Winds

Question 1

What are winds?

Answer 1

Air currents or moving masses of air from high pressure areas to low pressure areas.

Question 2

What are the different types of winds within the troposphere (under the tropopause)?

Answer 2

Surface and upper level winds.

Question 3

Why do we need to study winds?

Answer 3

Since there is a surplus of radiation in the tropics, winds help to redistribute it through the horizontal transfer of heat. It also contributes to rainfall formation.

Question 4

What affects the direction and speed of winds?

Answer 4

Pressure gradient force, coriolis effect and friction.

Question 5

What is pressure gradient force?

Answer 5

The force that moves air from an area of high pressure to low pressure.

Question 6

Why do high and low pressure areas exist?

Answer 6

The unequal heating of the earth’s surface. Cool air sinks forming high pressure areas, warm air rises forming low pressure areas.

Question 7

What are high pressure areas caused by?

Answer 7

High pressure areas are usually caused by air masses being cooled. As the air mass cools, it shrinks, becomes denser and sinks. This allows air from the surroundings to fill in above it, increasing the total mass of atmosphere above the surface, which then results in higher surface pressure. Sinking air is thus associated with high pressure areas.

Question 8

What are low pressure areas caused by?

Answer 8

Low pressure areas are usually caused by air masses being warmed. As the air warms, it expands upward, outward and rises. This removes some of the air from the surface, and thus reduces the surface air pressure, which results in lower surface pressure. Thus, rising air is associated with low pressure areas.

Question 9

What are the vertical transfers of heat?

Answer 9

Radiation, conduction, convection.

Question 10

What is conduction?

Answer 10

Radiation helps to transfer energy near the surface. As insolation is absorbed by the earth’s surface, the temperature of the surface becomes greater than the air above it. This results in the transfer of heat between the ground surface and the thin, adjacent layer of air.

Question 11

What is convection?

Answer 11

The mixing process by which energy conducted through the laminary boundary layer is distributed through the rest of the atmosphere. Convection circulates the heat between the very lowest and the remaining portions of the atmosphere and occurs when a localised parcel of air is heated more than nearby air. Because warm air is less dense than cold air, it is relatively buoyant and rises.

Question 12

How are velocity of winds affected by PGF?

Answer 12

The stronger the pressure difference between two places, the higher the velocity of the wind and vice-versa.

Question 13

What are isobars?

Answer 13

A measure of differences in pressure through lines joining areas of equal pressure, measured in millibars. Spacing between isobars indicates the intensity of pressure difference.

Question 14

How does the spacing between isobars indicate wind speed?

Answer 14

The closer the isobars, the steeper the pressure gradient and vice-versa. More spaced out isobars indicate a more gradual pressure gradient and lower wind speeds.

Question 15

What is the Coriolis Effect?

Answer 15

It is the psuedo force that causes an apparent deflection in the direction of moving objects that are not rooted to the ground. It arises due to the rotation of the earth and alters the direction of wind.

Question 16

How does the Coriolis Effect change in Northern and Southern Hemisphere?

Answer 16

Winds are deflected to the right in the Northern Hemisphere and deflected to the left in the Southern Hemisphere.

Question 17

What is the strength of the Coriolis Effect influenced by?

Answer 17

Latitude and wind velocity.

Question 18

How does latitude influence Coriolis Effect?

Answer 18

The strength of the Coriolis Effect increases with further distance from the equator. Winds experience maximum deflection at the poles and minimal/no deflection at the equator.

Question 19

How does wind velocity affect the Coriolis Effect?

Answer 19

Faster moving objects cover more distance in a given time, and hence undergo more deflection.

Question 20

How does friction affect wind speed?

Answer 20

Air in contact with the earth’s surface experiences frictional drag, which decreases wind speed. Air just above, and in contact with the slower-moving surface layer of air, likewise experiences fictional drag, but from the underlying air rather than from the earth’s surface. As this layer slows down, the air at higher levels is similarly affected. The effects of friction thus originate from the surface but are found throughout lower atmosphere.

Question 21

How does friction influence the speed and direction of winds within the planetary boundary layer (the lowest 1.5km of the atmosphere)?

Answer 21

Within this layer e.g. surface winds are influenced by surface friction, hence lowering the wind speed for a given pressure gradient. Coriolis Effect is also reduced, since the degree of deflection is less where wind velocity is lower.

Question 22

How does friction influence the speed and direction of winds within the free atmosphere (right above the planetary boundary layer)?

Answer 22

The effect of friction on winds e.g. upper level winds on this layer is negligible. In the absence of friction, due to Coriolis Effect, wind is deflected to a larger degree in the upper atmosphere and is able to move parallel to the isobars at a constant speed.

Question 23

What is a geostrophic wind?

Answer 23

When wind runs parallel to the isobars due to Coriolis Effect being equal to Pressure Gradient Force.

Question 24

What are jet streams?

Answer 24

They are examples of fast-flowing, high altitude geostrophic winds.

Question 25

What kind of winds redistribute surplus radiation in the tropics (surface winds and upper-level winds)?

Answer 25

Surface winds: Hadley Cell | Upper-level winds: Jet streams

Question 26

What is the Hadley Cell?

Answer 26

One of the types of semi-permanent pressure cells of the General Circulation Model formed due to differential heating of the earth's surface.

Question 27

How is the Hadley Cell formed?

Answer 27

The Hadley Cell is formed where air in the low latitudes is heated, expands and rises, which forms an area of low pressure known as the equatorial low. After rising to about 16km above the surface, the air diverges, moves towards the poles and descends over the subtropics.

Question 28

How is the Hadley Cell closed?

Answer 28

The cold, descending air over the subtropics forms belts of high pressure (subtropical high pressure belt) at around 30-40 degrees north and south of the equator. At the surface, due to PGF, air is drawn from the subtropical high to the equatorial low, completing Hadley Cell.

Question 29

How do winds operate within the Hadley Cell?

Answer 29

Air is deflected by the Coriolis Effect to form surface winds, northeast trade winds in the Northern Hemisphere and southeast trade wins in the Southern Hemisphere. As these winds move across the globe, they help to redistribute solar radiation in the tropics.

Question 30

How is the ITCZ formed?

Answer 30

Trade winds also converge along the low pressure belt. The combination of heating via conduction and convergence also forces air aloft and forms the intertropical convergence zone (ITCZ). The ITCZ is associated with bands of clouds and rainy weather.

Question 31

How does the ITCZ move?

Answer 31

The ITCZ shifts according to the position of the overhead sun which shifts seasonally. On 21st June, the sun is overhead at the Tropic of Cancer which causes the ITCZ to shift into the Northern Hemisphere. On 21st December, the sun is overhead at the Tropic of Capricorn and the ITCZ shifts into the Southern Hemisphere.

Question 32

Why is the shift of the ITCZ not uniformed?

Answer 32

Uneven heating of land and water, among other factors.

Question 33

How does the Hadley Cell move?

Answer 33

The Hadley Cell also shifts according to the position of the overhead sun. On 21st June, the sun is overhead at the Tropic of Cancer that causes the Hadley cell to shift into the Northern Hemisphere. On 21st December, the sun is overhead the Tropic of Capricorn and the Hadley cell will have moved from the Northern to the Southern hemisphere in the intervening time.

Question 34

How do the trade winds move?

Answer 34

In accordance to the Hadley Cell.

Question 35

What are jet streams?

Answer 35

Jet streams are high altitude geostrophic winds that are found very high up in the troposphere, at about 9-15km above the Earth's surface, near the tropopause.

Question 36

What do jet streams mark?

Answer 36

Jet streams mark the boundaries between warm and cold air. The greater the difference in temperature, the faster the jet stream.

Question 37

What direction are major jet streams?

Answer 37

Major jet streams on Earth are westerly winds flowing in an easterly direction.

Question 38

Do jet streams move at consistent speeds?

Answer 38

Jet streams do not move at consistent speeds. Like a river, the speed of the jet stream will vary from its core to its edges.

Question 39

How do jet streams vary in size?

Answer 39

Jet streams have varying width and depth, they can span across a few metres to kilometres.

Question 40

What are the three dominant jet streams?

Answer 40

Subtropical jet stream (STJS), tropical easterly jet stream (TEJS) and African easterly jet stream (AEJ).

Question 41

What is the subtropical jet stream (STJS)?

Answer 41

It is located about 30°N/S. They are generally associated with fair weather conditions on the ground.

Question 42

What is the tropical easterly jet stream (TEJS)?

Answer 42

It forms in the Northern Hemisphere summer, due to significant temperature differences between the land mass and ocean. Typically found 5-10°N from Southern India to the east coast of Africa.

Question 43

What is the African easterly jet stream (AEJ)?

Answer 43

It also occurs in the Northern Hemisphere summer, 10-15°N above West Africa. It develops because of temperature differences between Sahara Desert and Gulf of Guinea.

Question 44

Why do the winds in a jet stream converge and diverge?

Answer 44

Jet streams are inconsistent due to the temperature gradient between the equator and the poles and the presence of land masses on the earth's surface.

Question 45

How do convergences and divergences in the jet stream impact surface conditions?

Answer 45

Upper air convergence results in descending air that reinforces the high pressure at the surface. Upper air divergence results in rising air that reinforces the low pressure at the surface.

Question 46

How is surplus heat redistributed by jet streams?

Answer 46

Since the jet streams flow from east to west, there can be no direct poleward transfer of heat as in Hadley Cells. Instead, jet streams can help carry the heat from the equator further into mid-latitudes (30° to 60° N&S). Also, its influence on surface conditions which result in rising air may result in redistribution of heat via vertical transfers.