How does the atmosphere work?

[Peter Parker]

I've always wondered how the atmosphere repels or sets objects ablaze.

I thought at first that it may have something to do with preasure or the gas molecules, which in turn would repel objects that would try to penetrate its surface. Then I thought maybe solar energy (perhaps solar winds) was the reason why objects are set ablaze when they enter the atmosphere but Im not too sure and Im really curious why it happens. Can anyone explain this strange phenomenon to me?

[Eclogite]

Friction.

 

Meteors coming in from space are travelling very rapidly. 30km/second is a typical value. Striking the atmosphere at that speed is almost like striking a solid wall. Many objects distintegrate. The rest are heated by the friction against the air, so that their exteriors become white hot.

 

It would be misleading to say the atmosphere repels objects. Only if the object comes in at an oblique angle, so that it 'bounces' off the atmosphere, might it be said to repel.

[Boerseun]

An easy way to picture this is to stick your hand in water. It's cool, soft, and easily gives way to your fingers. Now go waterskiing and hit that same water at 60km/h! Hard as rock! Same with the atmosphere, kinda.

[Peter Parker]

Oh that makes sense...thanks

[Turtle]

Friction.

 

Meteors coming in from space are travelling very rapidly. 30km/second is a typical value. Striking the atmosphere at that speed is almost like striking a solid wall. Many objects distintegrate. The rest are heated by the friction against the air, so that their exteriors become white hot.

 

It would be misleading to say the atmosphere repels objects. Only if the object comes in at an oblique angle, so that it 'bounces' off the atmosphere, might it be said to repel.

 

Mmmmm.... My understanding is that attributing the heating of a meteor to friction is a mistaken idea. The high speed is a factor, but the heating is derived from the high compression of air at the leading 'edge' or face of the object. Think about filling a SCUBA tank, which if you've never done requires the tank be put in a tub of water to cool the tank as it fills; conversley, decompressing results in cooling. :(

[CraigD]

Mmmmm.... My understanding is that attributing the heating of a meteor to friction is a mistaken idea. The high speed is a factor, but the heating is derived from the high compression of air at the leading 'edge' or face of the object.

In this context, the distinctions between gas temperature, velocity, and pressure are blurred. An object moving through a gas – the atmosphere - undergoes physical stress and heating because of it’s atoms colliding with those of the gas. Temperature is a measure of the average kinetic energy of the gas atoms relative to the object, following the familiar [math]E = \frac12 M V^2[/math] formula, per unit volume, usually expressed in formulas like the Root mean square speed. If you use these equations to calculate the temperature equivalent of a 30000 m/s entry into the Earths atmosphere (considering it to be 14 AMU nitrogen), you get something like 500000° (C or K, it doesn’t much matter for temperatures this high), which doesn’t make much intuitive sense.

 

I find it most useful to visualize the pressure on and heat transferred to an object moving through a gas – be it a meteor or a high-speed aircraft – as resulting from the many collisions of its surface with the gas’s molecules.

[Turtle]

I find it most useful to visualize the pressure on and heat transferred to an object moving through a gas – be it a meteor or a high-speed aircraft – as resulting from the many collisions of its surface with the gas’s molecules.

 

Tha's some powerful careful wording there Craig. Try as you might however, you could not bring yourself to say it is 'friction', and try as I might I cannot help but say it is not.

[LJP07]

Is is true that if you drop from an aeroplane accidently say, and happen to be dropping in to sea from a relatively high distance, does the sea act like a concrete block and it's extremely painful.

 

I thought this may be true if the particles are held together and suddenly there is a bang just disturbing them thus causing harm, but can you confirm this?

[CraigD]

Is is true that if you drop from an aeroplane accidently say, and happen to be dropping in to sea from a relatively high distance, does the sea act like a concrete block and it's extremely painful.

Though it’s commonly described that way, no.

 

An object (hopefully not a person) striking the surface of a body of water at a high speed makes a (very big) splash and penetrates the surface in a way that, viewed in slow motion, most people wouldn’t at a glance find different from a much lower speed impact. The resulting forces and mechanical work (transfer of energy), however, are much greater. This is why falls from high places into water typically either fatally injure, or incapacitate their victims enough to cause drowning.

 

Mythbusters episode 5 showed contained some nice high-speed video of this sort of thing, as well as some pretty fancy crash-dummy sensor data.

[Turtle]

Is is true that if you drop from an aeroplane accidently say, and happen to be dropping in to sea from a relatively high distance, does the sea act like a concrete block and it's extremely painful.

 

I thought this may be true if the particles are held together and suddenly there is a bang just disturbing them thus causing harm, but can you confirm this?

 

The action of the water on a body falling into it, both human and otherwise, is also strongly affected by the shape of the body and its orientation to the water. Whenever possible, when one is falling from a great height into water the best (safest) position is feet first, vertical, legs crossed, chin tucked and arms locked around the chest.

 

The more streamlined an object that falls into water, and the closer its axis of streamlining to vertical, the less shock to that body on entry.

[Boerseun]

Jumping into a pool unstreamlined, say, a belly flop, is painful. This is just a matter of velocity - a fall into water at not very much higher speeds than could be obtained from jumping from an average-heigth diving board, would be more than sufficient to burst some really needed internal organs.

[LJP07]

Would the mass of the water affect this person dropping in, just say for example, you have 5 tonnes of water and anotehr pool with 50 tonnes of water. The person drops face first into both pools, will one effect him more than the other, if so, why is this occuring?

[InfiniteNow]

Would the mass of the water affect this person dropping in, just say for example, you have 5 tonnes of water and anotehr pool with 50 tonnes of water.

 

There are many factors, not just mass, which play a role. You can significantly increase your understanding by viewing the wiki article on surface tension.

 

Surface tension - Wikipedia, the free encyclopedia

[CraigD]

Would the mass of the water affect this person dropping in, just say for example, you have 5 tonnes of water and anotehr pool with 50 tonnes of water. The person drops face first into both pools, will one effect him more than the other, if so, why is this occuring?

No.

 

Provided that both pools are sufficiently large that an equivalent volume of water can be displaced by the object striking it – that is, as long as it’s not nearly the same size and shape of object’s cross-section, preventing water from escaping it when struck by the object – the forces on the body will be nearly identical.

 

Although the mass of the water doesn’t, the density of the water does have an effect on the forces experienced, greater density resulting in greater force. For example, when diving or jumping from the same height, salt water slaps you slightly harder than fresh.

 

Certain liquids, such as a thick corn starch suspension, can be much harder when struck at more than a specific speed, due to an effect known as the Non-Newtonian fluid. A popular demonstration of this effect involves slapping a shallow container of such a solution – a soft slap will splash it out of the container, while a hard slap will not. Jumping into a pool of such liquid from more than a low height feel the same as jumping onto a hard floor covered by a thin rubber mat – if you ever do a demonstration involving jumping into a tub of it from a step ladder, be careful to do so with bend knees!

[LJP07]

Understood, thanks again CraigD.

 

Just want to clarify a concept that many people believe. Is it true that when it fogs, the clouds come down to our level?

[InfiniteNow]

Is it true that when it fogs, the clouds come down to our level?

Fog is just a cloud that is at a lower altitude, usually touching the ground. That's the only difference as they are really the same thing. It's not so much that a cloud decends, it's just formed at ground level.

[LJP07]

Would cloud be formed at normal height when it is foggy? What causes the fog to occur at ground level?

 

Also, why is it when it's really cold at night, the sky conditions are perfect for astronomy? The sky is always perfect when the temp. is ultra cold.