AT 351 Lab 8 Air Pressure, Forces, and Upper Level Winds

1
AT 351 Lab 8Air Pressure, Forces, and Upper
Level Winds

• Due 31 October 2005

2
Two columns - same temperaturesame distribution
of mass
500 mb level
1000 mb
1000 mb
3
Cool the left column, warm the right column The
level of the 500 mb surface changes the surface
pressure remains unchanged
The 500 mb surface is displaced upward in the
warmer column
The level corresponding to 500 mb is displaced
downward in the cooler column
new 500 mb level in warm air
original 500 mb level
new 500 mb level in cold air
The surface pressure remains the same since both
columns still contain the same mass of air.
1000 mb
1000 mb
4
Air moves from high to low pressure in middle of
column, causing surface pressure to change.
Low
High
original 500 mb level
1003 mb
997 mb
5
Air moves from high to low pressure at the
surface
Low
High
original 500 mb level
Low
High
1003 mb
997 mb
6
Constant pressure charts

• Constant pressure (isobaric) charts are often
  used by meteorologists
• Isobaric charts plot variation in height on a
  constant pressure surface (e.g., 500 mb)

• In this example the 500 mb height is greater in
  the warm air region than the cold air region
• A map of the 500 mb surface will then show high
  height contours over this warm region and low
  height contours over the colder region
• We will come back to this

7
Forces and winds

• Pressure gradients produce air movement
• Multiple forces act simultaneously to cause the
  wind direction to differ from the direction of
  decreasing pressure
• Primary Forces controlling the wind (for this
  lab)
• Pressure Gradient Force
• Coriolis Force
• Friction

8
Forces expressed as vectors

• Forces have two properties
• Magnitude (Size)
• Direction
• Vectors have same two properties
• Length of arrow denotes magnitude
• Direction of arrow denotes direction
• For this lab, dont worry too much about magnitude

9
Pressure Gradient Force

• Direction
• Always directed toward lower pressure

10
Coriolis Force

• Apparent force due to rotation of the earth
• Direction
• The Coriolis force always acts at right angles to
  the direction of movement
• To the right in the Northern Hemisphere

11
Coriolis Force

• Acts to right in northern hemisphere
• Stronger for faster wind

12
Geostrophic Wind

• The Geostrophic wind is flow in which the
  pressure gradient force balances the Coriolis
  force.

Lower Pressure
994 mb 996 mb 998 mb
Higher Pressure
Note Geostrophic flow is often a good
approximation high in the atmosphere (gt1000
meters)
13
Friction is important near Earths surface

• Frictional drag of the ground slows wind down
• Direction
• Always acts in the direction opposite to the
  movement of the air parcel
• Friction can only slow wind speed, not change
  wind direction
• Important in the friction layer
• lowest 1000 m of the atmosphere

14
What happens when we add friction?

• In the northern hemisphere, if the wind speed is
  decreased by friction, the Coriolis force will be
  decreased and will not quite balance the pressure
  gradient force
• Force imbalance (PGF gt CF) pushes wind in
  toward low pressure
• Angle at which wind crosses isobars depends on
  surface roughness
• Average 30 degrees

15
Upper Level Winds

• Meteorologists look at constant pressure maps to
  assess atmospheric motion
• Trough An area of relatively low height on a
  constant pressure map
• Ridge An area of relatively high height on a
  constant pressure map
• A center of low height is marked with an L
  while a center of high height is marked with an
  H

16
Trough
Center of Low Height
Ridge
Center of High Height
17
Upper Level Winds

• Air flows counterclockwise around lows and
  clockwise around highs
• In terms of constant pressure map analysis, most
  weather is associated with areas of low height
• Low heights imply lower pressure at surface
• This leads to surface convergence and thus rising
  air

18
(No Transcript)