Chapter 5: Wind Flashcards
wind is
a horizontal movement of air
Differences in temperature create
differences in pressure
Wind is a horizontal movement of air. Differences in temperature
create differences in pressure. The pressure differences drive
a complex system of wind in a never ending attempt to reach equilibrium
Wind is a horizontal movement of air. Differences in temperature create differences in pressure. The pressure differences drive a complex system of wind in a never ending attempt to reach equilibrium. So the wind is the result of
horizontal differences in air pressure
The wind direction is measured by
the wind vane
The wind direction is measured by the wind vane. This instrument,
which is a common sight on
many buildings, always points into the wind
The wind direction is measured by the wind vane. This instrument,
which is a common sight on many buildings, always points into the wind,
sometimes the wind direction is shown on
a dial that is connected to the wind vane.
The dial indicates
the direction of the wind either by points of the compass-that is, North (N), Northeast (NE), East (E), southeast (SE), and so on or by a scale of 0 to 360
On latter scale.0 (or 360) is
north. 90 is east, 180 is south and 270 is west
If the wind blows from the west the wind direction is
westerly
If the wind blows from the west the wind direction is westerly, if it from the south east – its
south easterly and so on
The wind speed units are:
meter per second m/s
kilometer per hour km/hour
knot; one knot = 0.51 m/s or = 1.9 km/hour.
Factors affecting wind:
Force generating wind (pressure gradient force)
Coriolis force
Friction
Pressure differences must create
a force in order to drive the wind this force is pressure gradient force.
Pressure differences must create a force in order to drive the wind this force is pressure gradient force. The force is from
higher pressure to lower pressure and is perpendicular to the isobars or contours.
The larger pressure differences ( ……………………….) the……………………. the …………………….
( the closer the spacing between the isobars) the stronger the pressure gradient force, the stronger is the Wind.
The magnitude of the pressure gradient force is a function of
the pressure difference between two points and air density
PGF
It can be expressed as:
-(1/p)(Δp/Δx)
All free moving objects including wind are deflected to
the right of their path of motion in the Northern Hemisphere and to the left in the Southern Hemisphere.
All free moving objects including wind are deflected to the right of
their path of motion in the Northern Hemisphere and to the left in the
Southern Hemisphere.
The reason of this deflection is
the earth’s rotation
For example if a rocket moved from the North Pole toward a target on the
Equator and took one hour to arrive its target, then the earth will rotate
15o in this time
For example if a rocket moved from the North Pole toward a target on the
Equator and took one hour to arrive its target, then the earth will rotate 15 in this time. So the rocket would look as it
veers off its path and hit the ground 15o west of the target
Coriolis force
deflects the free moving objects to the right in the Northern Hemisphere because of its counter clockwise rotation, and to the left in the southern Hemisphere because of the clockwise rotation of the earth in the Southern Hemisphere
The mathematical formula of computing the Coriolis force is :
Fc= 2ων sinθ
Where Fc is
the Coriolis force measured in m/s2
ω Is
the angular rotation of the earth (which is equal 7.29x10-5 rad/s
The main properties of the Coriolis force are:
- Fc is directly proportional the latitude and wind speed.
- It deflects the free-moving object to the right in the N.H. and to the left in the S.H.
- It affects the wind direction only not affects the wind speed.
Friction
acts to slow a moving objects and to change the direction. Its direction is always opposite to the direction of the wind.
Friction as a factor affecting wind is important only within
the first few kilometers of the atmosphere
The roughness of the of the terrain determines
the angle at which the air flow will cross the isobar as will as influencing the speed at which it will move
Where the friction is low (over smooth ocean surface), air moves at an angle of
10 to 20 degrees with the isobar and at speeds roughly two-third of the geostroghic wind.
The geostroghic wind is
the resultant wind of the balance between the pressure gradient force and the Coriolis force on the free atmosphere.
On the free atmosphere the effect of the friction force is
negligible.
Over the rigged terrain the angle between the wind direction and the isobar is
high (may reach 30-45 degrees) and the wind speed is reduced by much as 50 % of the geostroghic wind.
As said earlier, friction significantly influences
airflow near the earths surface and negligible at a height of few kilometers above
Aloft where is no friction the Coriolis force
balances the pressure gradient force and deflects the wind direction to the right at an angle of 90 in the Northern Hemisphere to move in a direction parallel to the contours
Contours are
the lines of equal heights. This wind is called geostroghic wind
gradient wind:
this is assumption to calculate wind in uniform circular systems such as tropical cyclones.
object moving around a circle needs
a force that is overcoming the desire of the object to move at a straight line. the force is called centripital force.
define gradient wind
the wind which is a result of the balance of forces that are forcing the air to move in a uniform circle. those two forces are pressure gradient and corriolis
thermal wind is
the change in the amplitude or sign of the geostrophic wind due to a horizontal temperature gradient. it is the difference between the upper and lower geostrophic wind.
thermal wind is used to indicate
the thermal advection in a baroclinic atmosphere
thermal wind is parallel to
the thickness
the cold air (lower thickness) is to ………………………..
the left of the thermal wind
cold advection:
thermal wind causes the geostrophic wind to rotate counterclockwise with heigh (backing)
warm advection
thermal wind causes the geostrophic wind to rotate clockwise with heigh (veering)
ageostrophic wind
the difference between actual and geostrophic winds
ageostrophic wind is used to
include the effect of divergence in the upper air
ageostrophic wind is measured by
soundings (radiosondes) and geostrophoc
the ageostrophic wind is the value that is causing
the vertical motion
despite the fact that the midlatitude atmosphere is predominantly in
geostrophic balance
despite the fact that the midlatitude atmosphere is predominantly in geostrophic balance, all weatehr occuring there is a result of
this small porion of wind(the ageostrophic) so it is responsible for the distribution of cyclones, anticyclonic, clouds and precipitation in the atmosphere
the ageostrophic wind is used to measure the horizontal acceleration in the
curvature and jets. it is normal to them and directed to the left.
Jet streams:
Wind in the average increases with height throughout the troposphere cumulating in a maximum near the level of the tropopause.
Jet streams:
Wind in the average increases with height throughout the troposphere
cumulating in a maximum near the level of the tropopause. This maximum
winds tend to be
further concentrated in a narrow bands
Jet streams:
Wind in the average increases with height throughout the troposphere
cumulating in a maximum near the level of the tropopause. This maximum
winds tend to be further concentrated in a narrow bands. These narrow bands
of strong winds meandering through the atmosphere at a
level near the tropopause
Types of jet streams
polar jet stream which is concentrated in midlatitude between 40o and 60o.
- subtropical jet stream at 30o latitude.
Main characteristics of the jet streams:
- jet streams typically occur in the break of the tropopause.
- the form in area of intensified temperature gradients.
- the wind speed must be 50 knots or greater to cla ssify as a jet stream.
- the jet stream maximum is not constant rather its broken into segments.
- the polar jet shifts farther south and it is stronger in winter than in summer.
- jet stream segments move with the pressure ridges and troughs in the upper atmosphere.
Local terrain features such as mountains and shore lines influence
local winds and weather.
Mountain and Valley wind:
In the daytime air next to a mountain slope is heated by the contact with the ground as it receives radiation from the sun. The air usually becomes than air at the same altitude but farther from the slope. Colder denser air in the surroundings settles downward and forces the warmer air near the ground up the slope. This wind is a Valley wind, so called because the air is flowing up out of the valley.
At night the air in contact with the mountain slope is cooled by terrestrial radiation and becomes heavier than the surrounding air. It sinks on the slope producing the Mountain wind which flows l ike water down the mountain slope Mountain winds are usually stronger than valley winds, especially in winter.
Land and Sea breezes:
As we know land surfaces warm and cool more rapidly than do water
surfaces, therefore land is warmer than the sea during the day. So wind
blows from cool water to warm land.
The sea breeze so called because
it blows from the sea. At night the wind reverses, blows from cool land to warmer and creates the “land breeze”. Land and sea breeze develop only when the overall pressure gradient is weak.
Wind with stronger pressure gradient mixes the air so rapidly that local temperature and pressure gradient do not develop along the shore line.
the speed units
knot, meter per second, kilometer per hour and feet per second which they are related by: 1 kt= 0.515 m/s = 1.853 km/h =1.152 mi/h = 1.689ft/s
wind speed may be indicated in
makes use of familiar, natural phenomena connected with different wind speeds.
An anemometer is a device for measuring
wind speed and direction
An anemometer is a device for measuring wind speed and direction. The term is derived from the Greek word anemos, meaning
wind
An anemometer is a device for measuring wind speed and direction. The term is derived from the Greek word anemos, meaning wind. It is divided into:
Wind Vane. Wind vane measures the wind direction.
- A cup anemometer. The cup anemometer measures the wind speed. The wind speed is read from a dial much like the speedometer of an automobile.
Wind sock consists of
a cone shaped bag that is open at both ends. The degree to which the sock is inflated is an indication of the strength of the wind.
