Editing Hot air balloon (section)

===Generating lift===
Increasing the air temperature inside the envelope makes it less dense than the surrounding (ambient) air. The balloon floats because of the buoyant force exerted on it. This force is the same force that acts on objects when they are in water and is described by [[Buoyancy#Archimedes' principle|Archimedes' principle]]. The amount of lift (or [[buoyancy]]) provided by a hot air balloon depends primarily upon the difference between the temperature of the air inside the envelope and the temperature of the air outside the envelope. For most envelopes made of nylon fabric, the maximal internal temperature is limited to approximately 120&nbsp;°C (250&nbsp;°F).<ref>{{cite web
| url=http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/d7a71e4def72db7e862573e900555227/$FILE/A33CE.pdf
| archive-url=https://web.archive.org/web/20080624220610/http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/d7a71e4def72db7e862573e900555227/$FILE/A33CE.pdf
| archive-date=2008-06-24
| url-status=dead
| title=Department of Transportation, Federal Aviation Administration, Type certificate data sheet no. A33CE
| access-date=2008-06-16
}}</ref>

The melting point of nylon is significantly greater than this maximal [[operating temperature]]—about 230&nbsp;°C (450&nbsp;°F)—but higher temperatures cause the strength of the nylon fabric to degrade more quickly over time. With a maximal operating temperature of 120&nbsp;°C (250&nbsp;°F), balloon envelopes can generally be flown for between 400 and 500 hours before the fabric needs to be replaced. Many balloon pilots operate their envelopes at temperatures significantly less than the maximum to extend envelope-fabric life.

The lift generated by {{convert|100000|cuft|m3|order=flip|abbr=on}} of dry air heated to various temperatures may be calculated as follows:
: {| class="wikitable"
|-
! Air temperature
! Air density
! Air mass
! Lift generated
|-
| {{cvt|20|C}}
| {{cvt|1.2041|kg/m3|lb/cuft}}
| {{cvt|3409.7|kg}}
| 0&nbsp;lb, 0&nbsp;kg
|-
| {{cvt|99|C}}
| {{cvt|0.9486|kg/m3|lb/cuft}}
| {{cvt|2686.2|kg|lb}}
| {{cvt|723.5|kg|lb}}
|-
| {{cvt|120|C}}
| {{cvt|0.8978|kg/m3|lb/cuft}}
| {{cvt|2542.4|kg|lb}}
| {{cvt|867.3|kg|lb}}
|}

[[File:Infarot 9.jpg|thumb|left|Thermal image showing temperature variation in a hot air balloon]]
The [[density of air]] at {{cvt|20|C}} is about {{cvt|1.2|kg/m3|lb/cuft}}. The total lift for a balloon of {{cvt|100000|cuft|m3|order=flip}} heated to {{cvt|99|C}} would be {{cvt|723.5|kg|lb|0}}. This is just enough to generate neutral buoyancy for the total system mass (not including the heated air trapped in the envelope, of course) stated in the previous section. Liftoff would require a slightly greater temperature, depending on the desired rate of climb. In reality, the air contained in the envelope is not all at the same temperature, as the accompanying thermal image shows, and so these calculations are based on averages.

For typical atmospheric conditions ({{cvt|20|C|F|disp=or}}), a hot air balloon heated to {{cvt|99|C}} requires about 3.91&nbsp;m<sup>3</sup> of envelope volume to lift 1&nbsp;kilogram (equivalently, 62.5&nbsp;cu&nbsp;ft/lb). The precise amount of lift provided depends not only upon the internal temperature mentioned above, but the external temperature, altitude above sea level, and humidity of the air surrounding. On a warm day, a balloon cannot lift as much as on a cool day, because the temperature required for launch will exceed the maximum sustainable for nylon envelope fabric. Also, in the lower atmosphere, the lift provided by a hot air balloon decreases about 3% per 1,000&nbsp;m (1% per 1,000&nbsp;ft) of altitude gained.<ref>{{cite web
|url = http://www.overflite.com/thermo.html
|title = How to Calculate the Weight of Air and Model Hot Air Balloon Lift
|access-date = 2008-01-01}}</ref>