Wind

Question 1

Define: geostrophic wind.

Answer 1

Air that is in motion due to:

» pressure gradient force

» geostrophic force

which are in equilibrium.

Question 2

Define: geostrophic force.

Answer 2

Force due to the angular velocity of the Earth.

It acts at right angles to the pressure gradient force: to the right in the northern hemisphere and to the left in the southern hemishere.

Question 3

Define: Buys Ballot’s Law.

Answer 3

If an observer stands with their back to the wind, the lower pressure is:

» on their left in the northern hemisphere

» on their right in the southern hemisphere

Question 4

Define: pressure gradient force.

Answer 4

The force that moves the air from high pressure to low pressure.

Question 5

Define: gradient wind.

Answer 5

The resultant wind combined from:

» pressure gradient force

» geostrophic force

» centripetal force

Question 6

Describe how gradient wind changes as it approaches the surface.

Answer 6

Entering the friction layer means that:

» wind is slowed

» coriolis effect is reduced

» coriolis deflection reduces

» the wind backs in the northern hemisphere, veers in the southern hemisphere

Question 7

Define: isobars.

Answer 7

Lines joining points of equal atmospheric pressure.

Question 8

Principally, what is a deciding factor on whether a wind will be geostrophic wind or gradient wind?

Answer 8

» straight isobars: geostrophic wind

» curved isobars: gradient wind

Question 9

Define: hurricane.

Answer 9

A wind with a mean surface velocity of 64 kt or more.

Question 10

What does the geostrophic wind velocity formula signify?

Answer 10

That the magnitude of the geostrophic wind velocity increases with decreasing latitude.

Question 11

Define: wind veering.

Answer 11

A clockwise change in wind direction.

Question 12

What overall effect does friction layer have on wind?

Answer 12

It slows it down.

Question 13

What is the nature of friction layer turbulence?

Answer 13

Mechanical and thermal.

Question 14

Define: gust.

Answer 14

A short variation from the mean wind speed in which the wind speed varies by 10 kt or more.

Question 15

What is the primary cause of wind?

Answer 15

Pressure differences, which are usually caused by temperature differences.

Question 16

Is there a difference in depth of the friction layer between night and day?

Answer 16

Yes, at night the friction layer depth reduces to around 1,000 ft due to the absence of thermal turbulence.

Question 17

Define: gale.

Answer 17

A wind with a mean surface velocity of 34 kt to 47 kt.

Question 18

What are the requirements for pure geostrophic wind?

Answer 18

» steady pressure gradient force

» completely horizontal motion of air

» straight and parallel isobars

Question 19

Define: isotach.

Answer 19

A line of equal wind speed.

Question 20

Define the formula for:

(a) geostrophic force

(b) pressure gradient force

and identify the elements.

Answer 20

The formula is the same for both (a) and (b), because they are assumed to be in equilibrum in geostrophic wind.

GF/PFG = 2 · Ω · ρ · V_W · sin Φ

Ω = Earth’s angular velocity

ρ = air density

V_W = geostrophic wind velocity

sin Φ = latitude sine

Question 21

Define: wind backing.

Answer 21

An anti-clockwise change in wind direction.

Question 22

In which direction does the pressure gradient force act?

Answer 22

Towards the low pressure, perpendicular to the isobars.

Question 23

What is an alternative name for the geostrophic force?

Answer 23

Coriolis force.

Question 24

What does the GF formula signify?

Answer 24

That the magnitude of the geostrophic force increases with increasing latitude.

Question 25

Define: friction layer.

Answer 25

Turbulent layer of air that extends (generally) up to about 3,000 ft.

Question 26

Define the formula for the geostrophic wind velocity.

Answer 26

V_W = (PGF) / (2 · Ω · ρ · sin Φ)

Question 27

What does isobar spacing indicate?

Answer 27

The strength of the pressure gradient force.

Question 28

Define: wind.

Answer 28

Horizontal motion of air.

Question 29

How does wind speed affect TAS?

Answer 29

TAS increases with increasing headwind or decreasing tailwind.

TAS decreases with decreasing headwind or increasing tailwind.

Question 30

What is the relationship between isobar spacing and wind strength?

Answer 30

The closer the isobars, the stronger the wind.