Primary Chapter 5 Notes
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Transcript Primary Chapter 5 Notes
Chapter 5
Air Pressure
Driving Question
What is the significance of horizontal
and vertical variations in air pressure?
Air Pressure
Air pressure is a measure of the force that air exerts
on a surface
Weight per unit area of the column of air above that
location
– Weight = mass * acceleration due to gravity
Average air pressure at sea level
– 1.0 kg/cm2
– 14.7 lb/in2
The air pressure at any point is the same in all
directions
Air Pressure Measurement
Barometer
Aneroid
Mercury
Mercurial Barometer
More accurate than aneroid
Invented by Torricelli in 1643
One meter (39 inches) long glass tube sealed
on one end
Open end is inverted in pool of mercury
Height of mercury changes as pressure
changes
Adjustments are required for temperature and
latitude
Aneroid Barometer
Flexible evacuated
chamber with a spring
inside
As pressure changes
allow the chamber to
flex
This causes movement
in gears which display
the pressure
Air Pressure Tendency
The change in air pressure with time
– Rising: continuing fair or clearing weather
– Falling: approaching inclement weather
– Steady: no change
Barograph
– An instrument that provides a continuous trace of
air pressure with time
Altimeter
– An aneroid barometer that is calibrated to
measure altitude or elevation
Air Pressure Units
Millibars, inches of Mercury, Pascals
Air Pressure
Meteorologists often express altitude in
terms of pressure (850mb map)
Worldwide range in air pressure
averages between 970mb and 1040mb
Lowest: 870mb (25.69in Hg) in Typhoon
Tip near Guam (Peak Winds of 190
mph)
Highest: 1083.8mb (32.01in Hg) in
Siberia
Variations in Air Pressure With Altitude
The maximum air density is at the
surface
Number density: the number of gas
molecules per unit volume
– Decreases with altitude
Thinning of the air with altitude is
associated with decline in air pressure
50% atmosphere’s mass below 3 miles
99% atmosphere’s mass below 20 miles
The Standard Atmosphere:
a model of the real
atmosphere averaged
across all latitudes for all
seasons characterized by
1. Sea level air temperature
of 15oC
2. Sea level pressure of
1013.25mb
Variations in Air Pressure With Altitude
Denver (mile high city) has an average
air pressure 83% less than Boston
– Altitude sickness
– Lexington is about 900 feet above sea
level, so actual pressure is less than “fixed”
pressure
In very sparse air (thermosphere)
temperature is very high, but heat
transfer is low
Horizontal Variations in Air
Pressure
On a surface weather map, variations in
pressure due to altitude are removed by
determining what the pressure would be
at that point if that point were at sea
level
Easier to observe variations in pressure
from one place to another by day and
hour
Influence of Temperature and
Humidity
Generally, temperature has a greater
influence on density and pressure than
water vapor
Air pressure drops more rapidly with
altitude in cold (more dense) air than
warm (less dense) air
Dry air is more dense that humid air!!
Influence of Temperature and
Humidity
Cold, dry air masses are more dense
and produce higher surface pressures
than warm, humid air masses
Change in air pressure is usually
accompanied by a change in air mass
Influence of Winds
Divergence (convergence) are caused by
winds blowing away (toward) a location
The Gas Law (Equation of State)
Variables of State: temperature, pressure,
and density
p = ρRT
–
–
–
–
Pressure (p)
Density (ρ)
Gas Constant (R)
Temperature (T)
In the atmosphere these 3 variables are
constantly changing
The Gas Law
P = ρRT
Pressure is held constant
– If T increases then ρ decreases
– If ρ increases then T decreases
Temperature and Pressure are
inversely proportional
The Gas Law
P = ρRT
Density is held constant
– If T increases then P increases
– If P increases then T increases
Temperature and Pressure are directly
proportional
The Gas Law
P = ρRT
Temperature is held constant
– If P increases then ρ increases
– If ρ increases then P increases
Density and Pressure are directly
proportional
Basic Understandings (Ch. Review)
Important changes in weather often
accompany relatively small changes in
air pressure at the Earth’s surface.
High or rising pressure indicates fair
weather
Low or falling pressure indicates
inclement weather