Contours: the height of a level On a constant-pressure chart (say 500 hPa) each line joins places where that pressure level is at the same height. These contours (isohypses) act like isobars: low contours = low pressure at that height.
2. Units and spacing Heights are in geopotential metres (gpm; 1 gpm = 9.8 J/kg) and the labels drop the last figure: 528 means 5280 gpm. Contours are drawn every 40 gpm on the 700 and 500 hPa charts and every 80 gpm on the 300 and 200 hPa charts.
3. Thickness Thickness is the height between two pressure levels. A cold layer is thin, a warm layer is thick, so lines of equal thickness are the isotherms of the layer's mean temperature. 300 hPa contour about 9160 m: 300 hPa is the ISA 30 000 ft level.

Takeaway: Contours join places where a pressure level is at the same height: 40 gpm apart on 700 and 500 hPa charts, 80 gpm on 300 and 200 hPa. Thickness between two levels follows the layer's mean temperature.

How do you tell a trough from a ridge on a chart?

The five patterns

One pattern per step, northern hemisphere. Isobars every 2 hPa.

1. Low, depression, cyclone An area ringed by an isobar with the lowest pressure in the middle. Two or more closed isobars (2 hPa apart) make a depression; deeper still, a cyclone. Northern hemisphere: winds blow anticlockwise and converge towards the centre, so air rises: bad weather, but visibility is better than in a high. Winds back.
2. Trough A tongue of low pressure stretching out of a low. Pressure along it is lower than on either side and the isobars are V-shaped. Crossing it the wind changes suddenly and backs. Bad weather. The monsoon trough over the Indo-Gangetic plains is one.
3. High, anticyclone Highest pressure in the middle. Winds blow clockwise, veer and spread outwards. The sinking air gives fair weather, but traps haze, so visibility is poor. A high that sits still and blocks the path of passing lows is a blocking anticyclone. A cold anticyclone forms in the cold air behind a departing low; its sinking air warms it with time.
4. Ridge A wedge of high pressure pushing out from a high. Pressure along it is higher than on either side, the isobars are rounded (not V-shaped), the wind veers and the weather is fair.
5. Col The region between two highs and two lows, set crosswise. The pressure is nearly uniform, the winds light and variable, and the weather a mix of both. Trough: lower pressure than either side, winds back. Ridge: higher, winds veer.

Memory hook: Lows back, highs veer.

Takeaway: Low: anticlockwise, converging, bad weather, better visibility. Trough: V-shaped isobars, wind backs. High: clockwise, diverging, fair but hazy. Ridge: rounded isobars, wind veers. Col: between two highs and two lows, light variable winds.

What do highs, lows, troughs, ridges and cols look like?

Pressure as a landscape

Pressure drawn as height: highs are hills, lows are valleys. White lines are isobars, 2 hPa apart.

• Pressure as height Here the height of the surface is the sea-level pressure. The blue hills (H) are high pressure, the violet valleys (L) low pressure, and each white contour is an isobar.
• A low A low is a valley: lowest pressure in the middle. In the northern hemisphere the wind blows anticlockwise round it (follow the moving arrows) and converges, so air rises and the weather is bad.
• A high A high is a hill: the wind blows clockwise round it (follow the moving arrows) and spreads out, air sinks and the weather is fair.
• Trough and ridge A trough is a tongue of low pressure running out of a low: a valley line. A ridge is a tongue of high pressure: a spur off a hill.
• Col A col is the saddle between two highs and two lows. The pressure is nearly even and the winds light and variable.

Takeaway: A low is a valley, a trough a valley stretched out, a high a hill, a ridge a spur running out from it, and a col the saddle between two highs and two lows.

Which pressure pattern is it?

Highs, lows and cols

Tap a clue, then the pattern.

• Winds converge and air rises
• Bad weather, but better visibility
• Clockwise winds in the NH, diverging
• Fair weather, but poor visibility
• V-shaped isobars, winds back sharply
• Rounded tongue of high pressure, winds veer
• Between two highs and two lows, light variable winds
• The monsoon trough over the Ganga plains

Takeaway: Low: anticlockwise (NH), converging, rising air, bad weather but better visibility. High: clockwise, diverging, sinking air, fair weather but poor visibility. Trough: V-shaped, winds back, bad weather. Ridge: rounded, winds veer, fair. Col: between two highs and two lows, light variable winds.

What does the spacing of isobars tell you?

Gradient and tendency

Two charts and a tendency chart.

1. Pressure gradient The pressure gradient is how fast pressure changes across the isobars, measured at right angles to them and pointing from high to low. Isobars close together: a steep gradient and strong wind.
2. A flat gradient Isobars far apart: the pressure hardly changes, the gradient is flat and the wind is light.
3. Pressure tendency and isallobars The change of pressure over time is the pressure tendency: in India over the past 24 hours, in higher latitudes over the past 3 hours. Isallobars join places with the same change. The biggest fall is the isallobaric low: a surface low tends to move towards it and deepen. An isallobaric high means a system is weakening.

Takeaway: The pressure gradient is the change of pressure across the isobars, from high to low. Close isobars: steep gradient, strong wind. Isallobars join places of equal pressure change; lows move towards the isallobaric low and deepen.

Altitude, height or elevation: which is which?

Altitude, height, elevation

One picture for all the words. Tap Next.

1. Mean sea level Mean sea level (MSL) is the average level of the sea between high and low tide. It is the zero line that altitudes, elevations and charts are measured from, so every pilot and every chart starts from the same place.
2. Elevation Elevation is how high a point on the ground, such as an airfield or a peak, is above MSL.
3. Altitude and height Altitude is how far the aircraft is above MSL. Height is how far it is above a chosen datum, for example the airfield.
4. Transition altitude The transition altitude (TA) is the highest altitude at which aircraft always fly on the local QNH. At or below it, an aircraft's vertical position is given as an altitude.
5. Transition level and layer The transition level (TL) is the lowest flight level at which aircraft always fly on the standard setting, 1013.2 hPa. The airspace between the TA and the TL is the transition layer.
6. Flight levels and pressure altitude Flight levels are surfaces of constant pressure, counted from 1013.25 hPa and 500 ft of pressure apart: FL50 = 5000 ft, FL300 = 30 000 ft. With 1013.2 set, the altimeter shows pressure altitude, and no one needs to keep updating QNH. Pressure altitude 19 000 is written FL190: drop the last two zeros.

Takeaway: Altitude is measured from mean sea level (MSL). Height is measured from a chosen datum, often the airfield. Elevation is how high the ground is above MSL. The transition altitude (TA) is the highest altitude flown on QNH; the transition level (TL) is the lowest flight level flown on 1013.2. Between them lies the transition layer.

What does each pressure setting make the altimeter read?

QFE, QNH, QFF, QNE

An airfield 1500 ft above the sea.

1. QFE: zero on the ground The pressure at the aerodrome reference point (the highest point of the runway). With QFE set, the altimeter reads zero on the ground (or its own height above the ARP) and height above the field in the air. Also called the zero setting.
2. QNH: elevation on the ground The station pressure reduced to MSL assuming ISA conditions. With QNH set the altimeter reads the airfield elevation on the ground and altitude in the air. Used for separation from terrain and from other aircraft.
3. QFF: for the weather map The station pressure reduced to MSL using the actual temperature of the air column. It is the value plotted on synoptic charts to draw isobars. Hotter than ISA: QFF is less than QNH. At sea level QFF = QNH = QFE.
4. QNE and regional QNH QNE is the altitude the altimeter shows with 1013.25 hPa (29.92 in) set, used at high-altitude airfields. Regional QNH is the lowest QNH forecast for an altimeter setting region in the next hour: issued every hour, valid one hour, and safe for terrain clearance.

Memory hook: QFE zero, QNH elevation, QFF charts, QNE 1013.

Takeaway: QFE: field pressure, reads zero on the ground. QNH: reduced to MSL with ISA, reads the elevation. QFF: reduced with the actual temperature, for charts. QNE: the reading with 1013.2 set.

Which Q code does what?

QFE, QNH, QFF, QNE

Each pair looks alike. The rule underneath tells them apart.

• QFE: The pressure at the aerodrome (the highest point of the runway). Set it and the altimeter reads 0 on the field, and height above the field in the air. QNH: The aerodrome pressure brought down to MSL using ISA. Set it and the altimeter reads the field elevation on the ground, and altitude above MSL in the air. QFE: height above the field. QNH: altitude above the sea.
• QNH: Reduced to MSL with the ISA temperature and lapse rate. Used for flying: separation from terrain and other aircraft. QFF: Reduced to MSL with the actual temperature of the place. Used only by meteorologists to draw isobars on synoptic charts. QNH flies. QFF draws charts. At a sea-level field they are equal; in air warmer than ISA, QFF < QNH.
• QNE (1013.25): The standard setting, 1013.25 hPa = 29.92 in. Reads pressure altitude; at or above the transition level that is a flight level (FL190 = 19,000 ft pressure altitude). Regional QNH: The lowest pressure forecast in an altimeter setting region, issued every hour and valid for one hour. Keeps you clear of terrain. Above the TL: 1013. Below the TA: QNH.

Memory hook: QFE = Field Elevation zero. QNH = Nautical Height (MSL). QNE = Standard 1013.

What will your altimeter read?

Fly the altimeter

An altimeter is an aneroid barometer marked in feet. Turn the subscale, move the aircraft, change today's pressure and temperature, and watch the needles. The red aircraft is where you really are; the dotted one is where the altimeter says you are.

• Sea-level field
• Field at 600 ft
• Delhi 778 ft
• Leh 10,700 ft
• On the ground: QFE
• The teacher's example: field elevation 600 ft, QFE 990 hPa. With QFE set, the altimeter reads 0 on the field (the 'zero setting').
• On the ground: QNH
• QNH = QFE + 600 ÷ 30 = 990 + 20 = 1010 hPa. With QNH set, the altimeter reads the field elevation, 600 ft.
• On the ground: 1013
• With 1013 set it reads the pressure altitude of the field: (1013 - 990) × 30 = 690 ft, not the elevation.
• High to low: look out below
• You set 1013 and fly into an area where the pressure has fallen to 1000 hPa. At the same true altitude the altimeter now reads about 390 ft too high. Keep the reading constant and you will descend: High to Low, look out below.
• Low to high
• Flying from low to high pressure without resetting, the altimeter under-reads: you are higher than it says.
• A cold day
• 20 °C colder than ISA at 10,000 ft: the altimeter over-reads by about 7% (1% for every 3 °C). Warm to cold, look out below.
• A hot day
• 15 °C warmer than ISA: the altimeter under-reads by about 5%. You are higher than it says.
• Leh: QFE off the scale
• At a very high field the QFE (about 656 hPa here) is below the subscale's range, so it cannot be set. That is why QNE is used at high-altitude airfields.

Memory hook: QFE: zero on the field. QNH: field elevation on the field. 1013: flight levels. High to low (or warm to cold), look out below.

Takeaway: The altimeter only measures pressure and turns it into feet above whatever pressure is set on the subscale. Set QFE and it reads height above the field; set QNH and it reads altitude above MSL; set 1013.25 and it reads pressure altitude. If the pressure falls, or the air is colder than ISA, it over-reads: you are lower than it says.

Can you do the altimeter sums?

Worked sums

Worked sums, one per card. Use 1 hPa = 30 ft unless told otherwise.

1. QNH from QFE QFE is 1000 hPa at a field 90 ft above sea level. What is the QNH?
2. The 600 ft field Field elevation 600 ft, QFE 990 hPa. What is the QNH, and what does the altimeter read on the ground with 1013 set?
3. A 27 ft example QFE 1000 hPa, field elevation 270 ft, and use 1 hPa = 27 ft. QNH?
4. Metres QNH 1005 hPa at an aerodrome 160 m above MSL. Use 1 hPa = 8 m. QFE?
5. Without ATIS You are on the ground at Delhi (778 ft) with no ATIS. How do you find the QNH?

Takeaway: Height ÷ 30 = hPa. QNH = QFE + elevation ÷ 30. QFE = QNH - elevation ÷ 30. On 1013 the field reads (1013 - QFE) × 30.

Can you work QNH, QFE and QNE out in your head?

Worked sums

Each card is one worked example. Use 30 ft per hPa unless told otherwise.

1. QFE 950, QNH 1000 Elevation = (1000 − 950) × 30 = 1500 ft. With 1013 set on the ground the altimeter reads (1013 − 950) × 30 = 1890 ft: more than the elevation, because QNH is below 1013.
2. QFE 970, QNH 1020 Elevation = (1020 − 970) × 30 = 1500 ft. With 1013 set: (1013 − 970) × 30 = 1290 ft: less than the elevation, because QNH is above 1013. Set pressure below QNH: reads low. Set pressure above QNH: reads high.
3. QFE 1000, elevation 270 ft 270 ÷ 27 = 10 hPa, so QNH = 1000 + 10 = 1010 hPa.
4. Took off at A (400 ft, QNH 1010)... ...and landed at B (600 ft) without resetting. The altimeter reads 700 ft. It reads 100 ft high, which is about 4 hPa at 27 ft per hPa. Reads high means the pressure fell from A to B (high to low, over-read). QNH at B ≈ 1010 − 4 = 1006 hPa.
5. Alticor and D value The correction from indicated to true altitude, used for mountain flying. Alticor = indicated − true. D value = true − indicated; it decreases flying from high to low. Rough rules: add 30 ft per hPa that MSL pressure is above 1013.2 (subtract below). Add 1% of the indicated altitude for each 3 °C warmer than ISA (subtract colder). Add the two together.
6. Indicated 10 000 ft, MSL 1003 hPa, ISA − 6 °C Pressure: 10 hPa below 1013, so −300 ft. Temperature: 6 °C colder, 2% of 10 000, so −200 ft. True altitude ≈ 10 000 − 500 = 9500 ft.

Takeaway: Elevation ≈ (QNH − QFE) × 30 ft. With 1013 set on the ground the altimeter reads (1013 − QFE) × 30 ft. Alticor = indicated − true; D value = true − indicated.

Why does everyone switch to 1013 at the same height?

Transition altitude

Four cards. Tap Next.

1. Same height, different readings Two aircraft are both at 10,000 ft. The real QNH here is 1013, but one pilot still has 1000 hPa set from one airfield and the other 1020 hPa from another. Their datums differ by 20 hPa, about 600 ft, so their altimeters disagree by 600 ft at the same height. On different settings, ATC cannot keep them apart safely.
2. Everyone on 1013 So above a set height everyone sets the same standard pressure, 1013.25 hPa. Now the altimeters all read pressure altitude, expressed as flight levels (FL50 = 5,000 ft, FL300 = 30,000 ft), and separation works.
3. TA, TL and the layer Transition altitude (TA): the highest altitude at or below which you fly on QNH. Transition level (TL): the lowest flight level at or above which you fly on 1013.25. Transition layer: the airspace between them.
4. Only on 1013 A flight level exists only with 1013.25 set. Every other reading is a height or an altitude, never a flight level. Levels are spaced by the pressure for 500 ft, in hundreds of feet: 3,500 ft pressure altitude is FL35.

Memory hook: Climbing: at the TA, set 1013. Descending: at the TL, set QNH.

Takeaway: Below the transition altitude fly on QNH (altitudes, terrain clearance). Above it everyone sets 1013.25 and flies flight levels, so all aircraft share one datum. The transition layer lies between the TA and the TL.

Will the altimeter over-read or under-read?

Over or under?

Tap a situation, then what the altimeter does.

• Flying from high to low pressure
• Flying from low to high pressure
• Air colder than ISA
• Air warmer than ISA
• Wind from port (NH)
• Wind from starboard (NH)
• Subscale set higher than the real QNH
• Subscale set lower than the real QNH

Memory hook: High to Low, or Hot to cold: look out below (over-reads).

Takeaway: Pressure falling, or air colder than ISA: over-reads, you are lower than shown. Pressure rising, or air warmer than ISA: under-reads, you are higher than shown.

Quick test

10 questions to check this chapter. Each answer explains why.

1. Leaving Ahmedabad (QNH 1016) for a field with QNH 1004, you forget to reset the altimeter. On arrival it shows

   • less than your true height
   • your true height
   • more than your true height

   Answer: more than your true height. Into lower pressure the reading is too high: you are lower than it says.

2. On the ground at an airfield, which setting makes the altimeter read the airfield's elevation?

   • QNH
   • 1013.25 hPa
   • QFE

   Answer: QNH. QNH is the pressure reduced to mean sea level, so on the ground the altimeter shows height above the sea: the elevation.

3. Near sea level, roughly how much does pressure fall for each 30 ft you climb?

   • 1 hPa
   • 0.1 hPa
   • 10 hPa

   Answer: 1 hPa. About 1 hPa per 27-30 ft near the surface; the drop gets smaller with height as the air thins.

4. Two balloons rise from Leh in January and Chennai in May. Over which does the pressure drop off faster in the first kilometre?

   • Chennai, in the warm air
   • Leh, in the cold air
   • The same over both

   Answer: Leh, in the cold air. Cold air is dense, so its pressure levels are packed close together.

5. An aircraft holds FL100 all the way from Delhi into a much colder air mass over the Himalaya. Its true altitude

   • rises
   • falls
   • does not change

   Answer: falls. Over cold air the 'FL100' pressure level sits lower, so the aircraft descends with it while the altimeter reads the same.

6. Climbing out on QNH, you set 1013.25 hPa

   • at the transition altitude
   • at 1000 ft
   • at the transition level

   Answer: at the transition altitude. Climbing, you change to standard at the transition altitude; descending, you change to QNH at the transition level.

7. QFE is the pressure

   • at the airfield, so the altimeter reads zero on the ground
   • of the standard atmosphere
   • reduced to sea level

   Answer: at the airfield, so the altimeter reads zero on the ground. With QFE set, the altimeter shows height above the airfield.

8. At a coastal airfield at sea level, how do QNH and QFE compare?

   • They are the same
   • QFE is 10 hPa higher
   • QNH is always 1013

   Answer: They are the same. At sea level there is no height to correct for, so the airfield pressure and the sea-level pressure are the same.

9. The small twice-daily rise and fall of pressure in the tropics peaks at about

   • 6 am and 6 pm
   • 10 am and 10 pm
   • noon and midnight

   Answer: 10 am and 10 pm. The semi-diurnal wave has highs near 10 and 22 local time and lows near 04 and 16; it is clearest in the tropics.

10. A forecaster draws smooth lines through every station reporting 1008 hPa, 1012 hPa and so on. She is drawing

    • isobars
    • isallobars
    • isotherms

    Answer: isobars. Isobars connect equal pressure; isotherms connect equal temperature and isallobars equal pressure change.