Download Pressure, winds, convergence and divergence

Patterns of pressure and wind:
from the horizontal to the vertical
Mt. Washington Observatory
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
PRESSURE
100 km
(Barometer measures 1013mb)
PRESSURE
1013mb
pressure
500mb
pressure
 As you go up in altitude, there’s less weight (less molecules) above
you…
 …as a result, pressure decreases with height!
Pressure vs. height
p = p0Exp(-z/8100)
Pressure decreases exponentially with height
This equation plots the red line of pressure vs. altitude
Barometers: Mercury, springs, electronics can all
measure atmospheric pressure precisely
CAUSES OF
PRESSURE VARIATIONS
Universal Gas Law
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
•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 viceversa
Temperature, height, and pressure
•
Temperatures differences are the main cause of
different heights for a given pressure level
SEA LEVEL PRESSURE VARIATIONS RECORDS
Lowest pressure ever recorded in the Atlantic Basin: 882
Hurricane Wilma, October 2005
Highest pressure ever recorded in the world: 1086
Mongolia, December 2001
mb
mb
Pressure Differences cause Air to move
PRESSURE GRADIENT FORCE
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
Can you identify the equator?
Why have there been no hurricanes there?
ROTATION OF THE EARTH
ROTATION OF THE EARTH
ROTATION OF THE EARTH
ROTATION OF THE EARTH
ROTATION OF THE EARTH
CORIOLIS EFFECT
CORIOLIS EFFECT
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
What other force affects winds near surface?
How will this new force affect wind direction?
Jetstream in cross-section
•
•
The jetstream occurs where the strongest pressure gradient exists
This pressure gradient is caused by the overall temperature difference
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
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
WHAT ABOUT THE CLOUDS?
So, as the air moves towards the low pressure
it has nowhere to go but UPWARDS
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
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
What pressure level is this? Where are the highs and lows?
Where are the strongest winds?
What pressure level is this? Where are the highs and lows?
Where are the strongest winds?
Low pressure is clearly associated with clouds and
precipitation. High pressure is associated with clear skies
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
Identifying troughs and ridges: pressure or height?
Different methods to identify a front on a map