Wikipedia:Reference desk/Archives/Science/2016 February 25

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February 25

Where does the air go on a hot day?

On a hot day, the air gets thinner; less atoms per unit volume. I'm gonna go out on a limb here and say that atoms are not created or destroyed in the earth's atmosphere in sufficient numbers to make up the difference; where do they go, then? Does it follow the sun? Is this what meteorologists mean when they refer to "high pressure/low pressure" fronts; warm weather in one area pushing atoms out over hundreds of miles, or cold weather "sucking" them in? Do they "go" up (e.g., does the air density at 10km vary with the temperature at ground level? That seems somewhat hard to believe)? Apologies for the silly title, by the way! Riffraffselbow (talk) (contribs) 00:56, 25 February 2016 (UTC)

In a pinch yes, warm air has less atoms per unit volume BUT the volume is increased, the number of atoms stays the same, warm air expands, becomes less dense and "floats up" sucking in surrounding, often cooler air. Vespine (talk) 01:05, 25 February 2016 (UTC)

An article with pictures like this might help understand it better. Vespine (talk) 01:09, 25 February 2016 (UTC)

That article is a good schematic illustration of high and low pressure systems but those systems are caused by processes that are essentially unrelated to the daily cycle of heating and cooling. Shock Brigade Harvester Boris (talk) 03:14, 25 February 2016 (UTC)

Mostly they move to colder spots on the planet (hence wind), although some vertical motion occurs as part of the process. StuRat (talk) 01:18, 25 February 2016 (UTC)

When air is heated near the ground it expands. Somewhat paradoxically, this causes an increase in pressure aloft -- you can think of the whole atmosphere being lifted up, though that's not formally correct.

At locations near a coastline this can cause a sea breeze. The pressure surfaces are lifted aloft over land but not over the sea, since the surface temperature of the sea responds only very weakly to the daily cycle of light and dark. This difference causes a pressure contrast between land and sea, which can lead to a sea breeze.

The large scale high and low pressure systems seen on a weather map have very little relation to this daily cycle of heating and cooling. Shock Brigade Harvester Boris (talk) 01:44, 25 February 2016 (UTC)

Ok, i think some of the above is probably causing more confusion than necessary. When air is heated near the ground it expands. Somewhat paradoxically, this causes an increase in pressure aloft . Talking about "increase in pressure aloft" I think is confusing, areas where the air is heated create LOW Pressure zones as seen in weather maps, areas of low temperature creates high pressure, due to plain old equilibrium air travels from high to low pressure. Quote from the link I pasted above "Areas of high and low pressure are caused by ascending and descending air. As air warms, it ascends leading to low pressure at the surface. As air cools, it descends leading to high pressure at the surface. ". So StuRat saying some vertical motion occurs as part of the process. is also incorrect, the vertical motion is what starts the whole thing off, it's not a "byproduct". if wind was actually CAUSED by expanding air you would expect wind to travel from hot areas to cold areas as the hot air expands, but that's the opposite of what actually happens. Vespine (talk) 03:12, 25 February 2016 (UTC)

I didn't say it was a byproduct. I made no statement about which occurs first, the vertical motion or the horizontal. StuRat (talk) 05:23, 26 February 2016 (UTC)

(edit conflict)Firstly, high and low pressure systems of the type that we see on weather maps are caused primarily by baroclinic instability. The daytime solar heating cycle plays a secondary, indirect role at most. So talking about large-scale high and low pressure systems isn't really responsive to the original question.

The initial response to surface heating is that surface pressure remains constant (via the hydrostatic relation). Fundamentally this must be so because air in the column is not created or destroyed. Since the surface pressure remains constant, but the air near the surface expands, it follows (again by the hydrostatic relation) that any arbitrary level of constant pressure aloft becomes higher (in height) than it was before the air was heated. Saying that a given pressure takes on a higher altitude is exactly the same as saying that a given altitude aloft takes on a higher pressure. This is tough to get your head around at first -- even my class of very, very bright grad students last semester had trouble with it.

If the surface is not heated uniformly (which indeed it cannot be, what with Earth being round amongst other things) this will cause a pressure gradient aloft. It is this pressure gradient that then causes the air to move. The lateral transport of mass then leads to surface pressure changes. Shock Brigade Harvester Boris (talk) 03:36, 25 February 2016 (UTC)

If I may pitch again for the fantastic free textbook, Aviation Weather - the very first chapter introduces the atmosphere, and the very second chapter talks about the effect that temperature has on the atmosphere. High temperature reduces air density - so, all other things being equal, that means lower pressure. In actual reality, all other things are almost never equal - so temperature and pressure in Earth's atmosphere can vary as largely independent parameters. This may contradict some of the simplifications that you learn, such as the ideal gas law; although the ideal gas law does still apply, the key concept is that Earth's atmosphere is very big, and there are enormous sources and sinks for heat, mass flux, and so on; so simple models just don't work well. Nimur (talk) 03:21, 25 February 2016 (UTC)

I think I saw in my weather textbook (when I took weather my first year) contrasting surface lows and highs with upper air lows and highs. Warm air at the surface creates a surface low (not necessarily on a weather map / synoptic scale), which rises aloft -- the air column reaches higher altitudes for a given pressure, and is less dense, and "stretches out" more. This means that at the upper air level -- 500 mb, say -- high above the ground, this air column will actually be higher in pressure then the air around it. Likewise, a cold air column will be denser than a warm air column, will create a surface high and an upper air low. So air will flow in a convection-like manner -- at the surface it will flow from the cold air column to the warm one, but at the upper air level, it will flow from the warm air column to the cold one, forming a closed loop. Yanping Nora Soong (talk) 05:32, 26 February 2016 (UTC)

• Not disagreeing with answers above, but just to answer the question as simply as possible: yes, the air goes up. In fact when you have mountains, heating during the day tends to produce winds that blow upslope as the atmosphere expands; cooling during the day produces winds that tend to blow downslope as the atmosphere shrinks. Looie496 (talk) 15:52, 26 February 2016 (UTC)

What does a photon of visible light look like before you see it?

I have heard that the human eye can see even a single photon of visible light. But, before this photon is seen, and absorbed, what kind of physical properties does a photon have as it travels through space? Does it have any dimensions? Is it only visible if one is directly in it's path, or is a photon visible from a vantage point perpendicular to it's direction of travel? Would these answers change if a billion photons were involved? Honeyman2010 (talk) 23:25, 25 February 2016 (UTC)

See Photon and Photoreceptor cell - note that both articles are fairly complex. A photon is a unit (a quantum, to use the technical term) of electromagnetic radiation. It becomes visible when it interacts with the cells of the retina - for it to reach the retina, it has to pass through the eye, so you do have to be directly in its path. I'm not sure about the precise conversion between the number of photons and the intensity of the light (measured in lumens), but a billion probably still isn't enough to read by. Tevildo (talk) 23:44, 25 February 2016 (UTC)

Additionally, as a photon usually travels at the speed of light, the huge Time dilation makes it impossible to "watch" it. There is one exeption tho. With Photonic crystal, because they are able to "trap" photons, you can in a way. So if you for examply buy a nice Opal you can "watch photons" from any angle you like. --Kharon (talk) 01:03, 26 February 2016 (UTC)

Wow, this is a tough one... how much quantum mechanics do you want to learn? How comfortable are you with mathematical abstraction?

We want to treat photons as a point particle, but they just aren't.

Photons have a position - but it's weakly useful, because its position is uncertain. We have to solve for the position, by applying a position operator to a wave function; and then we just get a sort of statistical distribution with information about the position. There is no single point where the photon is - it's spread out over space. In that sense, the photon has extent that is inseparable from its location.

A little bit more abstractly, a photon is a localization of disturbances in the electric and magnetic fields. Where is the disturbance? Well... at a position that can only be described as above, using quantum mechanics. How big is the disturbance...? Well... even the magnitude of the field at any point can only be described using the wave function! So, we need to use a quantum mechanical wave equation just to get to the point where we can apply the electromagnetic wave equation.

The easiest and most intuitive way to walk away from this headache-inducing mess is to succinctly summarize photons this way: photons travel as a wave, and interact like a particle. So - after an event occurs, like when a photon slams into a camera-pixel or a retina-cell, then we can positively describe where it hit, and how much energy it brought. Before that event occurs, the photon is traveling - and the best way to describe it at that time is to fully define it using the wave equation. This is called quantum electrodynamics, and it involves very difficult equations. We'll need to start teaching you a lot of symbology before you can start making headway understanding the way those symbols relate to real observable things.

If you want to read what a famous physicist - specifically, George Gamow, had to say about this: try to find a copy of Mr Tompkins's voyages. This is a very old book, written by one of the founders of modern atomic physics, and it's intended to help interested readers visualize the weird effects of non-classical behaviors. The author presents a view of the universe if fundamental physical constants were different, and among the many adventures that the protagonist experiences, he gets to see the visual effects of relativistic dilations; he gets to see particles interact and collide with photons and with antiparticles; and all sorts of other fantastic things.

Nimur (talk) 01:59, 26 February 2016 (UTC)

Just to summarize the above, a photon is invisible until it strikes something. Now you may wonder why you can sometimes see a sunbeam. Some of the photons are actually striking dust particles in the air, and being deflected into your eyes. That wouldn't happen in a vacuum. There the ray of light would be invisible. StuRat (talk) 05:20, 26 February 2016 (UTC)

To put it a bit differently, when you see anything, it's because photons from the thing you're seeing are entering your eye. Vision is the interaction of photons with your eye. It doesn't even really make sense to talk about "seeing a photon" in the same sense that you see an apple. Mnudelman (talk) 22:12, 26 February 2016 (UTC)

This ties into a question I asked recently about Bohmian mechanics - see this reference I was given then. Apparently the degree to which a photon is a real particle moving along a real path is a matter of interpretations of quantum mechanics, which is to say, the same math predicting the same results can be taken to mean different things intuitively depending on how you look at it, without (I think!) a scientifically testable difference between them. Wnt (talk) 02:29, 27 February 2016 (UTC)