309M: Wind

What is the fundamental definition of wind?

Wind is defined as air in motion.

For the purposes of a METAR report, wind represents the horizontal flow of air at what specific height?

It represents the horizontal flow of air at a height of 10 meters.

What force is required to set a mass of air in motion from rest?

The pressure differential between high and low-pressure areas, known as the pressure gradient.

What is the primary cause of the pressure gradient that initiates the movement of air?

Density variation caused by temperature differences.

During the daytime, a _ breeze occurs because the land heats up more quickly than the sea, creating lower pressure over the land.

sea

Explain the formation of a land breeze.

At night, the land cools more rapidly than the sea, creating higher pressure over the land, causing air to flow from the land to the sea.

The pressure gradient force (PGF) always acts in which direction?

It acts in the direction from high to low pressure.

On a synoptic chart, what does it mean if isobars are closely packed together?

The pressure gradient is steep, and the resulting wind is strong.

On a synoptic chart, what does it mean if isobars are far apart?

The pressure gradient is slack, and the resulting wind is light.

What is the name of the 'force' that is due to the Earth's rotation and changes the direction of the wind?

The Coriolis Effect.

In the Northern Hemisphere, the Coriolis Effect always deflects the wind to the _.

right

In the Southern Hemisphere, the Coriolis Effect always deflects the wind to the _.

left

Does the Coriolis Effect create wind or affect its speed?

No, it only changes the direction of the wind.

Above the influence of surface friction, the combination of the Pressure Gradient Force and the Coriolis Effect causes air to flow in what orientation to the isobars?

The air flows parallel to the isobars.

In the Northern Hemisphere, in what direction does air circulate around an area of high pressure?

It circulates clockwise.

In the Northern Hemisphere, in what direction does air circulate around an area of low pressure?

It circulates counter-clockwise.

State Buys-Ballot's Law as it applies to the Northern Hemisphere.

If you stand with your back to the wind, the lower pressure will lie to your left.

Using Buys-Ballot's Law in the Northern Hemisphere, if you have a west wind, the area of lower pressure would be located to the _.

north

Using Buys-Ballot's Law, with the wind at your back, where would higher pressure (highs or ridges) be located?

To your right.

What term describes a clockwise change in wind direction?

Veering.

What term describes a counter-clockwise change in wind direction?

Backing.

If the wind changes from 230° to 290°, is this an example of veering or backing?

Veering (an increase of degrees).

If the wind changes from 130° to 90°, is this an example of veering or backing?

Backing (a diminution of degrees).

If a low-pressure system passes from west to east with its center passing to the north of your station, what change in wind direction would you expect?

The winds will veer.

If a high-pressure system passes from west to east with its center passing to the north of your station, what change in wind direction would you expect?

The winds will veer.

If a low-pressure system passes from west to east with its center passing to the south of your station, what change in wind direction would you expect?

The winds will back.

What is the general altitude of the boundary layer, where friction significantly impacts wind?

From the surface to approximately 3000 feet.

What third force, besides PGF and Coriolis, affects wind within the boundary layer?

Friction Force.

What is the definition of Friction Force in meteorology?

The force which results when the wind interacts with the Earth's surface.

What are the two primary effects of the Friction Force on wind?

It diminishes wind speed and modifies wind direction.

How does the Friction Force affect the Coriolis Effect?

It diminishes or weakens the Coriolis Effect.

How does surface friction alter the wind's direction relative to the isobars?

It causes the wind to cross the isobars at an angle, toward the area of lower pressure.

True or False: As surface friction increases, the angle at which the wind crosses the isobars decreases.

False. As surface friction increases, the cross-isobar angle increases.

Will the angle of wind across isobars be greater over rough terrain or over water?

It will be greater over rough terrain due to increased friction.

When an aircraft is departing and climbing out of the boundary layer, what change in wind can be generally expected?

The wind will veer and increase in speed.

When an aircraft is landing and descending into the boundary layer, what change in wind can be generally expected?

The wind will back and decrease in speed.

What is a katabatic wind?

A wind that carries high-density air from a higher elevation down a slope under the force of gravity.

A katabatic wind is also known as a _ wind.

drainage

What is an anabatic wind?

A warm wind that blows up a steep slope or mountainside, driven by heating of the slope from the sun.

An anabatic wind is also known as an _ flow.

upslope

Under what weather conditions do anabatic winds typically occur?

They typically occur during the daytime in calm, sunny weather.

The _ Effect describes how a mountain range can act like a dam, forcing air to either go over it or change direction to go around it.

Barrier

What topographical effect causes wind to gush through a pass in a mountain range with considerable speed?

The Funnel Effect.

What is the Valley Effect on wind?

In weak surface wind conditions, the wind tends to follow the direction of any well-defined valley rather than the isobaric pattern.

What causes the horizontal movement of air?

Horizontal pressure differences.

The combination of Pressure Gradient Force, Coriolis Effect, and _ determines the surface wind.

Friction Force

With strong friction, the surface wind direction will be influenced more by which force: Pressure Gradient or Coriolis?

Pressure Gradient Force.

With very little friction, such as over a large body of water, the surface wind direction will be more closely aligned with what?

It will be more parallel to the isobars, dominated by the Coriolis Effect.

Which two factors can diminish the effect of surface friction on wind?

Altitude (flying above the boundary layer) and daytime heating (which increases air mixture).

What is the general direction of travel for most pressure patterns?

They travel in a general west to east direction.