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Atmospheric
Pressure: Force
exerted by the weight
of the air
If the number of air molecules above a surface increases,
there are more molecules to exert a force on that surface
and consequently, the pressure increases.
Atmospheric pressure is measured with a "barometer",
which is why atmospheric pressure is also referred to as
barometric pressure.
Location of high or low
pressure centers provides
information about:
Wind direction
Wind speed
Cloud cover
Precipitation
We use metric unit for pressure called
a millibar.
the average pressure at sea level is
1013.25 millibars.
Differences in pressure from one location to
another causes:
1) Horizontal motions (wind)
2) Vertical motions (convection and
subsidence)
Location of high or low
pressure centers
provides information
about:
Wind direction
Wind speed
Cloud cover
Precipitation
The change in pressure measured
across a given distance is called a
"pressure gradient".
The pressure gradient results in a
net force that is directed from high
to low pressure.
In Northern Hemisphere
Winds blow clockwise out
of a high pressure center.
Winds blow
counterclockwise into a
low pressure system
Pressure Gradient Force directed from
high to low pressure.
High Pressure
Centers
A high pressure center is
where the pressure has
been measured to be the
highest relative to its
surroundings.
Wind strength is proportional to the
pressure differences.
Strong gradients = strong winds
Weak gradients = low wind speeds
That means, moving in
any direction away from
the "High" will result in a
decrease in pressure.
A high pressure center is
indicated on a weather
map by a blue "H".
Low Pressure Centers
A low pressure center is
where the pressure has been
measured to be the lowest
relative to its surroundings.
That means, moving in any
horizontal direction away
from the "Low" will result in
an increase in pressure.
A low pressure center is
indicated on a weather map
by a red "L" and winds flow
counterclockwise around a
low in the northern
hemisphere.
As air flows out of a high pressure center, air
from higher in the atmosphere sinks to
replace it.
This subsidence produces warming,
dissipation of clouds and precipitation
As air converges in a low pressure center, it
rises and cools. If moist, cooling will
cause moisture to condense and form
clouds.
Rising pressure readings: approach of a high
pressure center and fair weather.
Falling pressure readings: approach of a low
pressure center and stormy weather
Mountain barriers
influence the
development of high
and low pressure
centers
Intense low or high
pressure systems
often occur on lee
side of mtn. barriers
Pressure decreases with height
Falling pressure readings: approach of a low
pressure center and stormy weather.
Pressure gradient maps = semi-horizontal
layer
As surface of constant pressure descends,
the pressure measurement rises
Rule of Thumb
10 m height change
= 1 mb
Top of Space
Needle to Base:
184 m --- 22 mb
Fixed standard pressure level
maps (rather than heights)
 Surface maps, at Earth surface
 850 mb - available moisture for
precipitation, ~1500m level
 500mb - middle of atmosphere, troughs
(lows) and ridges (highs), ~5500 m level
300 mb - jet streams at ~9100 m level
[The average atmospheric pressure at sea
level is ~1000mb]
Mt. Everest: 8848 m w/ 314 mb pressure at summit
31% of atmosphere is present above this level.
A line connecting points of
equal pressure is called
an isobar.
At every point along a
given isobar, the values of
pressure are the same.
The black numbers are
station reports of sea-level
pressure in millibars.
The small blue numbers
are contour labels, which
identify the value of an
isobar.
These isobars have been
generated from pressure
observations.
For example, pressure
reports in Washington
State are between 1014
mb and 1024 mb.
An area of relatively high
pressure is centered in
western NE Nevada, while
the pressure increases
outward from this region.
An area of lower pressure
is located in western
Alberta.
Troughs: Elongated low
pressure centers
Ridges: Elongated high
pressure centers.
The black numbers are
station reports of sea-level
pressure in millibars.
The small blue numbers
are contour labels, which
identify the value of an
isobar.
Wind Direction, Isobars
and
surface maps
Ships on ocean example
Small wave length waves
or ripples are called shortwave troughs or ridges.
500 mb forecast for 1/21/1999.
Troughs = red; solid lines = 500
mb height contours.
High and low pressure centers and
Wind direction
Primary force = pressure gradient
force (differences in pressure are
proportional to pressure gradient
Pressure gradient force is directed
from high pressure to low pressure,
crossing isobars at right angles.
Secondary force = Coriolis force, apparent
force from earth’s rotation
Deflects winds at 90° to the right in
direction of travel (N. hemisphere)
Above 1000m, the balance of pessure forces cause winds to blow
parallel to isobars or pressure height contours
High and low pressure centers and
Wind direction
Path of air parcel starting out from rest
in upper levels.
Air flows towards low pressure
Air turns to the right by CF
When CF and PGF balance, winds
blow parallel to the isobars.
These winds are called geostrophic
winds.
Geostrophic wind blows parallel to the
isobars because the Coriolis force and
pressure gradient force are in balance.
A. Aloft & B. at the ground
Different wind directions and speeds
relative to height contours (aloft) and
pressure contours (ground).
High and low pressure centers and
Wind direction
Near Ground surface
Third force - frictional drag
Slows wind down (fig 5.4b)
Lower wind speeds weaken Coriolis
force, causing winds to turn left,
across isobars at angles 10° to 35°.
Winds at ground blow more directly into
low pressure centers.
Monitor changes in wind direction, can
see changes of pressure patterns and
movement of high and low pressure
centers
Monitoring changes in wind direction.
A. Winds aloft from west, low to the
north
B. Several hours later, winds aloft from
the south, the low is now driven
southward, west of observer.
General Circulation
Unequal heating between equator and
pole causes circulation cells
Location of cells correspond to
alternating belts of high and low
pressure regions.
Cells also correspond to wind.
Easterly winds from equator to 30°
latitude (trade winds) and 60° to
poles.
Westerly winds from 30° to 60°.
Jet Streams
Strong air currents produced by
pressure gradient between poles and
equator.
Location, strength and orientation
vary with season and day to day.
Summer and Winter positions
Air Masses
Regional scale volume of air with
horizontal layers of uniform
temperature and humidity.
Form during episodes of high
pressure
Note location name = origin
M = maritime
C = continental
T = tropical and P = polar
Surface Maps
Uses available surface data
Shows:
 Temperature
 Weather
 Dew point temperature
 Cloud cover
 Pressure
 Wind direction and
speed (wind barb)
Surface Maps
Use available surface data
Shows:
Temperature
Weather
Dew point temperature
Cloud cover
Pressure
Wind direction and
speed (wind barb)
Observed
Temperature
station
reporting
symbol
Observed Dew Point
Temperature
Dew Point and RH
When the dew point temperature and air
temperature are equal, the air is saturated.
Dew point temperature is NEVER
GREATER than the air temperature.
If the air cools, moisture must be removed from the air.
this is accomplished through condensation.
Condensation is the formation of tiny water droplets that can lead to the
development of fog, frost, clouds, or even precipitation.
Relative Humidity can be inferred from dew point values.
When air temperature and dew point temperatures are very close, the air has a
high relative humidity.
The opposite is true when there is a large difference between air and dew point
temperatures, which indicates air with lower relative humidity.
Dew Point and RH
Dew points indicate the amount moisture in
the air.
The higher the dew points, the higher the
moisture content of the air at a given
temperature.
Dew point temperature is defined as the temperature to which the air
would have to cool (at constant pressure and constant water vapor
content) in order to reach saturation.
A state of saturation exists when the air is holding the maximum amount
of water vapor possible at the existing temperature and pressure.
Cloud Cover:
Surface Maps
Use available surface data
Shows:
Temperature
Weather
Dew point temperature
Cloud cover
Pressure
Wind direction and
speed (wind barb)
The the amount of cloud
cover observed at the
time the observation is
taken. In this case,
broken clouds were
reported.
Sea Level Pressure:
The last three digits
of the sea level
pressure reading in
millibars (mb).
Wind Barb:
The wind barb. The wind
barb indicates wind
direction and wind speed.
Surface Maps
Use available surface data
Shows:
Temperature
Weather
Dew point temperature
Cloud cover
Pressure
Wind direction and
speed (wind barb)