Air Temperature (Honoree)

• If you heat up a gas it expands.

• In the case of a hot air balloon, when the air inside the balloon expands the extra
air is pushed out the bottom of the balloon, meaning that there are fewer atoms inside
the balloon, meaning that the air in the balloon is lighter than the air outside the balloon.

• The amount of lifting power is controlled by how hot the air is. If you heat the air inside
the balloon 100 degrees F hotter than the outside air temperature, then the air inside the balloon
will be about 25 percent lighter than the air outside the balloon.

• So a cubic foot of air weighs about 35 grams at 32 degrees F. A cubic foot of hot air at 132 degrees
F will weigh 25 percent less, or about 26.5 grams. The difference is 8.5 grams or so. So a hot air
balloon has to be much bigger to support the same weight, but it will float because hotter air is
lighter than cooler air.

• Suspended beneath the envelope part of the balloon is the gondola or wicker basket which carries
a source of heat capable of producing a sufficient temperature gradient between the air inside the
envelope and the surrounding air mass to give enough lift to keep the balloon and its passengers aloft.

• The opening of the parachute valve allows some of the trapped heat to escape into the atmosphere,
causing the air temperature inside the envelope to fall. Since the temperature outside the envelope
and the temperature inside the envelope are now equal, the ascent of the balloon slows.

• This is how the pilot controls how high or low the balloon will go.

Sources:
http://www.thetartan.org/2007/12/3/scitech/work
http://science.howstuffworks.com/helium3.htm

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