# Does “wind” include the air molecules involved, or merely the kinetic energy?

This question is about a riddle in which the statement "I am weightless" was made, and the answer was "wind".

I want to know, by definition, does the wind include the air being moved, and thus have a weight, or is it only the kinetic energy, and thus doesn't have a weight.

• $E_k = 1/2 m v^2$ so yes it depends on the mass of the object. Kinetic energy it is directly proportional to the mass of the object and to the square of its velocity. The integral of momentum with respect to velocity is the kinetic energy, mathematically. – Isopycnal Oscillation Apr 20 '15 at 19:16
• Wind (or air) has mass and weight, but that is annihilated by bouyance (though that is a bit tautologoical) – Hagen von Eitzen Apr 20 '15 at 21:21
• @HagenvonEitzen Except when it isn't and thus we get up/down drafts. – nHaskins Apr 20 '15 at 21:40
• It's striking how compact mathematical language is to describe nature. The formula given by @Isopycnal Oscillation answers the question for those of us who read math. – Tactopoda Apr 21 '15 at 7:22

In meteorology, wind refers to the velocity of air. Its magnitude is called wind speed and has units of m s$^{-1}$. Related quantities are momentum (mass times wind) and kinetic energy (wind times half momentum), but they are not wind.

Air has mass and thus it has weight as well. Wind does not.

• Thank you for adding this perspective. It's very true that wind, on land and sea, when defined and measured is the speed and the density of the gas is not concerned. However, I thought that OP referred to the wind in everyday language, as the phenomena we can observe. Maybe the question should be edited and answers updated accordingly. – Tactopoda Apr 25 '15 at 14:37
• @Tobbe Well, we are on the Earth Science website after all. Ideally, scientific and everyday language should refer to same physical concepts. I think it's part of out job to communicate these concepts accurately. :) – milancurcic Apr 25 '15 at 14:55
• Agree, but it's difficult sometimes... We should be better at editing the questions to be more precise and still understandable. – Tactopoda Apr 25 '15 at 15:02
• @Tobbe After reading the question again it does seem like OP was meaning wind as described by milancurcic. I did not even think about it at the time, but looking back it makes sense. Afterall, a riddle often hinges around double-speak or veiled meaning. – Isopycnal Oscillation Apr 25 '15 at 23:58

Kinetic energy is the work needed to accelerate a mass. Wind defined as the velocity air (not the same as energy, as noted in comment).

However, the air moved by wind is a good example of kinetic energy, molecules are accelerated by a difference in pressure and moved. When the pressure difference decreases, the movement of molecules stop. So yes, the wind moves matter with weight.

However, weights of gases are somehow difficult to sense, as any gas would replace the weight of the air that normally surrounds us. The weight of the air can therefore be understood as the pressure generated by the weight of all the air above. Air particles moves from high pressure to low pressure and so the wind is formed.

A way to show how efficiently the wind is actually moving the air masses, is to consider how the wind can transport solid matter, sailing boats, wind mills and even sand and rocks. For example the Harmattan, a wind from the North that transport huge amounts of dust from Sahara to the coast of West Africa and the Atlantic Ocean. Other air borne sediments can form thick formations of Loess or other aeoloian formations as sand dunes. Also consider how the wind can move clouds and how all water in rivers and lakes therefore have been transported by the wind.

I hope that this, very simplified answer will make you more interested in meteorology and Earth science in general.

• Completely agree with your concern. Even if semantic, we should use the terms correctly. – Tactopoda Apr 26 '15 at 0:25

Looking up the definition of "wind" in the Oxford English Dictionary, the relevant definition is:

The perceptible natural movement of the air, especially in the form of a current of air blowing from a particular direction

It is the movement of the air, rather than the air itself, that is referred to. It is thus reasonable to say that while air has mass and hence weight, wind does not.

Whether dictionary definitions are important in the context and spirit of a riddle is, of course, up for debate :-)

There's no need to get into details about the energy in wind:

Let's assume there is wind in a volume of air.
Now, the wind stops.
The mass of the air in the volume did not change*.

So the mass of air plus the mass of wind is the same as the mass of air. Therefore, the mass of wind must be 0:

mair + mwind = mair

mwind = 0

*) Assuming the volume is large enough that the effects on the border of the volume are relatively small enough.

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