WIND IN THE ATMOSPHERE

Q: What is a force? A force is a pushing or pulling that will result in motion if it is unopposed.

Newton’s Laws of Motion

1. In the absence of forces an object at rest will remain at rest and an object in motion will remain so with the same velocity.

2. Force equals mass times acceleration (F = m·a)

3. To every action there’s an equal and opposite reaction.

Forces that affect winds in the atmosphere

1. Pressure gradient force – the greater the pressure gradient the stronger the wind. This is the force that pushes air out of an inflated balloon.

2. Gravity – acts downward, toward the center of the Earth. Gravity balances the pressure gradient in the vertical.

3. Coriolis force – due to earth’s rotation. The Earth rotates counter clockwise in the Northern Hemisphere. The Earth rotates under moving air; therefore, the air appears to curve to the right. The opposite is true in the S.H.

4. Centrifugal force – from Newton’s first law: objects want to move to move in straight lines.

5. Friction – acts in direction opposite motion, and degrades wind motion into heat.


Q: Why do we not notice the affect of the Coriolis force in everyday life?

Answer: the Earth rotates too slowly. For example, if you throw a ball and it takes the ball 10 seconds to land, the earth has turned only minutely underneath the ball during those 10 seconds because it takes the earth 24 hours to make a complete rotation. Thus the affect is not felt or seen.

Q: Which of the five forces can produce winds from air that is initially at rest?

Answer: only pressure gradient force and gravity.

Q: What forces act to produce the horizontal winds? –

We can neglect gravity since it acts in vertical. Forces left are pressure gradient, Coriolis, friction, and centrifugal. Away from earth’s surface we can ignore friction; now we have pressure gradient, Coriolis, and centrifugal: in absence of curvature, there is no centrifugal force, and we are left with the pressure gradient and Coriolis forces to balance each other.

Geostrophic balance – When the pressure gradient force is just balanced by the Coriolis force. The resultant wind is called the geostrophic wind. Much of the flow in the atmosphere is very nearly in geostrophic balance.

Gradient wind balance – Wind resulting from balance of Coriolis, pressure gradient, and centrifugal forces. This is the case for curved flow around highs and lows.

Cyclostrophic balance – Wind resulting from a balance of pressure gradient force and centrifugal force (found in tornadoes and the eye wall of hurricanes).

Hydrostatic balance – In the vertical we have a balance between the vertical pressure gradient and the force gravity exerts on the air. It can be expressed by:

the change in pressure = - density x gravity
change in height

or

where r = density of the air, and g = the acceleration due to gravity.
Near the Earth’s surface (in the boundary layer) friction results in reduction of wind and a net flow into low-pressure areas and out of high-pressure areas.