Wind Flashcards
What is Wind?
Air in motion over the earths surface
Wind Velocity is a … quantity
Vector
What is a vector expressed as?
Magnitude and Direction
The wind magnitude is its speed in
Knots
Direction of wind refers to?
The direction FROM which the wind is blowing, expressed in degrees relative to North
Wind velocity is measured at
10 meters over open level terrain for 10 minutes
What is a gust?
Sudden increase in wind speed lasting a short time and dying suddenly
What is a squall?
Sudden increase in wind speed by at least 16kts or more, lasting at least one minute and reaching top speeds of 22kts
What is a veer?
Wind direction changes in clockwise manner
What is a back?
Wind direction changes in an anticlockwise manner
What is pressure gradient force?
Causes air to tend to flow from high to low pressure across isobars
What does the strength of pressure gradient depend on?
Spacing of isobars and is measured perpendicular to isobars
What is Coriolis effect?
Due to earths rotation, moving air is deflected to the left in the southern hemisphere
Where is Coriolis effect zero?
Equator
Where is Coriolis effect maximum?
Poles
What does the strength of Coriolis depend on?
Wind Speed
- The stronger the wind, the stronger the deflection
- The relative strength of these forces governs wind direction
Buys Ballots Law
With your back to the wind the lower pressure system will be on your right (in the southern hemisphere)
The overall pattern of air circulation around the globe results from…
The distribution of pressure systems
Pressure systems are formed by…
Uneven heating of the earths surface
Why is there a large scale convection over the equator?
Air in contact with the earths surface at the equator is warmed and rises
What is the equatorial trough?
The rising air generates a low pressure system
What is the equatorial trough also called?
Inter-tropic convergence zone or the doldrums
At higher levels the air begins to move horizontally towards the ….
Poles
As the air cools….
What happens in the subtropical region?
It descends in the subtropical regions
Subsiding air forms a belt of high pressure known as…
Subtropical ridge
What is the polar high?
Over the poles cold dense air results in a region of high pressure
What is the sub-polar low?
As converging air meets it is forced upwards, this forms a low pressure belt
What generates the world’s major airstreams?
Movement of air near the surface from the high-pressure belts towards the low-pressure regions
In the southern hemisphere, Coriolis deflects moving air …
Left
Why do we experience seasons?
Due to the tilt of the Earths axis
Does the equatorial trough remain stationary over the equator all year round?
No
In summer in the southern hemisphere what happens to the Equatorial trough?
It moves south to lie across Northern Australia
What is the North West Monsoon?
Air flowing towards the equatorial trough crosses the equator and is deflected left due to Coriolis force
In winter over northern Australia winds are southeasterly due to..
The influence of the southeast trades. Resulting in dry conditions over northern Australia except the east coast
The Southeast trade winds blow from the subtropical ridge towards…
The equatorial trough
Due to continual large-scale subsidence over the subtropical ridge, a widespread inversion exists extending towards…
The Equatorial trough
What are the roaring ’40s?
The roaring ’40s prevail around 40 degrees S latitude as air flows from the subtropical ridge towards subpolar low
In relation to the roaring ’40s;
The direction is northwesterly in the southern hemisphere due to….
Coriolis deflection
What do local pressure patterns have a strong influence on?
Wind Velocity
What does wind direction depend on?
Relative strength of the gradient force and Coriolis effect
Gradient wind flows parallel to…
Curved Isobars when coriolis force matches the gradient force
Geostrophic flow is parallel to …
Straight Isobars when the forces are matched
What is the lower 3000ft of the atmosphere known as?
Friction Layer
In the friction layer wind speed is…
reduced
Due to the friction with the surface
Over the land wind speed is reduced by…
2/3rds
Over the Sea wind speed is reduced by…
1/3rd
Reduction in wind speed reduces Coriolis; reduced Coriolis causes
Wind in the friction layer to veer compared to wind aloft
Above the friction layer, layer wind tends to flow….
Parallel to isobars (Gradient wind)
At the surface the wind veers to flow cross isobars towards what?
Low pressure
Surface wind veers across isobars at what degrees?
30 degrees over land
10 degrees over water
Why at the equator does wind flow perpendicular to the isobars?
Lack of Coriolis deflection
During the day does land or sea heat faster?
Land
Rising air over the land draws in cooler air from the sea; at the surface this is called?
Sea Breeze
At night the sea is warmer due to what?
Specific heat
What is land breeze?
The opposite of sea breeze, cool air from the land flows to the sea
What is katabatic wind?
With clear night skies; air in contact with a mountain side may be cooled by conduction, increased density causes the air to flow down the mountain generating katabatic winds
What are Anabatic winds?
During the day; air in contact with a mountain side may heat due to conduction, reduced density causes the air to rise, flowing upslope.
