Getting cold feet
Why does kicking a football hurt your foot on a cold day but not on a warm one?
鈥� Human bodies conserve heat in cold weather by reducing the circulation of blood in the skin and minimising the movement of muscles and corresponding joints. After exposure to excessive cold, muscles go into a state similar to a partial spasm, otherwise known as a shiver. This is nothing more than the muscles rapidly contracting and expanding to generate heat.
A sudden violent activity such as kicking a football, lifting weights or running without adequate warm-up is bound to cause excessive load on the muscles, and this is felt as pain. This is true not only in the cold but also following a period of inactivity of the joint. The excruciating morning pain of rheumatoid arthritis, which worsens in winters, is also partly due to this.
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Vivek Jain
Gujarat, India
鈥� If you are cold you will have a decreased blood flow to your foot or any part of your body. If your skin is cold on your thigh, for example, and a football strikes it hard, it seems to hurt more than if your thigh is warm. You also get a red mark pretty much immediately, because of the sudden increased blood flow.
The reason for this increased blood flow is twofold. Damaged tissue releases chemical mediators that increase blood flow to the tissue and activate C-fibres, a type of neuron triggered by painful stimuli.
The increase in blood flow warms the skin, causing a tingling sensation that even under normal circumstances is not very pleasant, and is similar to sticking cold hands into a basin of warm water. If the tingling occurs in a region that is already sensitive from a smack with a ball, it increases the perceived pain.
Simon McMullan University of Bristol, UK
鈥� If the ball is made of synthetic material rather than leather, the plastic will become stiffer and more brittle at a lower temperature. This means that it will deform less readily when a player鈥檚 foot hits it. When a plastic deforms well, such as foam rubber, it can absorb high forces, but in its brittle state it returns a higher reaction force, a bit more like kicking the goalposts.
Chris Taylor Leeds, West Yorkshire, UK
In the summertime
At school I recently learned about evaporation and I wondered why it does not rain more in summer than in winter. Because it is so much hotter in the summer, surely the heat should evaporate more water than in winter, when it is much colder. Therefore it makes sense to me that more clouds will be likely to form in the summer and there would be more rain. What is wrong with this assumption?
鈥� During the summer, when it is hotter, evaporation does occur at a faster rate. The reason that you don鈥檛 see an increase in cloud formation or rain is that warmer air can hold more water vapour. Because evaporated water can travel very long distances, this actually has little effect on local areas. You can see this in photographs of the Earth from space: clouds cover a vast portion of the planet鈥檚 surface, but they are not all generated locally.
For clouds to form, an area of the atmosphere with a given humidity percentage must come into contact with another part of the atmosphere that is too cold to hold the same amount of water vapour, so it condenses into clouds. The same effect can be seen when you pour yourself a glass of cold water, and water condenses onto the outside of the glass. (It is also why your mum tells you to use the coasters on the table.)
Joe Thurston
Richmond, Virginia, US
鈥� As a child, I too wondered why rain wasn鈥檛 proportional to the evaporation rate of the water.
Only now, as a scientist, do I realise that to have rain we need more than water in the sky. Consider a single water molecule in a river. Just as with any liquid, water has a vapour pressure at the interface with the air above. This means there is a chance that a single molecule might escape from the strong intermolecular interaction within its liquid state and move to the atmosphere as a gas. Because the vapour pressure increases with temperature, more molecules will escape from the river in the summer.
Where does an escapee water molecule go? Because the water vapour is now lighter than the heavy gases that surround it in the atmosphere 鈥� nitrogen and oxygen 鈥� the molecule rises until it reaches a place where the density of the air is the same as that of the molecule. At this altitude, the molecule will meet many other molecules. If the concentration of these molecules is higher than the vapour pressure at this point they will condense to form a big, white cloud 鈥� a suspension of very small droplets of water in the air.
However, for these drops to fall as rain, the cloud needs to cool 鈥� the situation found in a cold front 鈥� a huge mass of cold air that is able to condense more water, decrease the kinetic energy of the water droplets and produce drops large enough to be denser than the surrounding air.
This happens more frequently in winter because more cold fronts occur. Also, both temperature and water vapour pressure are higher in summer, which means fewer clouds because fewer water molecules condense. And because the diffusion of the water molecules is proportional to the temperature, warm molecules can spread easily without forming clouds.
Edson Minatti
Department of Physical Chemistry University of Bordeaux, France
This week鈥檚 questions
In a pickle
The process of pickling in vinegar has been used for centuries to preserve food without refrigeration. As I understand it, harmful bugs can鈥檛 survive in the acid. So why does it say on my pickled onions and other pickles: 鈥淩efrigerate after opening and consume within six weeks鈥�?
Doug Fenna
Middlesbrough, UK
Mellow mallow
If you puffed up marshmallows with helium instead of ordinary air, could you make them so light that they would float out of the bag and around the kitchen?
Caleb Huntington
Boston, Massachusetts, US