How hot air balloons work

Hot air balloons are an ingenious application of some basic scientific principals.

The basis of warm air rising in cooler air is the principal on which ballooning works. The warm air is lighter than cooler air and has less mass per unit of volume (the mass can be defined by the measure of how much matter something contains).

A hot air balloon is made up of three main parts: the fabric balloon which holds the air; the burner which propels heat up into the envelope and the basket which contains the pilot and passengers.

The actual balloon is known as the envelope, and has to be large as it requires a large amount of heated air to lift it off the ground. The envelope is made of fireproof material to prevent the balloon from being ignited by the fire used to propel it from the burners.
The burners are attached atop the basket and are attached to the compressed liquid propane gas that is stored in lightweight cylinders in the basket by means of hoses. The propane travels through the hose to a heating coil that circles the burner. When the burner is lit the gas is drawn out and ignited resulting in the metal coil being heated and the propane changing into a gaseous state in which it burns. The burner needs to be fired at regular intervals throughout the flight to ensure that the balloon continues to be stable.

When the pilot opens the valve for the propane and fires the burner it creates heat and the balloon rises as the air inside is warmed. Similar to that of a grill, the more gas released, the larger the flame and the higher the heat which results in the balloon rising faster.
The parachute valve at the top of the balloon is used to control downward movement. This is essentially a circle of fabric cut out at the top of the envelope which is controlled with a long cord that runs to the basket. By pulling on the cord the hot air is allowed to escape and the inner air temperature is decreased which results in the balloon descending.
Although mainly limited to vertical movement, horizontal movement can be achieved to some level by accessing the wind blowing in different directions at different altitudes and ascending or descending to the appropriate level.

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