Viscosity of Air: What Makes Calm Possible on Earth?

[limarodessa]

Hello all !

Why on a surface of the Earth the calm is possible? The Earth rotates about (аround) the axis. Why air molecules are motionless concerning the Earth?

 

[klimatos]

Hello all !

Why on a surface of the Earth the calm is possible? The Earth rotates about (аround) the axis. Why air molecules are motionless concerning the Earth?

Whether or not a parcel of the atmosphere is "calm" depends upon the sensitivity of your sensing instruments. If your instruments show no NET movement of air, then we say the air is calm.

All that "calm" means is that the number of molecules moving in anyone direction is essentially the same as the number moving in the exact opposite direction, and that their mean speeds in each of those two directions are the same.

Hence, no net movement does not mean no molecular movement. It just means that all direction movements and speeds cancel out.

The molecules of the air are in continual movement, both relative to one another and relative to the surface of the Earth. The various distributions of molecular velocities are Maxwell distributions. The mean of and single distribution depends upon molecular masses and the air temperature.

For dry air at a temperature of 25°C, the mean molecular speed will be some 468 meters per second along the molecules' true paths.

 

[limarodessa]

All that "calm" means is that the number of molecules moving in anyone direction is essentially the same as the number moving in the exact opposite direction, and that their mean speeds in each of those two directions are the same.

Hence, no net movement does not mean no molecular movement. It just means that all direction movements and speeds cancel out.

But when we go by the open car (cabriolet) we feel a motion of molecules of air - a wind to the face

Why it is not so when Earth rotates ?

 

[Ophiolite]

But if you close the car window the air in the car moves along with you, just as the atmopshere moves along with the Earth. There is nothing opposing the atmosphere's motion with the Earth, so it moves with it and relative to it is stationary - except, of course, for all the thermal activity that generates wind.

 

[LudusRex]

I can explain that the viscosity of air is what allows for calm conditions on the Earth's surface. Viscosity is a measure of a fluid's resistance to flow, and air has a relatively low viscosity compared to other substances. This means that air molecules are able to move and flow easily, allowing for calm conditions on the Earth's surface.

Additionally, the Earth's rotation about its axis plays a role in creating calm conditions. The rotation causes the Coriolis effect, which deflects air currents and creates high and low pressure systems that contribute to calm or turbulent weather patterns.

It is important to note that air molecules are not completely motionless concerning the Earth. In fact, they are constantly in motion, but the low viscosity of air allows for smooth and gradual movement rather than chaotic turbulence.

Overall, the combination of low viscosity and the Earth's rotation are the key factors that make calm conditions possible on the Earth's surface.