Hot air swells and rises

Have you watched a hot air balloon take off? Perhaps you have been for a ride in one. A hot air balloon rises into the sky because of the way gases behave when they are heated.

Gases are made of billions of freely moving particles. The power for their movement comes from the energy they possess. Gas particles do not stick together as solid particles do, or slide together as liquid particles do. They have space between them in which to move. Heat is a form of energy. When a gas is heated, its particles take up this extra energy. They use it to move faster and further. As a result of this they spread out more. There is more space between them. When a gas cools down it loses heat. Its particles have less energy to move and so they move more slowly and stay closer together.

The weight of a balloon of gas depends on the number of particles it contains. When the air in a hot air balloon is heated, the air particles spread out. They push on the materials which make the balloon, and the balloon inflates. Some air particles may escape from the bottom of the balloon. The air in a hot air balloon is lighter in weight than the air outside because it has fewer particles and they are more spread out.

When two liquids, like water and oil, are put together the one that is lighter in weight floats above the other. It is the same when two gases differ in weight. The one that is lighter in weight rises through the other and floats above it. In the hot air balloon the lightweight gas is trapped. This means that as it rises through the cooler air around it, the hot air lifts the balloon and the basket which is hanging from its base.

If the pilot of the balloon shuts off the gas burner, the hot air in the balloon will gradually cool as it rises. As this happens, the particles in the air in the balloon will lose energy and come closer together. Air particles may enter the balloon from the bottom. These two processes make the air in the balloon heavier and it rises less quickly.

Eventually the air in the balloon has cooled so much that it weighs the same amount as the air around it. When this happens the balloon stops rising. If the balloon is allowed to cool down more, the weight of the air inside it and the weight of the material in the balloon and basket make the balloon heavier than the air around it. When this happens, the balloon sinks towards the ground.

The pilot controls the height of the balloon by using the burner. If the pilot wishes to go higher the burner is switched on and the air is heated more strongly to make it lighter in weight than the surrounding air. When the balloon is coming into land the burner is used again to slow down the speed at which it falls. By careful use of the burner as the balloon descends the pilot can avoid a crash landing.

Does hot air make a flame go upwards?
Yes, it does. A flame is made from tiny particles of soot, which are so hot that they glow. The hot air rising from a burning material carries the glowing soot particles with it and makes the flame go straight up. The rising hot air also carries soot particles into the air, which are not glowing. We call them smoke. The smoke rises vertically unless it is blown by the wind.

Does a chimney help a fire?
Yes, it does. Although the first purpose of a chimney was just to take the smoke out of a room. When a fire burns, the hot air rises and cooler air is drawn in from around the fire. The chimney helps to channel away the hot air more efficiently and this in turn draws air in more efficiently to the fire and improves the rate of burning. You may see tall chimneys on the roofs of old houses. These were designed to make the fires burn efficiently in the rooms below. New houses today may have a special tile on their roof rather than a chimney. When a gas fire is fitted to the house, the engineer has to check that the flow of air through the chimney reaches a certain speed. Otherwise a harmful gas called carbon monoxide may enter the room when the gas fire is burning.

If the ground is warmed by the Sun, can it make the air hot?
Yes, it can. When the Sun has been shining on the ground for a few hours the ground becomes warm. Some of its heat escapes into the air closest to the ground. The heat gives the air particles extra energy and they move further apart. This makes the air lighter in weight than the air above it. The lightweight air then rises through the cooler air. As the air rises, it creates space for cooler air to move in close to the ground. This air also becomes warm and rises too. In a short time a current of rising warm air flows straight up from the ground. It may reach a few thousand metres in height. The column of rising air is called a thermal.

Are thermals useful?
They are useful to vultures and glider pilots. A vulture is a bird that lives on carrion - the bodies of dead animals. It lives in parts of the world, such as deserts, where thermals are common. The vulture has large broad wings, which it holds out to catch the current of rising air in the thermal. The rising air carries the bird high into the air. The vulture circles in the thermal to keep rising. As it moves round it gets a good view of its habitat so it can look for food. The glider pilot uses thermals to keep flying. As a glider moves forwards it gradually sinks. If a pilot can find a thermal the glider can be turned in a circle in the rising air. The long wings catch the current of air and the glider is pushed upwards. When the pilot reaches the top of the thermal the glider can be steered away into a dive until the next thermal is reached.

Can rising warm air make wind on the ground?
Yes, it can. At the coast there is land on one side of the beach and sea on the other. On a sunny day the land warms up faster than the sea. The warm air above the land rises and cool air from over the sea rushes up the shore to replace it. This movement of air makes a strong current of air called a sea breeze. At night the land cools down faster than the sea. Warm air now rises from the sea and cool air rushes down the shore to replace it. This air current is called a land breeze.

How can clouds start to appear out of a clear sky?
A sunny day may begin with a cloudless sky. During the morning you may see small wisps of cloud develop. They are formed at the top of thermals. The rising warm air carries water vapour in it. As the air rises it cools. At a certain height it is too cool for water to exist as vapour, so it changes back to billions of tiny droplets of water and makes a cloud. During the afternoon several thermals may produce quite a large cloud. By evening as the land cools and thermals stop clouds simply fade away leaving a clear sky.