ATS 113--chapter 4

What is centripetal acceleration?

Centripetal acceleration is the acceleration associated with the flow in a curved direction. It is associated with the Centrifugal force. The centripetal acceleration is inward directed..

With which pressure system (High pressure or low pressure) would you expect the usually get clouds?

Cloud with low pressure because of convergence and rising motion. Which one would you expect to usually get clear skies? High pressure because of sinking motion and divergent flow.

What happens to density as you go up in the atmosphere?

Decreases

What happens to pressure as you go up in the atmosphere?

Decreases

What does friction do the wind speed, the Coriolis force, the pressure gradient force?

Friction slows the wind, it weakens the Coriolis force (see the Coriolis force equation), and it does nothing directly to the pressure gradient force.

What is pressure? Pressure is a force per unit area.

In our atmosphere, it is the force per unit area exerted by the gases.

If you are above the surface, what force is non-existent?

In the free atmosphere, friction is essentially non-existent. In the planetary boundary layer (the first 1.5 km), the friction force is important.

What are supergeostrophic and subgeostrophic flow?

Supergeostrophic flow is the gradient flow found around high pressure systems where the forces result in a wind speed faster than would be expected from the pressure gradient force. Subgeostrophic flow is the gradient flow found around low pressure systems where the forces result in a wind speed slower than would be expected from the pressure gradient force.

When is the Coriolis force equal to zero?

The Coriolis force is the force that appears to deflect motion. It is an apparent force that arises because of the earth's rotation. It is always zero at the equator and when the wind speed is zero.

What is Guldberg-Mohn balance? What forces are in balance?

The Guldberg Mohn balance is a balance that arises because of the effect of friction on the wind. The forces in balance here are the friction force, the pressure gradient force and the Coriolis force. If we have a closed low pressure or high pressure system, we would add the centrifugal force.

Identify the direction of air flow around a high pressure in the Northern Hemisphere.

The air flow around an area of high pressure in the northern hemisphere is clockwise.

Identify the direction of air flow around a low pressure in the Northern Hemisphere.

The air flow around an area of low pressure in the northern hemisphere is counterclockwise.

In the Guldberg-Mohn balance, how do we explain the cross isobar flow?

The cross isobar flow is the result of the friction reducing the wind speed which in turn weakens the Coriolis force. This cause the flow to cross isobars toward low pressure and cross isobars outward from high pressure.

What are the forces that move the air?

The forces are the pressure gradient force, the Coriolis force, the gravitational force, the centrifugal force and the friction force.

What is geostrophic balance? What forces are in balance?

The geostrophic balance is the balance found where the air is not accelerating and the forces in balance are the pressure gradient force and the Coriolis force.

What is gradient balance? What forces are in balance?

The gradient balance occurs where the air flows parallel to the isobars in a curved manner. The air accelerates as in the curved path. (Why?. This is left for you). The forces in balance are the pressure gradient force, the centrifugal force and the Coriolis force.

What is the hydrostatic equilibrium? Why is it important?

The hydrostatic balance is the balance between the vertical pressure gradient force and the gravitational force. Without it, the atmosphere would have rapidly ascending air in all areas.

What is the planetary boundary layer, the free atmosphere?

The planetary boundary layer (PBL) is that part of the atmosphere within the first 1.5 km (approximately) where friction is important. The free atmosphere is the region above the PBL where friction is no longer important.

Pressure gradient force

This force is the result of pressure differences. The greater the pressure difference, the more closely drawn are the isobars. This means that the wind is very strong. The weaker the pressure gradient, the more spread are the isobars. This means that the wind is weak. This force is always pointed toward low pressure.

Friction force

This force is the result of the character of a surface. It causes the wind to slow down in speed and can change the direction of the wind.

Geostrophic Balance

This is a balance between the horizontal pressure gradient and the Coriolis force. In this balance, the horizontal pressure gradient force is pointed toward low pressure (it always is) and the Coriolis force is pointed in the opposite direction. The resultant wind is the geostrophic wind. This flow is generally occurring in flow that is not curved (that is non-accelerating).

Hydrostatic Balance

This is the balance between gravitational force directed down and the pressure gradient force which is pointed up.

Gradient Balance

This is the balance between the Pressure gradient force, the Coriolis force, and the Centrifugal force. This balance occurs in curved flow (accelerating flow) where the centrifugal force is always directed outward from the center, pressure gradient force points toward low pressure and the Coriolis force points in the opposite direction to the pressure gradient force.

Gravitational force

This is the downward directed force that is the result of the Earth's gravitational field.

Coriolis force

This is the force that occurs because the earth rotates. It causes a deflection of the air in a rightward direction in the northern hemisphere.

Why do we need to reduce the pressure to sea level?

To plot maps of pressure, we have to take into account the terrain. Because pressure decreases with height, if we didn't reduce pressure to sea level, there would be constant areas of low pressure in elevated locations.

Gulberg-Mohn balance

balance between the friction force, coriolis force, and the pressure gradient force. This balance occurs in the planetary boundary layer and accelerates the wind by changing the speed and direction (velocity). It has no impact on the pressure gradient force.

Centrifugal force

represents the effects of inertia that arise in connection with rotation and which are experienced as an outward force away from the center of rotation.