| Cold air falls and warm air rises. Why? Discuss! | | | | prevented from escaping through the top of a |
| Many of us experience the effects of falling cold | | | | building. As well as conserving heat, the |
| air and rising warm air on a regular basis. It is | | | | movement of colder air downwards must also be |
| happening all the time in the air above and around | | | | considered when designing refrigerators and |
| us and is one of the components in our weather | | | | refrigeration systems. |
| systems. You may notice that when there is no | | | | If you have a few minutes, try this experiment. |
| heating or air conditioning operating in your house, | | | | Firstly make sure nothing has been put into your |
| the rooms upstairs are slightly warmer than those | | | | refrigerator for at least 2 hours. Then, leave a |
| downstairs. Another example is the hot air balloon | | | | thermometer inside the refrigerator, making sure |
| that works precisely to this principle. By heating | | | | it is placed at the bottom and then close the door. |
| the air inside the balloon, the craft will be lighter | | | | After 10 minutes, open the door and straight |
| than the surrounding air and will rise. This is | | | | away and note the reading on the thermometer. |
| actually the result of cold air gushing downwards | | | | Then place the thermometer on the top shelf and |
| around the balloon at the same time as it is rising. | | | | close the door. After 10 minutes, take the reading |
| So why does cold air fall? That is simple: it is | | | | as soon as you open the door. You will notice a |
| heavier than warm air. And why is it heavier? | | | | difference - perhaps as much as 1 to 2 degrees. |
| That is slightly less simple, but only slightly. As | | | | This may not be such a big issue for us at home |
| with any gas, the air (a generic term for the | | | | most of the time. For commercial kitchens, |
| mixture of the gasses in our atmosphere), | | | | however, this difference maybe critical when |
| contains molecules that move (or agitate). This | | | | ensuring food is kept at an optimum temperature. |
| movement (or agitation) is greater as the | | | | Commercial fridges are often fitted with a fan |
| temperature rises. The molecules move in ever | | | | that evens out the colder and warmer air, thus |
| greater orbits, taking up more space. This causes | | | | negating the tendency for cold air to fall and |
| the mass of the air to expand. Although the total | | | | warmer air to rise. |
| mass of a lump of air has not changed, the mass | | | | If you take large freezer stores - the ones that |
| is more spread out and so any given cubic area | | | | hold thousands of boxes of stock - the |
| of it will be lighter. An analogy is found with | | | | movement of air around the facility is an |
| popcorn. A half pound of popcorn before being | | | | extremely important factor. The fans that blow |
| popped may fit into a cup. After popping, the | | | | frozen air into the store are always situated near |
| same corn would fill a large saucepan. Its total | | | | to the ceiling, allowing it to diffuse downwards. |
| weight will be more or less the same half pound | | | | Eventually, of course, the goods in the freezer |
| that it always was, but if you filled up the original | | | | store will need to be taken out and moved to |
| cup with the popped corn, it would weigh less | | | | another location, typically loaded onto a truck. If |
| than the unpopped corn as the rest of it would no | | | | frozen goods are loaded onto a frozen truck this |
| longer fit into the cup. Expanding hot air is similar. | | | | is no problem. Occasionally though, only a small |
| A cup of cold air would weigh more than a cup of | | | | quantity of frozen goods may be needed and the |
| hot air. | | | | use of a large truck whose temperature is set at |
| As we are playing the why game, let's continue. | | | | a frozen temperature would be wasteful if only a |
| Why do the molecules move about more when it | | | | few boxes were being despatched. This is where |
| is warmer? They absorb energy through | | | | insulated pallet shrouds or roll cage covers come |
| electro-magnetic waves that smash into the | | | | into play. These enclose the pallet or roll cage, |
| molecules. In short, this is energy transfer by | | | | protecting frozen goods for up to 8 hours within |
| radiation. So we have a collection (several trillion, | | | | an ambient environment (they also protect |
| lets say) of molecules that are very agitated and | | | | ambient goods such bakery products and bananas |
| another collection which are far less agitated. The | | | | in a chilled or frozen environment). When a roll |
| agitated collection is spread out and thus light. The | | | | cage is used, the insulated roll cage cover works |
| collection that are less agitated is heavier. The | | | | at its best when the cage is full of products. |
| heavier stuff falls downwards, while the light stuff | | | | When it is half full then - you guessed it - the cold |
| rises. | | | | air falls to the bottom. This is fine at first |
| As well as being the main process behind hot air | | | | (assuming the goods are in the bottom half), but |
| balloons, the movement of air according to its | | | | after a while the warmer air that has risen to the |
| temperature is a critical factor with the weather. | | | | top will start to affect the top layer of goods. |
| Forecasters must ensure that these movements | | | | This is where a temperature insulated divider |
| are factored into their modelling systems in order | | | | must be used to make a seal and protectthe |
| to produce a decent weather forecast. Air | | | | goods in the half full roll cage. |
| conditioningdesigners and must also take these | | | | So now you know the whys and hows of cold |
| factors into account as must architects. In order | | | | and warm air and now you also know why your |
| to preserve valuable heat, warm air must be | | | | feet get cold in winter! |