Balloons

Air at 15 °C weighs 1.225 kg a cubic metre at sea level. A balloon lifts that weight minus the weight of the gas inside:

• Hot air at 100 °C weighs about 0.95 kg/m³, so each cubic metre lifts 0.28 kg; at 120 °C, 0.33 kg.
• Helium lifts 1.06 kg/m³; hydrogen 1.14 kg/m³.
• Height and heat cut lift: at 2,000 m the air is thinner and every figure is about 18 % less; on a hot day, a hot-air balloon lifts less.

Worked example: a person of 80 kg with a light basket and envelope, 230 kg in all, needs about 820 m³ of air at 100 °C: a sphere about 11.6 m across.

The Montgolfiers' first balloon, in June 1783, was 10 m across, of cloth lined with paper, and filled over a fire on the ground; it rose between 1,600 and 2,000 m and stayed up 10 minutes (Linda Hall Library). The first people to fly carried their fire with them. Today's balloons carry propane burners and fly in calm air near dawn and dusk.

• An envelope of gores, each tapering to the crown, sealed at every seam.
• Heat-resistant fabric at the mouth, where the heat is fiercest (Cameron Balloons).
• Keep flame and sparks off the fabric: paper and cotton burn, and so does nylon.

The tethered model balloon is the safe way to see it work.

Hydrogen lifts about four times what hot air does, cold, for as long as the envelope holds it. It is made from iron and dilute sulphuric acid (acid): Fe + H₂SO₄ → FeSO₄ + H₂, about 0.4 m³ of hydrogen for every kilogram of iron dissolved.

• It burns in air at any mixture from 4 % to 74 %, and a spark of 0.02 mJ — far less than a static spark from clothing — lights it (US DOE).
• Make and fill it outdoors, with no flame, no smoking, and no one in wool or synthetic clothing near.
• Helium lifts nearly as much and does not burn, but it comes from natural gas wells, which is a later tier.

A tethered balloon holds a position above the ground: an observer's eye, a signal lamp, or an aerial. Armies used tethered balloons for observation through the First World War; the US Army's 1918 manual covers the work (Balloon observation).

• A winch to haul down, and a cable rated for the lift plus the wind's pull.
• Calm air only: wind blows a tethered balloon down and over, and it drags.
• The same airspace rules as a large kite: in the US, no higher than 500 ft and no nearer than 5 miles to an airport (14 CFR 101.13).

Every day at 00:00 and 12:00 UTC, weather services release balloons from nearly 900 sites; each lifts a radiosonde that radios back temperature, humidity and pressure, and is tracked for the wind (NOAA). A rebuilding community can do the simplest version: a small gas balloon followed by eye through a theodolite gives the wind at each height it passes.

• Power lines: a hot-air balloon struck power lines near Lockhart, Texas, on 30 July 2016 and all 16 aboard died (NTSB). Every launch and landing field is checked for lines first.
• Fire: never release a hot-air balloon with a flame aboard. Brazil makes making, selling or releasing balloons that can start fires a crime, with one to three years' detention (Law 9.605/1998, art. 42).
• Weather: a balloon goes where the wind goes; it cannot steer away from anything.
• People who fly in a balloon learn from an instructor first: the FAA's Balloon Flying Handbook is the standard text.