EVS 113- Chapter 4 Review Questions

What is the 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 to 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 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.

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. In the planetary boundary layer, friction causes a drag that slows wind speed and therefore reduces pressure gradient force as well as Coriolis force.

Forces in the upper atmosphere

In the upper atmosphere the force of friction is nearly negligible. The pressure gradient force in the upper atmosphere causes the air to move faster from high pressure to areas of low pressure which causes them to deflect more to the right via the Coriolis force. This movement continues until the air parcel reaches a point where the pressure gradient force and the Coriolis force are equal and there's no net force working on the air parcel and moves parallel to the height contour with constant speed This is geostrophic flow.

What is pressure?

Pressure is a force per unit area. In our atmosphere, it is the force per unit area exerted by the gases. Total pressure is the sum of the partial pressures or the pressure exerted by each gas in the atmosphere which is expressed by Dalton's law. There are two things that can influence pressure: 1.) an increase in density will cause the pressure to go up (pressure is highest near the surface because this is where the atmosphere is most dense) 2.) an increase in temperature will cause the pressure to increase because the air molecules will move more rapidly. Thus, the pressure is directly related to temperature and density. Further, pressure is exerted in all directions equally. Air constantly moves to establish an equilibrium between areas of high pressure and areas of low pressure.

What are supergeostrophic and subgeostrophic?

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. sub is low and slow. super is high and fast.

Coriolis Force

The Coriolis force causes a deflection to Earth's surface over time. In the northern hemisphere, objects appear to deflect to the right because of the rotation of the Earth's surface. The Coriolis force is strongest at the poles and decreases to zero at the equator. The magnitude of the Coriolis force increases with wind speed. A fast moving object that covers a long distance over the same period of time as a slow moving object that covers a smaller distance will be subject to greater deflection. However, the Coriolis force only changes the direction of an object and never the object's speed. 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.

What is the Coriolis force? When 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. The Coriolis force causes an apparent deflection to the right in the Northern Hemisphere and an apparent deflection to the left in the Southern Hemisphere.

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

The Gulberg 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 low pressure in the Northern Hemisphere.

The air flow around an area of low pressure in the northern hemisphere is counterclockwise. This is a cyclone: air converges toward low pressure centers which are characterized by ascending air that cools to form clouds and possibly precipitation.

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 causes the flow to cross isobars toward low pressure and cross isobars outward from high pressure.

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

The flow around an area of high pressure in the Northern Hemisphere is clockwise.

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. Geostrophic flow only occurs in the upper atmosphere where the Coriolis force and the pressure gradient force enter a balance.

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. 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 because the pressure is concentrated near the surface and pressure gradient force causes air to flow from high to low pressure. Hydrostatic Equilibrium is when the vertical pressure gradient force is balanced by the force of gravity. Because the forces balance, there is no acceleration upward or downward. Occasionally, the vertical pressure gradient force exceeds the gravitational force which causes powerful updrafts that can cause strong thunderstorms. On the other hand, sometimes the gravitational force slightly exceeds the vertical pressure gradient force which causes downward movement of air.

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. The planetary boundary layer is the lowest 1.5 km of the atmosphere, where friction is an important force in affecting wind. The free atmosphere is the region of the atmosphere far enough above the surface that friction can be neglected, generally 1.5 km and higher.

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.

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.

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.

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. Also, pressure decreases rapidly as altitude increases so the variation makes it hard to compare atmospheric pressure. This also means that vertical pressure gradients are greater than extreme examples of horizontal pressure gradients.

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.

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

density decreases with height because the atmosphere is compressible and there is a greater concentration of air molecules near the surface.

The Role of density in hydrostatic equilibrium

gravitational force is proportional to mass. This means that a dense atmosphere needs greater gravitational force to remain in balance. For warm air, this equates to smaller vertical pressure gradients leading to hydrostatic equilibrium. For cold air, this equates to larger vertical pressure gradients leading to static equilibrium.

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

pressure decreases as you go up in the atmosphere because the density is the highest near the surface because the atmosphere is compressible. This compressibility causes pressure to decrease more rapidly at low elevations and gradually taper off at greater altitudes. Also, in the troposphere the temperature decreases with height and higher temperature is associated with higher pressure.

Centrifugal force

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

Friction force

this force is the result of the character of a surface. It causes 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 which is pointed up.