SLIDE 1 Patterns of pressure and wind:
from the horizontal to the vertical
- Mt. Washington Observatory
SLIDE 2 ATMOSPHERIC PRESSURE
- Weight of the air above a given area = Force (N)
- p = F/A (N/m2 = Pascal, the SI unit of pressure)
- Typically Measured in mb (1 mb=100 Pa)
- Average Atmospheric Pressure at 0m (Sea Level) is
1013mb (29.91” Hg)
- A map of sea level pressure requires adjustment of
measured station pressure for sites above sea level
Your book
SLIDE 3
PRESSURE
100 km (Barometer measures 1013mb)
SLIDE 4 As you go up in altitude, there’s less weight (less molecules) above
you…
…as a result, pressure decreases with height!
PRESSURE
500mb pressure 1013mb pressure
SLIDE 5 Pressure vs. height
p = p0Exp(-z/8100) Pressure decreases exponentially with height
This equation plots the red line of pressure vs. altitude
SLIDE 6
Barometers: Mercury, springs, electronics can all measure atmospheric pressure precisely
SLIDE 7 CAUSES OF PRESSURE VARIATIONS
Universal Gas Law
- Number of Moles, Volume, Density, and Temperature affect
the pressure of the air above us
- If temperature increases and nothing else changes, either
pressure or volume must increase
- At high altitude, cold air results in lower pressure and vice-
versa PV = nRT Rearrangement of this to find density (kg/m3): ρ=nM/V=MP/RT (where M=.029 kg/mol for air) Then P= ρRT/M
SLIDE 8 Temperature, height, and pressure
- Temperatures differences are the main cause of
different heights for a given pressure level
SLIDE 9
SEA LEVEL PRESSURE VARIATIONS - RECORDS
Lowest pressure ever recorded in the Atlantic Basin: 882 mb Hurricane Wilma, October 2005 Highest pressure ever recorded in the world: 1086 mb Mongolia, December 2001
SLIDE 10
Pressure Differences cause Air to move
SLIDE 11
PRESSURE GRADIENT FORCE
SLIDE 12 EARTH’S ROTATION and Coriolis Force
- Coriolis force results from Earth’s rotation
- Causes rightward deflection in NH, leftward in SH
- It depends on latitude and speed of movement
- It only is important when it can act for a long time
- Fcoriolis=2vΩsinφ where Ω=7.3x10-5
SLIDE 13
Can you identify the equator? Why have there been no hurricanes there?
SLIDE 14
ROTATION OF THE EARTH
SLIDE 15
ROTATION OF THE EARTH
SLIDE 16
ROTATION OF THE EARTH
SLIDE 17
ROTATION OF THE EARTH
SLIDE 18
ROTATION OF THE EARTH
SLIDE 19
CORIOLIS EFFECT
SLIDE 20
CORIOLIS EFFECT
SLIDE 21 Geostrophic balance
- The balance develops gradually. As PGF increases
velocity, coriolis force gets stronger until they reach equal strength and wind is parallel to isobars
SLIDE 22 What other force affects winds near surface?
How will this new force affect wind direction?
SLIDE 23 Jetstream in cross-section
- The jetstream occurs where the strongest pressure gradient exists
- This pressure gradient is caused by the overall temperature difference
SLIDE 24
SUMMARY UP TO THIS POINT
1) Differences in atmospheric pressure across the globe lead to winds across the surface of the planet 2) The PRESSURE GRADIENT FORCE pushes air from high to low pressure (WIND) 3) The BIGGER the difference between high and low pressure the STRONGER the wind 3) As air moves away from high pressure and towards low pressure, the CORIOLIS EFFECT deflects it to its right (NH) 4) The above leads to air circulating COUNTER-CLOCKWISE around LOW PRESSURE and CLOCKWISE around HIGH PRESSURE (NH) 5) South or North winds will result in higher or lower temps
SLIDE 25
WHAT ABOUT THE CLOUDS?
Remember that air converges towards low pressure and air diverges away from high pressure and FRICTION due to the surface of Earth “enhances” this
SLIDE 26
WHAT ABOUT THE CLOUDS?
So, as the air moves towards the low pressure it has nowhere to go but UPWARDS
SLIDE 27
WHAT ABOUT THE CLOUDS?
So, as the air moves away from high pressure it must be replaced by air from above that is moving DOWNWARDS
SLIDE 28 Covergence/Divergence is different at high altitude
- High altitude divergence: air diverges and pulls up air from below. diverging, rising air
creates colder air and lower pressure and plentiful cloudiness
- High altitude convergence: air piles up and is forced to sink. Piling, sinking air creates
warmer air and higher pressure and clear skies
SLIDE 29
What pressure level is this? Where are the highs and lows? Where are the strongest winds?
SLIDE 30
What pressure level is this? Where are the highs and lows? Where are the strongest winds?
SLIDE 31 Low pressure is clearly associated with clouds and
- precipitation. High pressure is associated with clear skies
SLIDE 32 Fronts are located in “troughs” of low pressure
- How can you identify a trough or
ridge?
- Troughs are relative minima in
pressure: They look like troughs, sagging downward
- Ridges are relative maxima in
pressure: They look like ridges, bulging upward
SLIDE 33
Identifying troughs and ridges: pressure or height?
SLIDE 34
Different methods to identify a front on a map
