Why Does Cold Air Fall and Warm Air Rise?

Cold air falls and warm air rises. Why? Discuss!

Many of us experience the effects of falling cold air and rising warm air on a regular basis. It is happening all the time in the air above and around us and is one of the components in our weather systems. You may notice that when there is no heating or air conditioning operating in your house, the rooms upstairs are slightly warmer than those downstairs. Another example is the hot air balloon that works precisely to this principle. By heating the air inside the balloon, the craft will be lighter than the surrounding air and will rise. This is actually the result of cold air gushing downwards around the balloon at the same time as it is rising.

So why does cold air fall? That is simple: it is heavier than warm air. And why is it heavier? That is slightly less simple, but only slightly. As with any gas, the air (a generic term for the mixture of the gasses in our atmosphere), contains molecules that move (or agitate). This movement (or agitation) is greater as the temperature rises. The molecules move in ever greater orbits, taking up more space. This causes the mass of the air to expand. Although the total mass of a lump of air has not changed, the mass is more spread out and so any given cubic area of it will be lighter. An analogy is found with popcorn. A half pound of popcorn before being popped may fit into a cup. After popping, the same corn would fill a large saucepan. Its total weight will be more or less the same half pound that it always was, but if you filled up the original cup with the popped corn, it would weigh less than the unpopped corn as the rest of it would no longer fit into the cup. Expanding hot air is similar. A cup of cold air would weigh more than a cup of hot air.

As we are playing the why game, let’s continue. Why do the molecules move about more when it is warmer? They absorb energy through electro-magnetic waves that smash into the molecules. In short, this is energy transfer by radiation. So we have a collection (several trillion, lets say) of molecules that are very agitated and another collection which are far less agitated. The agitated collection is spread out and thus light. The collection that are less agitated is heavier. The heavier stuff falls downwards, while the light stuff rises.

As well as being the main process behind hot air balloons, the movement of air according to its temperature is a critical factor with the weather. Forecasters must ensure that these movements are factored into their modelling systems in order to produce a decent weather forecast. Air conditioning

designers and must also take these factors into account as must architects. In order to preserve valuable heat, warm air must be prevented from escaping through the top of a building. As well as conserving heat, the movement of colder air downwards must also be considered when designing refrigerators and refrigeration systems.

If you have a few minutes, try this experiment. Firstly make sure nothing has been put into your refrigerator for at least 2 hours. Then, leave a thermometer inside the refrigerator, making sure it is placed at the bottom and then close the door. After 10 minutes, open the door and straight away and note the reading on the thermometer. Then place the thermometer on the top shelf and close the door. After 10 minutes, take the reading as soon as you open the door. You will notice a difference – perhaps as much as 1 to 2 degrees. This may not be such a big issue for us at home most of the time. For commercial kitchens, however, this difference maybe critical when ensuring food is kept at an optimum temperature. Commercial fridges are often fitted with a fan that evens out the colder and warmer air, thus negating the tendency for cold air to fall and warmer air to rise.

If you take large freezer stores – the ones that hold thousands of boxes of stock – the movement of air around the facility is an extremely important factor. The fans that blow frozen air into the store are always situated near to the ceiling, allowing it to diffuse downwards. Eventually, of course, the goods in the freezer store will need to be taken out and moved to another location, typically loaded onto a truck. If frozen goods are loaded onto a frozen truck this is no problem. Occasionally though, only a small quantity of frozen goods may be needed and the use of a large truck whose temperature is set at a frozen temperature would be wasteful if only a few boxes were being dispatched. This is where insulated pallet shrouds or roll cage covers come into play. These enclose the pallet or roll cage, protecting frozen goods for up to 8 hours within an ambient environment (they also protect ambient goods such bakery products and bananas in a chilled or frozen environment). When a roll cage is used, the insulated roll cage cover works at its best when the cage is full of products. When it is half full then – you guessed it – the cold air falls to the bottom. This is fine at first (assuming the goods are in the bottom half), but after a while the warmer air that has risen to the top will start to affect the top layer of goods. This is where a temperature insulated divider must be used to make a seal and protect

the goods in the half full roll cage.

So now you know the whys and hows of cold and warm air and now you also know why your feet get cold in winter!