Chapter 10 - Wind

Question 1

Wind

Answer 1

Movement of air in horizontal plane

Also known as advection

Question 2

Conversion units

Answer 2

1 kt = 0.52 m/s
1 kt = 1.85 km/h
1 kt = 1.15 mph

Question 3

Measuring surface wind

Answer 3

Using anemometer
Situated 10m/33 ft above ground level

Question 4

Calm wind

Answer 4

Less than 1 kt

Question 5

Gale force wind

Answer 5

34-47 kts

Question 6

Storm force wind

Answer 6

48-63 kts

Question 8

Hurricane force wind

Answer 8

Mean surface wind greater than or equal to 64 knots

Question 9

Gust

Answer 9

Sudden increase of 10 kts or more above average wind speed
Lasts less than 1 min

Question 10

Squall

Answer 10

A gust lasting longer than 1 min

Question 11

Lull

Answer 11

Very sudden drop in wind speed

Question 12

Wind direction

Answer 12

Wind direction always described as the direction it’s COMING FROM

Question 13

Backing of wind

Answer 13

When wind changes direction and stabilises at a new direction
In the anticlockwise direction the angle decreases eg goes from 180 to 90 hence the wind is backed

Question 14

Wind veering

Answer 14

When wind changes direction and stabilises at a new direction
In the clockwise direction the angle increases eg goes from 180 to 270 hence the wind is veering

Question 15

All ATC reports are given in degrees ____________

Answer 15

Magnetic

Question 16

When magnetic variation is west ________

When magnetic variation is east ________

Answer 16

Magnetic is best (bigger)

Magnetic is least (smaller)

Question 17

What causes wind

Answer 17

Horizontal pressure differences between areas of high and low pressure
Generates a pressure gradient force (PGF) as air moves from high pressure to low pressure

Question 18

Widely spaced isobars

Answer 18

Pressure CHANGING more slowly
Weaker PGF

Question 19

Narrowly spaced isobars

Answer 19

Pressure CHANGING more quickly
Stronger PFG

Question 20

Air deflects to left when moving from high to low pressure when you’re in

Answer 20

The southern hemisphere
Due to earth rotating + coriolos effect

Question 21

Air deflects to right when moving from high to low pressure when you’re in

Answer 21

The northern hemisphere
Due to earth rotating + coriolos effect

Question 22

Coriolis force equation

Answer 22

Coriolis force = 2 x angular rotation of earth (Ω) x density (⍴) x wind speed x sin theta

Theta being latitude

Question 23

Coriolis force acts at 90 degrees to _________ of wind direction in ___________ hemisphere

Answer 23

Right
Northern

Question 24

Coriolis force acts at 90 degrees to _________ of wind direction in ___________ hemisphere

Answer 24

Left
Southern

Question 25

Geostrophic wind needs

Answer 25

PGF CF must be above the friction layer (approx 2000-3000ft) Latitude greater than 15 degrees Straight isobars

Answer 26

V is proportional to 1 / sin theta Theta = latitude PGF proportional to speed

Question 27

Gradient wind

Answer 27

Wind that flows parallel to curved isobars Requires PGF CF Centrifugal force

Question 28

Comparing gradient and geostrophic wind

Answer 28

Geostrophic wind blows parallel to straight isobars Gradient wind blows parallel to curved isobars With EQUAL isobar spacing at SAME latitude Gradient wind around low is slower than Geostrophic wind Gradient wind around high is faster than Geostrophic wind Eg lower around a low and higher around a high

Question 29

The friction layer

Answer 29

Turbulent layer close to surface where laminar flow is deflected into vertical components Average depth 2000ft - 3000ft Will reduce to 1500ft at night due to reduced convection currents Depth affected by wind speed, terrain, stability

Question 30

Variation in wind velocity land and sea

Answer 30

Over land the friction force is greater Wind speed reduces by approx 50% Over sea the friction force is less Wind speed reduces by approx 30%

Question 31

What is the effect of friction

Answer 31

Increase in friction = greater reduction in speed = greater reduction in coriolis force (coriolis force is proportional to speed) cause greater angular deflection

Question 32

In the northern hemisphere Geostrophic wind

Answer 32

Backs and slacks (decrease speed)

Question 33

In the northern hemisphere Geostrophic wind

Answer 33

Veers and slacks (decrease speed)

Question 34

Diurnal variation of surface wind velocity

Answer 34

Thermal turbulence mixes slow moving surface air with fast moving free air above during the day No thermal activity at night = no mixing So in the northern hemisphere Surface wind backs + slacks overnight reaching min speed 30 mins after sunrise Surface wind veers + increases in speed during day reaching max speed at approx 15:00 OPPOSITE WAY AROUND IN SOUTHERN HEMISPHERE