What is the difference between moving air and still air

Such expansion results in loss of heat and consequent fall in temperature. Similarly, when air descends the air compresses and its temperature rises. The rate at which air on expansion cools is called the adiabatic lapse rate, and for dry air it is equal to 9. Adiabatic means that the air exchanges no heat with its surroundings, a condition very nearly true for rising and descending packets of air. If the rate at which the surrounding air temperature falls is less than the adiabatic lapse rate, a rising packet of heated air will cool faster, lose its buoyancy, and sink back to its original position.

In this case the atmosphere is said to be stable. If the rate at which the surrounding air temperature falls is greater than the adiabatic lapse rate, the packet of heated air will continue to rise. The atmosphere in this circumstance is said to be unstable. When the air is saturated with water vapour, the processes are similar to those described above for dry air, but the adiabatic lapse rate is different. When saturated air rises and cools, condensation of water vapour begins, releasing latent heat.

Consequently the temperature in rising moist air falls less than it otherwise would. This approaches the value for the dry adiabatic lapse rate for much cooler air carrying little water vapour. More usually in the atmosphere, unsaturated air rises, cooling at the adiabatic lapse rate until it reaches its dew point. Thereafter, it behaves like saturated air.

The moisture condensing out of the air becomes visible as cloud. Assuming that land heats up faster than the sea, at what time will a sea breeze along the coast be strongest? In the figure below indicate the direction of katabatic air flow on an otherwise still night, and mark the region of coldest temperatures.

Convection is the term commonly applied to vertical movement of air, whilst advection is used in the context of horizontal displacement of air.

This will occur when the incoming solar radiation balances the outgoing terrestrial radiation. At this time, the temperature of both the land and the sea reaches a maximum, but because the land heats up much faster than the sea, this is also the time of maximum temperature differences between land ad sea.

The warmer air over the sea then rises, pulling out the colder air over the land to generate a land breeze. The regions of coldest temperature are marked in the figure. This is greater than the dry adiabatic lapse rate 9. Air heated at the earth's surface will therefore continue to rise high into the atmosphere, which is said to be unstable. The Movement of Air Information Sheet 3.

Wind due to differences in pressure Movement of air caused by temperature or pressure differences is wind. Since it's the gradient of a scalar, it is curl free. As we just established, that means that the thing that it acts on the fluid parcel always keeps the same total energy. To state the connection correctly, you must not look at different places across the fluid; you must pick a flow line and follow it along in conditions of laminar, i.

As you move along this flow line, if the pressure goes up then the velocity goes down, and if the pressure goes down then the velocity goes up. Put like this, it becomes, I think, reasonably intuitive: when moving from a lower to a higher pressure, there is a pressure gradient opposing the flow, so the flow will naturally slow down.

To calculate this one calculates the work done by the pressure forces on either side of any particular parcel of fluid. The parcel of fluid gains or loses kinetic energy equal to the difference between the two amounts of work. This connection goes by the name Bernoulli's principle. I think the reason why it puzzles people is because it is often presented the other way round: it is said that when the fluid speeds up its pressure falls, or it is said that the fall in pressure is owing to the faster speed.

I think this way of putting it introduces a muddle between cause and effect. It is clearer to say that if there is a pressure difference then the fluid must respond accordingly.

But the emphasis on velocity can be useful when one is doing detective-work to find out what a given fluid is doing. When one observes laminar flow and one finds that the fluid is accelerating along some of its flow lines, then one can deduce that there is a pressure gradient along those flow lines.

All this does make a modest contribution to the physics of flight, but in that case there are other issues to think about as well. In particular, it is debatable whether the laminar flow approximation is ever good enough to describe an aerofoil, even to first approximation.

Since I have made several programs for lift and drug force acting on airfoil computation, I can explain my suggestion about how lift force generates. First of all, there is wrong explanation in Wikipedia about it. When we compute force we put some boundary condition for air on the top and bottom surface of airfoil.

Therefore there is no difference in velocity at all and it breaks the theory about Bernoulli principle and energy conservation. Nevertheless we have some pressure difference on the top and bottom surface, but this difference is mostly due to flow structure around airfoil.

It depends on angle of attack and mostly looks like on pictures presented on. Drag and lift force acting on an airfoil. How can we improve transonic flow visualization? Plotting Joukowski Airfoil Streamlines using conformal maps. On the last picture from the last post we can see red zone past to airfoil and this is main reason for the pressure difference.

When an aircraft starts from the ground it has very higher angle of attack and flow around wings looks like on the left picture below, while on the right picture there is pressure distribution shown. Apparently we have low pressure on top and high pressure on bottom surface, but velocity distribution contradicts to Bernoulli principle. Because when a stream of moving air attached to a curved wing is made to change direction, it sucks. The air wants to move in a straight line, but the surface underneath it is curving away from the flow.

So it tends to create a partial vacuum. Here is something to help us imagine the reason from first principles. We can assume that the air above the paper and the air below have equal temperatures. This can happen if the velocity of the 'blow' isn't too high.

Then by the time the moving air reaches the area above the paper, its temperature has equalized with the surroundings. However, since molecules in the air above the paper have a horizontal component of velocity, they'll have a lower vertical component, on average, than molecules under the paper so that the combined speed, from Pythagoras theorem, is the same. The ones above the paper then hit less hard on the paper vertically than the ones below and the paper is pushed upwards.

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K 1, 3 3 gold badges 17 17 silver badges 26 26 bronze badges. It does not apply significantly in open air. Add a comment. Active Oldest Votes. Improve this answer. Mike Dunlavey Mike Dunlavey Carl Witthoft Carl Witthoft 9, 2 2 gold badges 17 17 silver badges 28 28 bronze badges.

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