Asked in Meteorology and WeatherHistory of ScienceErosion and Weathering
Why is it colder in higher places than in lower places?
June 21, 2008 7:45AM
A (very) rough guide to the physic involved "why there is snow on mountains" - by Wayne Naylor of www folgarida.co.uk A snowball will melt if it is taken near to a fire and the nearer it gets the faster it melts, but climb a mountain (getting closer to the sun, which is very hot), you get colder and snow doesn't melt, WHY? Fires produce light, hot gasses and radiation which warms the air around it, when you take some snow into this area of radiation, gas and heat it will start to melt back into water and eventually evaporate. If you move away or into a shadow the melting process slows down as the gases and heat diffuse (thin out) with distance. In contrast, the heat and hot gases produced by the sun pass through the vacuum of space, so there is no direct warming of the earth's atmosphere (or your lump of snow). The sun releases (it does "not" create) energy due to gravity and pressure forcing hydrogen nuclei to fuse together to form a helium nucleus (well not exactly, but near enough for this) . As the new helium nucleus has less mass than the combined hydrogen nuclei it gets rid of the extra mass in the form of energy. Each second the sun converts around 680 million tons, (that's a "lot" of atoms) of hydrogen in helium and so… well lets just say it goes "bang". This electromagnetic energy radiates outwards in various forms such as heat, x-rays, light, gamma, radio waves, etc. One form energy can be converted into another, so when this "Sun-Light" comes into contact with the earth it warms the ground, oceans and everything else and this energy is converted and re-radiated upwards as heat which (like the fire), warms the air. There is more air closer to the surface than higher up so, as you climb a mountain, the warming effect is lessened. When you're far enough away from the earth's surface the warming effect will be so reduced that the air is either the same temperature or lower than the snow and the snow will not melt. Now all this is all well and good but: Why is snow cold and fire hot? What is the difference between snow and ice? Why is there more air lower down? Where do snow and water go to (and where did they come from in the 1st place)? Why is it colder in a shadow?
Why is a fire hot and snow cold? Fire (in the above example) is a exothermic chemical reaction (that is it releases energy) which occurs when a fuel (normally containing carbon atoms) is heated to a high temperature and it comes into contact with an oxidizing agent, normally oxygen from the air (oxygen itself does not burn as nothing can oxidize oxygen). It does "not", as is commonly thought, turn matter into heat energy, it simply releases the energy stored within the matter. Ok it does this very quickly sometimes but it's still not creating or destroying anything. -On this point, when the universe was created there was a certain amount of matter/energy created and it is still here, and this "exact" same amount will be around until the end of time. The 1st law of Thermodynamics (conservation of energy) states the energy/matter can not be destroyed (although Einstein tell us they are interchangeable) - Everything is made up of atoms which are continuously moving around, the application of heat causes molecules to move around more than normal and, at a high enough temperature, atoms will break free, releasing high amounts of light radiation and infrared energy (infra means below or before - so infrared is light below red in the spectrum). We see the light radiating out and the protons (in the infrared energy) causes the air around the fire to heat up (producing more infrared energy). Our nerves can detect the increase in temperature caused by the released energy which we feel as heat. This is why fire is hot. Snow is water that is cold, but in this case we're talking about snow feeling cold - which very different from a lack of heat (don't ask, it's complicated). So, snow is cold and your hand is hot. Heat will (without a heat pump) always pass from a hotter object to a colder object. This is one of the very few general laws of the universe that everyone agrees on (the 2nd law of Thermodynamics). When you touch the snow some of the kinetic energy (of the atoms) in your hand is transfered to the snow. This added energy relaxes the molecular bonds of the ice crystals allowing some of them (near the surface) to flow and the snow starts to melt. Heat is a form of energy and your hand has just lost some (into the snow) so your hand cools. This is why snow is cold. The 1st law (basically) says that you get out what you put in. As energy can not be destroyed the total energy that a system can produce is equal to the energy put in (eg. petrol into a car, a cheese pasty into you or the Big Bang itself). But, as anyone who has ever run out of petrol will know, whilst this may be "technically" possible, it just doesn't work in the real world. You can't start a machine, then run it from its own exhaust - that would be a perpetual motion machine and these simply ain't possible. This is due to the 2nd law which states that heat/energy will "always" move from a hot object to a cold so some of the heat/energy will flow out of the system (entropy) and it will eventually run out of fuel. QED; perpetual motion ain't possible. The 2nd Law of T explains everything from why your chips go cold to answering "will the universe end one day" - the answer to that by the way is dependent on whether it is a (en)closed system, ie. Is there anything outside the universe and this is a religious question so I'm going nowhere near that one. The 3rd law is all to do with the way matter reacts at (or just above) the temperature of absolute zero and is a bit mad!! QED = "Quot Erat demonstrandum" = "which was to be demonstrated" (or simply = "so there you go").. What is the difference between snow and ice? Ice is made when liquid water freezes and snow is made when airborne moisture (gas) freezes. Dihydrogenoxide (that's water to you and me) is unlike almost all other matter as it's volume (size) increases when it freezes, even though the mass (the amount of matter) stays the same. This is due to the molecule consisting of two hydrogen atoms (slight negative charge) and one oxygen atom (slight negative) which, when bonding attract the oppositely charged ends of other molecules (like magnets) forming hexagonal (six sided) spherical crystals which have lots of empty space inside (9% less dense than water - which is why ice floats) so the size of body of water/ice increases. Snow is not (as is often claimed) frozen rain, that is sleet. Airborne moisture will freeze (coalesce) around airborne particles such as evaporated sea salt, bacteria, pollen, etc. forming hexagonal crystals which grow larger as more moisture coalesces around it. As the corners stick out from the sides of the crystal (at a molecular level this is a vast distance) any airborne moisture hitting the crystal will arrive at the corners more often than the sides, so they grow faster. As all six corners are in the same packet of air/moisture they grow the same, forming an elegant and symmetrical six pointed star. Fluffy snow is made when, through friction with air molecules, the crystals thaw slightly making them sticky so they adhere to other crystals. Snow is white because the accreting process does not line up the molecules in any regular manner, so any light (and some other types of energy) hitting it is scattered in all directions - like lots of small mirrors (which is why snow doesn't let on a warm day) with all the colors mixed up and this is seen as white as no part of the spectrum is focused enough to give it a color. Ice, on the other hand, is normally made from simply frozen water with all the molecules in regular slots and so it is translucent (not transparent) as light does not reflect from the lined up molecules in the same way as snow. Light passes through the ice and then back again off whatever is behind it (which may be just more ice in a glacier). Of course the light's photons do mess about a little on the way through which gives the blurred translucent effect. If they did go right through without inter-reacting with a least some of the water then it really would be transparent. As the light passes through the ice the lower (red) part of the spectrum is absorbed so the eventual reflected light is bluer - which (contrary to what "that bloke in the bar" will tell you), is almost exactly opposite from the reason the sky is blue. The deeper the ice the more red is absorbed and the bluer the ice. Test it for yourself - Put a glass of water in the fridge. As the temperature decreases the water will shrink until, at around 4c, it starts to enlarge as the molecules start to bond into crystals. Of course if you open the fridge door to have a look you will increase the temperature and so affect (by allowing warm air into the fridge) the process you're meant to be studying - this is a brilliant example of the Uncertainty Principle, which, contrary to what Hollywood would have you believe, has nowt what-so-ever to do with Jurassic epoch dinosaurs. Why is there more air lower down? It's all to do with gravity. Basically everything attracts everything else (no, don't go there!). The amount of stuff (matter) in a thing is called the Mass and the more Mass there is, the more that thing will (according to Einstein) distort the space around it, causing other stuff to move towards it. The pulling force is weakened by distance, so the further apart two lots of "stuff" are the weaker the pull (it's not really a "pull" but that space-time is curved so mass "falls" towards other mass, like a having a mattress with a heavy object in the center then rolling a small ball over the mattress). If you double the distance between things the pull becomes 4 times weaker ( Newton's Inverse Square Law). Imagine everything has a cloud around it, the more mass the thing has, the bigger and denser the cloud. This cloud pulls other things towards its centre. As you move away from the centre the cloud thins and the pulling power become less strong. So everything is falling towards everything else but of course the bigger the thing (more Mass), the more it will pull. The earth (at just under 6.9 Billion Trillion metric tones), compared to a molecule of air (weighing almost nowt or about 29AMU's = just over twice that of a carbon atom), is pretty big, so it pulls the molecule towards it more than the molecule pulls the earth so the air is pulled downward towards the centre of the Earth's mass. This means that the density of air (the pressure) becomes lower the further you are from the centre of the earths Mass (higher up). Test it for yourself - Drink a cup of tea in a café at the base of a mountain and then another cup in a higher café - the cup of tea higher up will be colder. This is due to the weight of air pressing downwards forcing the water molecules to form into a liquid. Some of the molecules have a higher than average kinetic energy (energy caused by their motion) and are able to escape the molecular bonds holding the liquid together and they escape from the surface as gas (evaporate). By heating the liquid the average energy is increased so more of the molecules can evaporate. When the pressure of the air (pressing down on the surface) and that of the evaporating gas is equal, the water will boil as, at this point, the molecules have enough energy to evaporate not just from the surface but also from inside the body of the liquid. This forms bubbles of gas which rise to the surface and escape (boil). Obviously if there is less air (like there is at altitude) pressing down on the surface of the water then less heat is required to increase kinetic energy before the two pressures (air pressing down and the gas escaping from the surface) become equal, so the water boils at a lower temperature (approx. 1c for each 350m). Where do snow and water go to and where did they come from? In any body of water (from a single raindrop to an Ocean) the molecules are constantly moving and some will escape from the surface - evaporate. So, sooner or later, even without the addition of extra heat, if no extra molecules are added, the whole body will evaporate. Of course if extra heat is added, the average energy of the molecular structure (of the body of water) is increased and the process is speeded up and the water disappears faster. Where water comes from is a "very" long story. Each time you drink a glass of water you are swallowing one of the oldest things in the universe - somewhere between 15 and 20 Billion years old. The hydrogen atoms you have just gulped down were created within the very stages after the Big Bang itself. 1st came gravity and other forces next came heat and then came the lighter elements - one of which is hydrogen. Only slightly less amazing is the other "bit" of the water molecule - the oxygen. Stars "burn" by fusing hydrogen into a heavier element, helium, which in turn is converted into another even heavier element, carbon and so on, each time converting one element into something heavier (oxygen to silicon, etc) until it reaches Iron. This conversion of one element into another normally produces energy and it is this that (basically) pushes outwards against the gravity (weight) of the star (geeky people love the phrase Hydrostatic Equilibrium that is used for this). Iron does not produce energy when converted, it consumes it, so there is nothing to push outwards and the whole lot goes hurtling downwards towards the center of the star. If a star is "big" (many times larger than our Sun) the center of the star may collapse forming a neutron star and, in less than a second, the rest of in-falling solar matter hits this - well lets just say it goes bang! Not only producing a brief flash of light, that sometimes outshines the whole galaxy, it also throws of all the other bits - including the oxygen - which billions of years later, ends up in your kettle. So next time you make a cup of tea, savor it - it took a extremely long time to make. Oh and don't forget that "everything" is made of atoms, including you - 63%Hydrogen, 25%Oxygen, 9.5% Carbon, 1.5%Nitogen, .5% other weird stuff - so happy birthday. You, well some bits of you anyway, have just turned 15billion years old. If heat comes from the ground why is it colder in a shadow? Our nerves are better at detecting "changes" in temperature than just simple heat (I'm not talking about too much "heat" by the way). Whilst you are in a shadow your nerves are detecting the ambient temperature. But step out of the shadow and some of the "sun-light" (energy) that warms the earth also hits you (giving you a sun tan), and even some of the air molecule floating around causing them to move around faster and so "bump" into each-other more often producing heat energy, your nerves detect this temperature difference and you'll feel warmer and get a tan. Test it for yourself - Jump into a hot shower and it will feel very hot, but quite soon you'll get used to it. This is because you will be warmed up by the hot water so your nerve ending are not able to detect such a large temperature difference. Test it for yourself. Sit in the sunshine and your body will react by producing pigments such as Eumelanin (brown) and Phaeomelanin (red), these pigments are able to absorb more UV and so reduce skin damage. The longer you sit in the sun the more pigments are produced and the browner you get - or the redder you get, which is "very" funny. ----
Bibliography * Some bloke in the pub * QI * FHM - High Street Honeys section 2007 & 2005 * Wayne's 1st law = Understanding is better than recital.