Pressure and height

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 many feet is one hectopascal?

Feet per hPa

In ISA. Tap a card for the detail.

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 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.

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.