Transcript Air Pressure Notes Presentation

Air
Pressure
The Force of the Air Above
Pressure
What is Air Pressure?
The force that is exerted on a
surface by atoms and
molecules.
Surface
How Does Density relate to Air Pressure?
More dense air = More Pressure
More particles of air in a specific space (volume), therefore
more mass pushing on a surface.
Less dense air = Less Pressure
Less particles of air in a specific space (volume), therefore
less mass pushing on a surface.
Remember in class - When more people piled up, the
person on the bottom felt the most pressure, because as
the mass increased over the same area, the force
increased.
F = M/A or F=W/A
Force = Mass/Area or Force = Weight/Area
If the area is = (Constant) and the Mass Increases,
the Force Increases
If the area is = C(Constant) and the Mass Decreases,
the Force Decreases
More Mass = More
Force
Density
Less Molecules
in Same Volume
Less Pressure
More Pressure
More Molecules
in Same Volume
Surface
Surface
How Does Altitude relate to Air Pressure?
Gravity pulls air molecules closer to the earth, therefore
there are more molecules in the lower part of the
atmosphere resulting in more mass spread over a small
volume of space (More Dense Air)
In general air pressure...
•Increases as altitude decreases (since gravity pulls
more air molecules closer to the earth)
•Decreases as altitude increases (since there are less air
molecules higher in the atmosphere)
How Does Altitude relate to Air Pressure?
air pressure
decreases as
altitude
increases
credit: National Weather Service
How Does Temperature relate to Air Pressure?
We learned that in a closed container,
•If the volume is kept the same (constant) - as
temperature increases, pressure increases
(Gay-Lussac)
•If the temperature is kept the same (constant) as pressure increases, volume decreases
(Boyle)
•If pressure is kept the same (constant) - as
However, the atmosphere is not a closed
container!!!
•As temperature of an air mass increases, air molecules
spread out and the volume of air becomes less dense.
•Less dense air has less molecules and less mass.
•Less dense air applies less pressure on the surface
•Areas of high pressure can be caused when cool air is
sinking and pressing on the ground. At this time, the weather
is usually dry and clear.
•Areas of low pressure can be caused when warm air rises,
pressing less on the ground. With low pressure, the weather
is often wet and cloudy.
Air Pressure on a Can
Remember
•When the temperature inside the can increases, the air molecules rose - volume
of gas increased - leaving less air molecules in the can since the can was not
closed.
•This less dense air had less pressure than the air outside the can.
•The greater outside pressure caused the can to crush.
How is Air Pressure Measured?
Pascals (Pa), Millibars (MB), Inches of
Hg (mercury)
Air pressure at weather stations is
converted to a value that would be
observed if that instrument were located at
sea level.
Air pressure at sea level is about 14.7
pounds per square inch (psi), (1.03
2
kg/cm ), or...
Common units used by meteorologists
Inches of Mercury
Millibars
29.92 (760 mm) in of
Hg (mercury)
1,013.25 millibars
Height of a column of
mercury measured in
hundredths of inches
credit: National Weather Service
Values used in
meteorology range from
about 100 to 1050
credit: National Weather Service
Mercury Barometer
•Consists of a glass tube open at the bottom end and
partially filled with mercury.
• The air pressure pushing down on the surface of the
mercury in the dish is equal to the pressure exerted by the
weight of the column of mercury in the tube.
• When the air pressure increases, it presses down more
on the surface of the mercury.
• Greater air pressure forces the column of mercury higher.
Mercury Barometer
credit: National Weather Service
Aneroid Barometer
•The word aneroid means “without liquid”
• An aneroid barometer has an airtight metal chamber
that is sensitive to changes in air pressure
• As the shape of the chamber changes, the needle on
the dial moves.
Aneroid Barometer
credit: National Weather Service
How Does Air Pressure Affect Weather?
LOW PRESSURE
•Areas of low pressure can be caused when warm
air rises, pressing less on the ground.
•Not enough force, or pressure, to push the clouds
and storms away.
•Results in cloudy, rainy, or windy weather.
•Winds push in a counterclockwise direction.
(northern hemisphere)
credit: National Weather Service
How Does Air Pressure Affect Weather?
HIGH PRESSURE
•Areas of high pressure can be caused when cool air
is sinking and pressing on the ground.
•Higher pressure in the atmosphere pushes all the
clouds away.
•Usually creates cool, dry air and clear skies.
•Winds push in a clockwise direction (northern
hemisphere)
credit: National Weather Service
1. In the atmosphere, as the temperature
of air increases, the air molecules
spread apart.This causes the pressure at
the surface to:
•Increase
•Decrease
•Stay the same
•None of the above
2. In general, areas of high pressure
usually indicate:
•Fair to good weather
•Rainy, stormy weather
•Tornado may be approaching
•Hurricane may be approaching
Reading Surface Pressure Maps
Surface pressure maps are marked with H's and L's which
indicate high and low pressure centers. Surrounding these
"highs" and "lows" are lines called isobars. "Iso" means
"equal" and a "bar" is a unit of pressure so an isobar means
equal pressure. These areas of equal pressure are marked
with a line. Everywhere along each line is constant pressure.
The closer the isobars are packed together the stronger the
pressure gradient is. Pressure gradient is the difference in
pressure between high and low pressure areas. Wind speed is
directly proportional to the pressure gradient. This means the
strongest winds are in the areas where the pressure gradient
is the greatest or where the isobars are closest together.
Isobars & Isotherms
• Curved lines on a weather map that group
areas or locations together.
• Isobars – areas of equal air pressure
• Isotherms –areas of equal temperature
• Iso means equal
• Therm means heat
• Bar is the unit used to measure air pressure
Isobars
www.weather.com
L = Lowest Pressure
H = Highest Pressure
credit: National Weather Service
Links to Current Surface Pressure
Maps
HAM WEATHER