Winds

Question 1

The primary cause of wind is the -

Air flows from -

Creating a force called -

Loosely defined, the pressure gradient is the pressure difference between consecutive isobars, the implication being that the air flows 90° to the isobars which is in fact not the case because -

Answer 1

variation in air density caused by differences in air temperature.

high pressure to low pressure

The pressure gradient

The earth rotates

Question 2

The coriolis force deflects the air flow -

It is directly proportional to -

Answer 2

90 degrees to the direction of movement

the velocity of the wind.

Note: The coriolis force is caused by the earth’s rotation. Picture drawing a straight line over a record player with a ruler, it will appear like you are drawing a straight line but on the record it will be a spiral.

Question 3

The magnitude of the Coriolis force may be calculated by means of the formula:

Coriolis Force =

Answer 3

2vpw sin latitude

2 x wind velocity x density x earth rotational velocity x sine of latitude

Question 4

Explain the geostrophic force:

Answer 4

The combined effect of the pressure gradient force and the Coriolis force.

Question 5

Buys-Ballot’s law states that -

Answer 5

An observer who stand with his back to the wind in the southern hemisphere, will have the low pressure on his right.

Question 6

Where a pass or valley is found, the air will be forced through with considerable force, where the velocity may be raised to a gale force, due to -

Answer 6

the Venturi effect(Decrease in static pressure = increased velocity)

Question 7

The funnel effect defined is -

Answer 7

Forcing of air through valleys

Question 8

The two types of wind that are characterised as being hot and dry?

And what causes the hot characteristics?

Answer 8

The FoHN wind and Berg wind

Air at ground level is forced to rise over a mountain(called topographical uplift) and will cool adiabatically due to expansion, initially at the DALR of 3°/1000ft. Once the condensation level have been reached, the air will cool and a lower rate(SALR of 1.5°/1000ft). The lower cooling rate will cause the air to be warmer on the leeward side than on the windward side.

Question 9

The heating of hill slopes by the sun, causes which wind and why?

Answer 9

Anabatic wind. Due to the higher temperature the density reduces causing it to rise. Cooler air from below the slope(high pressure) will rise to replace the heated air(lower pressure)

Question 10

During night time the ground loses heat due to radiation, this will cause which wind and why?

Answer 10

Katabatic wind. The cool air, being more dense, will sink lower down the valley.

Question 11

What causes a sea breeze?

Answer 11

Land heats up more quickly than water, this will result in a lower pressure. Air from over the water, being more dense and at a higher pressure, will attempt to equalise by moving toward land. This will occur during day time.

Question 12

What causes a land breeze?

Answer 12

This is the opposite of a sea breeze. Water retains its heat longer than land. This will result in a lower pressure over the water which will, logically, cause an on shore wind.

Question 13

Between the poles and the equator, which will provide a higher Coriolis force?

Answer 13

The poles.

Question 14

Explain gradient wind:

Answer 14

The geostrophic wind is only applied where to isobars are straight, but where these a curved, as they usually are, centrifugal force has to be taken in account.

Gradient wind is added to the PGF and the actual wind will be stronger with an anti-cyclone or high pressure system.

Gradient wind opposes the PGF and the actual wind will be less with around a depression or low pressure system.

Question 15

Explain PGF(pressure gradient force):

Answer 15

Pressure gradient force is the effect of high pressure directly moving to low pressure. The spacing of the isobars determines the magnitude of this force - The closer they are the stronger the force, the farther they are the weaker the force. Buys law states that if your back is to the wind the low pressure will be on your right(in the southern hemisphere) so another factor must be present.

The other factor is the Coriolis force.

Note: wind speed is directly proportional to the PGF

Question 16

Explain Coriolis force:

Answer 16

Coriolis force is caused by the rotation of the earth. This will make the PGF wind move parallel to the isobars I.E 90 degrees to the wind. CF is directly proportional to wind speed and latitude, the formula is:

CF = 2vpw sin latitude

2 x wind velocity x density x earth rotational velocity x sine of latitude

The Coriolis force combined with the PGF will equal what is called the Geostrophic wind. Geostrophic wind is only valid where the isobars are straight, if not, other force will alter the wind such as the gradient force. The Geostrophic wind will always blow parallel to the isobars.

Question 17

Explain surface friction:

Answer 17

For wind to blow parallel to the isobars Coriolis force must equal the PGF I.E, the two must be balanced. Coriolis force is proportional to wind speed. Frictional retard exist below 2 000ft which will cause the Coriolis force to decrease resulting in the wind to flow towards the low or depression. The wind will “veer” in a clockwise direction.

Question 18

Explain Geostrophic wind:

Answer 18

Geostrophic wind is the result of the Coriolis force combined with the PGF and acts parallel to the isobars and is only valid if the isobars are straight. The Coriolis force and PGF must be equal for this to occur.

Question 19

Explain the ITCZ:

Answer 19

The Intertropical convergence zone is caused due to the equator heating faster than the cold polar regions. This warm area expands and rises, leading to the more dense air to sink(within the Hadley cell) to the equator in order to equalise the pressure difference. However, the Coriolis force deflects the air masses to the left when viewed from the southern hemisphere and to the right when viewed from the northern hemisphere. And this results in the trade winds which blows from the NE in NH and from the SE in SH. The region where these winds converge is called the ITCZ. The ITCZ logically follows the sun. Thunderstorm activity is pronounced in this region.

Question 20

The geostrophic wind will only be valid where(and why) -

Answer 20

The isobars are straight, otherwise the gradient wind will alter its movement.