Download Physics behind winds

Physics of Winds
Wind Energy Lectures –WRA1
Jami Hossain
[email protected]
Wind
• Wind is caused by:
– the circulation of atmospheric air between
areas of high energy and low energy intensity
• Rotation of earth around its axis results in
a an observed but not real force on wind –
Coriolis Force
• Air molecules from areas of high pressure
move towards areas of low pressure.
• Normally winds flow from cold regions to
hot regions
Pressure Readings
• When reading the
•
station pressure,
altitude corrections to
sea level must be made
to make the data useful
Comparing the
corrected sea level
pressure values across
a map allows us to see
pressure patterns by
drawing isobars
Surface Pressure Charts
•
•
•
•
H represents areas of high pressure, or anticyclones
L represents areas of low pressure, or cyclones
Solid lines are isobars
Wind blows from High toward Low pressure systems
Setting
Settin
g tthe
he Air in Motion
• Pressure
Gradient Force
(PGF) is
determined by
the amount of
pressure change
over a given
distance
• The PGF is
directed from
higher to lower
pressure at right
angles to the
isobars
Setting the Air in Motion (cont.)
• The magnitude of the
Preassure Gradient
Force (PGF) is directly
related to the
pressure gradient
• Closely spaced isobars
indicate a strong
pressure gradient
Coriolis Force
• An apparent force due to the rotation of Earth
• All free moving objects seem to deflect from a
straight line because Earth rotates under them
Coriolis Force (continued)
• The stronger the wind, the greater the
deflection produced
• Coriolis acts only at right angles to the
wind, deflecting to the right in the
northern hemisphere and to the left in the
southern hemisphere
• It affects wind direction but not its speed
• Coriolis Force is ZERO at the Equator
Geostrophic Wind
• When Coriolis balances the PGF, the wind blows in a
•
•
straight path parallel to the isobars
Low pressure is always to the left and high pressure to
the right
Speed of the geostrophic flow is directly proportional to
the pressure gradient force (PGF)
Gradient Wind
• Wind blows parallel to curved contours above the level
•
•
of frictional influence
North to south (or S to N) winds are known as
meridional
Winds blowing west to east (or E to W) are called zonal
Surface Winds
• Due to friction they do NOT blow parallel to the isobars, but across
at a 30°
30° angle into low pressure and out of high pressure
• Friction reduces the wind speed, which reduces the Coriolis force so
that it no longer balances PGF
• Wind rotates counterclockwise or cyclonically around low pressure
• Wind rotates clockwise or anticyclonically around high pressure in
the northern hemisphere (backwards in the southern hemisphere)
Vertical Air Motion
• As air moves into low pressure it converges, has
•
nowhere to go but up, rises, cools, and leads to
clouds and precipitation
Aloft air diverges, eventually sinking to generate
highs