Wind Flashcards
What causes wind?
Pressure differences (caused by temperature differences) over a large distance. Which always want to equalise
What are the 4 forces affecting wind? Breifly describe them.
Pressure Gradient Force - initiates air movement and needs horizontal pressure difference. Force= change in pressure/distance.
Coriolis Force
an effect viewed by earthbound observers that shows objects apperently moving off course (due to the rotation of the earth) in the NH: 90* to the right. SH: 90* to the left.
Centrifugal Force
Caused by an objects inertia and acts away from the centre of the circle. Proportional to V2. Inversely proportional to Radius. CF=V2/R
Friction
Resisitive force caused by the surface of the earth. High friction = low speed = low coriolis = wind direction change due to unbalanced forces (always towards the low pressure)
What is the geostrophic wind?
An initial PGF between straight parallel H and L isobars is veered off course by Coriolis. coriolis increases as wind speed increases. Eventually the forces balance and the wind is travelling parallel to the isobars and perpendicular to the pressure gradient.
Needs latitudes greater than 15* to experience enough coriolis and to be above the friction layer
What is the Gradient wind?
Gradient wind introduces centrifugal force to the mix. Occurs when isobars are not parallel (vast majority of the time). Gradient wind blows parallel to curved isobars.
PGF, coriolis and centrifugal now must balance each other out and cause the wind to rotate around aq H or L pressure system.
in NH: high pressure: wind rotates clockwise
in NH: Low pressure: wind rotates anti-clockwise
opposite in SH
Must be above the friction layer to work!
What is the surface wind?
Surface wind introduces friction caused by the surface into the forces. Friction recudes speed and so coriolis and so wind will blow across isobars at an angle towards the low pressure area.
The friction layer can extend upt 3000-4000 feet. Below which, wind changes from geostrophic or gradient wind to surface wind as speed decreases and so wind direction changes!
NH: wind will VEER (turn clockwise) with increasing altitude. which means it will BACK with decreasing height.
Opposite is true in SH.
Closer to the surface you get, the more the wind direction will change due to reduce speed.
This can vary depending on the surface below.
High pressure: divergence at surface. (low pressure seeking)
Low Pressure: Convergence at surface. (low pressure seeking)
What is a cyclostrophic wind?
blows in low latitiudes (low coriolis) with sharply curved isobars. Balance between PGF and centrifugal.
Typcially tropical revolving stroms, tornados, dust devils.
What are the typical values of wind and direction change due to friction?
Over water: reduces 30%: Deflects 10* Over land: reduces: 50%: deflects 30* Over land (night): Reduces 75%: deflects 45*
Night time difference due to cold dense air being harder to accelerate
What is a land breeze/sea breeze?
A localised wind system upto approx 20nm which occurs in a coastal areas and switches direction depending on time of day (due to temperature differences in land and sea masses).
Due to small scale, coriolis has no time to take effect and so wind is direct and approx 10kts.
SEA BREEZE during day: wind blows from sea to land. As warm air over land rises, air from sea flows in to replace it. At altitude the warm over over land replaces the sinking sea air and thus the cycle continues.
LAND BREEZE At night, land cools much quicker than sea. Therefore warm sea air rises and is replaced by cool land air. Small weak high forms aloft over sea.
What is an Anabatic wind/ Katabatic wind?
Occurs on a mountainside during the day. Solar radiation heats the mountainside which heats the local airmass via conduction which rises up along its side. Approx 5kts.
Katabatic wind occurs at night. Landmass is now releasing energy and thus surface air cooled quicker than free air. The cooler denser air sinks down mountainside into the valley. Approx 10 kts. Can contribute to valley inversion.
What is a Valley wind?
Venturi affect of the mountains sides causes wind speed to increase.
possibility of wind reversing direction completely.
high speeds and rough terrain causes low alt turbulences.
Upper winds?
Pressure gradients aloft control the upper winds.
Related to the temperature difference between air masses. aka: thermal winds.
