Winds Flashcards
What are winds?
Air currents or moving masses of air from high pressure areas to low pressure areas.
What are the different types of winds within the troposphere (under the tropopause)?
Surface and upper level winds.
Why do we need to study winds?
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.
What affects the direction and speed of winds?
Pressure gradient force, coriolis effect and friction.
What is pressure gradient force?
The force that moves air from an area of high pressure to low pressure.
Why do high and low pressure areas exist?
The unequal heating of the earth’s surface. Cool air sinks forming high pressure areas, warm air rises forming low pressure areas.
What are high pressure areas caused by?
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.
What are low pressure areas caused by?
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.
What are the vertical transfers of heat?
Radiation, conduction, convection.
What is conduction?
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.
What is convection?
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.
How are velocity of winds affected by PGF?
The stronger the pressure difference between two places, the higher the velocity of the wind and vice-versa.
What are isobars?
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.
How does the spacing between isobars indicate wind speed?
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.
What is the Coriolis Effect?
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.
How does the Coriolis Effect change in Northern and Southern Hemisphere?
Winds are deflected to the right in the Northern Hemisphere and deflected to the left in the Southern Hemisphere.
What is the strength of the Coriolis Effect influenced by?
Latitude and wind velocity.
How does latitude influence Coriolis Effect?
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.
How does wind velocity affect the Coriolis Effect?
Faster moving objects cover more distance in a given time, and hence undergo more deflection.
How does friction affect wind speed?
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.
How does friction influence the speed and direction of winds within the planetary boundary layer (the lowest 1.5km of the atmosphere)?
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.
How does friction influence the speed and direction of winds within the free atmosphere (right above the planetary boundary layer)?
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.
What is a geostrophic wind?
When wind runs parallel to the isobars due to Coriolis Effect being equal to Pressure Gradient Force.
What are jet streams?
They are examples of fast-flowing, high altitude geostrophic winds.
What kind of winds redistribute surplus radiation in the tropics (surface winds and upper-level winds)?
Surface winds: Hadley Cell
Upper-level winds: Jet streams
What is the Hadley Cell?
One of the types of semi-permanent pressure cells of the General Circulation Model formed due to differential heating of the earth’s surface.
How is the Hadley Cell formed?
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.
How is the Hadley Cell closed?
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.
How do winds operate within the Hadley Cell?
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.
How is the ITCZ formed?
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.
How does the ITCZ move?
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.
Why is the shift of the ITCZ not uniformed?
Uneven heating of land and water, among other factors.
How does the Hadley Cell move?
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.
How do the trade winds move?
In accordance to the Hadley Cell.
What are jet streams?
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.
What do jet streams mark?
Jet streams mark the boundaries between warm and cold air. The greater the difference in temperature, the faster the jet stream.
What direction are major jet streams?
Major jet streams on Earth are westerly winds flowing in an easterly direction.
Do jet streams move at consistent speeds?
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.
How do jet streams vary in size?
Jet streams have varying width and depth, they can span across a few metres to kilometres.
What are the three dominant jet streams?
Subtropical jet stream (STJS), tropical easterly jet stream (TEJS) and African easterly jet stream (AEJ).
What is the subtropical jet stream (STJS)?
It is located about 30°N/S. They are generally associated with fair weather conditions on the ground.
What is the tropical easterly jet stream (TEJS)?
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.
What is the African easterly jet stream (AEJ)?
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.
Why do the winds in a jet stream converge and diverge?
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.
How do convergences and divergences in the jet stream impact surface conditions?
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.
How is surplus heat redistributed by jet streams?
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.
