Hot air balloons float in the sky for the same reason that ships float in the sea. A ship floats because it is supported by the water below it: the weight of the ship (pulling down) is exactly offset by the pressure of the water below it (pushing up). A ship does not float perfectly on the surface of the water, but partially sinks in the water depending on its weight. The larger the boat, the greater the area of ​​water under it, the greater the force of the water pressure pushing it upward, and the more weight it can support.Here's another way of looking at it: Generally speaking, an object will float if it is less dense than water (in other words, lighter than an equal volume of water) and will sink if it is denser (heavier than an equivalent volume of water). Water). Imagine a block of lead the size of your arm in a bathtub filled with water.A "arm full of lead" weighs much more than a "arm full of water," so the lead sinks to the bottom of the tub immediately. But a "plastic-filled arm," the plastic arm of a mannequin, for example, floats because it weighs less than the same volume of water.The balloon burner heats the air and makes the air inside the balloon less dense, producing an upward force or lift.1) A hot air balloon stays on the ground (or descends) when the air inside it is too cold. In this case, the weight of the balloon is greater than the elevation.2) When the air inside is a little warmer, the balloon floats at a constant height because the lifting force and the weight are now the same.3) When the air is even hotter, the balloon rises because the lifting force is greater than the weight.A hot air balloon is not like a rubber balloon tied in a knot: it is open at the bottom so air can move in and out. That means that the density of the air in the balloon can change while the inner and outer pressure is essentially the same. Hot air balloons float because the air trapped inside the balloon is heated by a burner, making it less dense than the air outside. As the burner heats the air, it expands and some of the air escapes; that's what makes it less dense. Here's another way of looking at it. You've probably heard people say that heat rises, by which they really mean warm air rises. When you see clouds of dirty gray gas rising from chimneys, it is because the air coming out of them is warmer than the ambient (surrounding) air. If you could wrap a bag around hot air entering the bottom of a fireplace and seal it, the entire bag would shoot up and out the top before moving away and rising into the air. Indeed, you would have made a small hot air balloon!However, the little balloons are not really very useful. If you want to carry a heavy weight at sea, you need a large boat - one that can displace more water can carry more cargo.In exactly the same way, you need a large hot air balloon to lift a large weight, because you need to create more lift with a greater volume of hot gas. Just to lift the weight of a grown man, you would need a balloon of about 4 m in radius with the air inside heated to a temperature of about 120 ° C. That explains why hot air balloons are generally so large.Discover more in this podcast.