Pressure

1
Pressure

• Chapter 9

2
Atmospheric Pressure
3
Measuring Pressure
4
Measuring Pressure
5
Measuring Pressure
6
Pressure changes aloft due to temperature
variations
7
Pressure changes aloft due to temperature
variations
8
Horizontal Versus Vertical Variation of Pressure
9
Pressure Defined
10
Ideal Gas Law

• Air can approximately be regarded as an "ideal
  gas which obeys the law P CrT
• Pressure constant x density x temperature
• How can pressure change?
• By a change in density or temperature.

11
Pressure Versus Density
12
Pressure Versus Temperature
13
The Surface Pressure Chart
14
Elevation Differences

• One very important source of error when
  generating a surface pressure chart is that not
  all stations are at sea level.... Remember that
  pressure decreases with height.

15
Reducing Pressure to Sea Level

• In the lower part of the atmosphere, pressure
  changes by about 10 mb for every 100 meters of
  elevation change.
• Using this rule, we reduce all pressure
  measurements to sea level, producing a constant
  elevation sea-level pressure chart (SLP) ,
  commonly referred to as a surface weather map

16
Pressure Above the Surface
17
Isobaric Charts

• The 500 mb surface will be located at higher
  levels further south and at lower levels further
  north.
• On an isobaric chart (e.g., 500mb) we plot
  isopleths of the height of the surface.

18
Surface Map

• Surface maps show areas of high and low
  pressure.
• The solid lines are the isobars drawn at 4mb
  intervals.
• The arrows represent wind direction.

19
500-mb Chart
A 500mb chart for the same day. Solid dark
lines are the contour lines in meters. Dashed red
lines are isotherms in 0C. Arrows show the wind
direction. Notice the relationship between the
highs and low with the ridges and troughs.
20
Ridges and Troughs
21
What creates wind?

• What are these forces in the atmosphere?
• Pressure gradient force
• Coriolis force
• Centripetal force
• Friction

• Wind is the result of different forces acting on
  a parcel of air, summarized nicely by Newton's
  second law
• Fma (force equals mass times acceleration)
• Keep in mind that all forces have a magnitude
  and a direction.

22
The Pressure Gradient Force
23
The Pressure Gradient Force (PGF)
The pressure gradient force, like any other
force, has a magnitude and a direction.
The pressure gradient force direction is always
perpendicular to the isobars.
The pressure gradient force direction is always
directed from high to low pressure.
24
The Pressure Gradient Force (PGF)

• The pressure gradient force, like any other
  force, has a magnitude and a direction.
• Magnitude is determined by computing the
  pressure gradient.

25
Isobar Spacing Pressure Gradient
26
The Coriolis Force

• arises due to the fact that the earth is
  rotating.
• acts on objects not rigidly attached to the
  earth.
• always acts to deflect an object to the right
  (left) of it's direction of motion in the
  northern (southern) hemisphere.
• magnitude is zero at the equator, maximum at the
  poles.
• magnitude depends on the rotation rate of the
  earth - the magnitude would increase if the
  earths rotation rate increased.
• if the earth were not rotating, the Coriolis
  force would be zero.

27
The Coriolis Force

• is larger for parcels moving at faster speeds,
  it's zero if a parcel is not moving.
• is not that large for slow-moving objects or for
  those moving over short distances.
• is an "apparent" force that arises solely due to
  the fact that the earth is rotating.
• can only change a parcel's direction, it can not
  affect its speed.

28
Geostrophic Flow
29
Gradient Flow
30
The Centrifugal Force
This is called the centrifugal force and is equal
and opposite to the centripetal force.
31
Highs and Lows
32
Effect of Friction
33
Effect of friction