Local Wind Systems

Question 1

what is the diurnal variation?

Answer 1

the suns heating effect on earth will vary throughout the day - heating the surface during the day and cooling at night. This variations causes changes in the way the air moves over the surface. These changes will follow a 24 hour cycle. this is called diurnal variation

Question 2

How big is the diurnal effect at sea?

Answer 2

its very small due to a lack of convection current, but other unexpected air movements may occur when sea comes into contact with the coast.

Question 3

the land has a lower Specific Heat’ than the sea and will therefore absorb the sun’s radiation at its surface, faster than the sea. Whereas the sea will take longer to heat up but will then retain its ‘Specific Heat’
for longer than the land? True or False

Answer 3

True

Question 4

what is normally produced over the land after sunrise? and why?

Answer 4

a thin very warm surface layer is produced, as the solar heat energy is transferred to the surface air by contact through conduction and convection.

Question 5

what is a sea breeze?

Answer 5

reduction in surface density causes a decrease in surface (barometric) pressure and the heated surface air then starts to ascend until it reaches a layer of equal density.

creating an area of higher Pressure over the land, typically at 2000 feet. The Pressure over the sea is now higher than the land surface Pressure, the Pressure Gradient (blowing from High to Low Pressure), causes a breeze to flow from the sea to the land, bringing the cooler denser air – this
is called a Sea Breeze

Question 6

how does the continuous circulation pattern reinforce sea breeze?

Answer 6

The Circulation pattern is reinforced as air in the High-Pressure area above the land, flows towards the Lower- Pressure area above the sea and the colder air then descends to replace the air flowing towards the land.

Question 7

what are the significant times of sea breeze?

Answer 7

The Sea Breeze starts at about 10 am, peaks at about 3 pm, and dissipates around sunset.

Question 8

how fast can sea breezes get?

Answer 8

It can reach speeds of around 10 – 15 knots

Question 9

how high and far can sea breezes get?

Answer 9

it can extend up to 3000 feet high and between 25 – 40 kilometers inland

Question 10

what is the most associated cloud with sea breeze?

Answer 10

With continuous moisture carried inland from the sea to land, the associated cloud is Cumulus and often referred to as Fair Weather Cumulus

Question 11

what happens to cloud formation in a sea breeze when there is a peninsular?

Answer 11

In areas where there is a peninsular, a Sea
Breeze either side can develop, a convergence zone can then exist that raises the Cumulus into
Towering Cumulus (TCu) or even Cumulonimbus (Cb)

Question 12

is there any precipitation with a sea breeze?

Answer 12

no unless cloud formation conditions such as TCU or Cb exist.

Question 13

what are flight conditions like in a sea breeze?

Answer 13

turbulent, even in mild conditions, expect a light to moderate turbulence and associated windshear, as you move from sea breeze to normal wind above the sea breeze or from a higher level into the sea breeze

Question 14

how is land breeze formed?

Answer 14

At night, the land cools more quickly than the sea, because the land has a lower specific heat than the sea.

As air in contact with the land cools, a higher pressure develops above the land, and a lower pressure over the higher temperature – the sea’s surface.

This causes the direction of airflow to reverse, because at the surface the wind is now blowing from the land towards the sea.

Question 15

how fast is land breeze usually and how does its speed reinforce the circulation of the land breeze?

Answer 15

The wind speed is around 3 - 4 knots. As a result, the air tends to move from the sea to the land at a
higher level and this reinforces the Land Breeze circulation in the lower layers

Question 16

how can land breezes affect coastal regions?

Answer 16

Airports in coastal regions may be affected by Sea Fog - and the effect that these breezes have upon it. If Sea Fog exists - the cool air moving in off the sea may bring it ashore, causing you problems with visibility

Question 17

what is a slope updraught/anabatic wind?

Answer 17

On clear sunny days, mountainsides are warmed
by the sun’s radiation, and in turn, this heats the
overlying air layer. As the air becomes less dense it will begin to ascend. The air moves away from the valley floor and upwards towards the summit. this is called anabatic wind

Question 18

what is a slope downdraught/katabatic wind?

Answer 18

During clear nights, the mountain sides radiate
their heat and so cool the surrounding air, making it denser. This heavier air now sinks down the mountainside into the valley. This current of air is known as the Slope Downdraught - or Katabatic wind.

Question 19

what is the easy way to distinguish katabatic and anabatic wind?

Answer 19

you put the cat out at night’! (katabatic goes down)

Question 20

when a strong wind is blowing against a mountain range and its movement is impeded how will it find its way across?

Answer 20

A strong wind blowing against a mountain range will be forced to find an alternative route - normally over the top. But if a narrow gap or valley is available then the wind will be channelled through it

Question 21

what happens to wind speed when the wind is blowing against a mountain range? and why?

Answer 21

The speed of the wind blowing through the valley will be greater than the wind in the regions on either
side.
Due to the Venturi Effect, which occurs within the
confines of the valley.

Question 22

the shape of the mountain affects the wind speed, what else can it affect?

Answer 22

The shape of the mountain pass or valley can also produce a change in the vertical direction of
the wind.

Question 23

as a pilot what is expected if flying through a valley?

Answer 23

you should expect to encounter strengthening head or tailwinds and also downdraughts

Question 24

the speed of the wind blowing over the top of the mountain is also increased due to what? and what does the result produce?

Answer 24

it is caused by the Venturi Effect and produces a strengthened downdraught on the Leeward side

Question 25

what are the effects of wind being deflected by obstructions?

Answer 25

change in wind speed and direction (especially in high ground) and if the obstruction is similar to a coastal headland, then wind streamlines near to it, become closer increasing wind speed

Question 26

what is a good example of wind funneling/channeling in NZ?

Answer 26

The mountains and valleys of the Southern Alps in the South Island of New Zealand provide a good example of Funnelling - particularly in Westerly conditions

Question 27

what is the most obvious effect that a mountain range has on wind?

Answer 27

it will deflect it

Question 28

the deflection on wind on mountains ranges is dependent on what factors?

Answer 28

direction, strength and lapse rate of wind

Question 29

what are some other examples of wind funneling?

Answer 29

Other examples of Terrain Channelling (or Funnelling) are where the atmosphere is stable, such as in an anti-cyclone and the wind tends to be lighter and to
move around rather than over mountains. Also, topographical gaps such as river valleys means
that the air can more easily move through the valley

Question 30

how are mountain waves formed?

Answer 30

When the wind speed at the summit exceeds 25 knots and the air is stable, you may see the
formation of Mountain Wave

Question 31

during the effects of mountain waves what occurs on the lee side of the mountain?

Answer 31

On the Leeward side the air wants to return to its equilibrium altitude. Like a pendulum - it will
do so and damps oscillations. The induced wave motion can reach to altitudes as high as the
Tropopause and extend hundreds of miles downstream from the mountain.

Question 32

how are cap clouds formed?`

Answer 32

The ascending moist air on the windward side of the mountain often causes the formation of Cap clouds.

Question 33

what happens downwind of a mountain during the effects of mountain waves?

Answer 33

Immediately downwind of the summit is an area of strong downdraught, followed very quickly
by a strong updraught. This turbulence continues downwind, resulting in the formation of a
Roll cloud at lower altitudes.

Question 34

what type of clouds are expected at the bottom of the mountain during the effects of mountain waves?

Answer 34

Ragged clouds may also be visible below the height of the summit. This cloud shows a very hazardous area of rotating air that will quite likely exceed the limitations of your aircraft.

Question 35

how are rotor zones formed?

Answer 35

Rotor clouds form at lowest levels and are a form of lee eddy. The air in the cloud rotates around an axis parallel to the mountain range

Question 36

what type of clouds are expected at higher altitudes during the effects of mountain waves?

Answer 36

Lenticular clouds at higher altitudes in the waves,

downwind of the mountain, signifying ascending air on their upwind edge.

Question 37

as a pilot what is expected when flying over the lee side of a ridge?

Answer 37

you should allow for a large margin above safety height and provide suitable clearance when flying over the Lee side of the ridge.

spend as short a time as possible in the downdraught region, by selecting a flight path that is at right angles to the mountain range, on the Leeward side.

Question 38

as a pilot of a light aircraft (limited performance) when flying over a ridgeline what considerations should you take into play when flying over a ridge?

Answer 38

crossing a ridgeline at high altitudes may not be possible. Stay on windward side of the hill to benefit from updraught conditions, achieved by approaching the ridgeline at an angle which allows you to turn away from the ridge - before committing to crossing the lee ward side.

avoid lee side, where down draught and turbulent conditions exist ASAP.

Question 39

as a pilot of a light aircraft (limited performance) when flying over a ridgeline on the leeward side what considerations should you take into play when flying over a ridge?

Answer 39

approaching a ridge from the Leeward side, take particular care as the stronger the wind, the worse the downdraught and turbulence will be.

The stronger the wind, the worse the conditions will be. If in doubt – do not cross the ridgeline.