Transcript high pressure - emilybu13usm

Unit 1
Section 3 Air Pressure
What is Air Pressure?
How do you measure atmospheric
pressure?
A Look at Air Pressure
If you really want to know how the weather
will be tomorrow keep an eye on the
barometer.
The barometer is an instrument used to
measure air pressure.
Changing air pressure is the single best clue to
forecasting tomorrows weather.
Altitude and Pressure
What do you think happens to air molecules as
height increases? If you have ever spent time at
high altitudes (over 2400 meters, or 8000 feet)
you know that breathing becomes more difficult
the higher you go. Some climbers even succumb
to acute altitude illness that can cause death.
Reduced atmospheric pressure (less molecules)
at high altitudes creates a lack of oxygen in the
body.
At lower altitudes, gravity pulls air molecules
towards the Earth's surface, creating denser air
(more molecules). We may be unaware of the
pressure that air exerts but it is a critical and
essential component of living on Earth's
surface. Without atmospheric pressure liquid
water would not exist (it would boil away), nor
would there be any weather as we know it.
Pressure vs. Altitude
Atmospheric Pressure
On average, air exerts a
constant pressure of 14.7
lbs./sq. in. at sea level.
This is equivalent to a
column of mercury 29.92
inches in height or 1013
millibars. One inch of
mercury = 33.8 millibars.
29.92 inches of mercury =
33.8 millibars of pressure
Barometers
Measuring High and Low Pressure
Mercury vs. Aneroid Barometer
A single sheet of paper at
sea level when laid flat
experiences about 1,400
pounds of atmospheric
pressure! Barometric
pressure is measured in
units called millibars.
Most barometers use
mercury in a glass
column, like a
thermometer, to measure
the change in air
pressure.
Why Does Warm Air Rise?
The Sun's heat is the driving force behind weather. It causes warm air masses to
form. As we have seen, Earth's atmosphere is 78% nitrogen, with oxygen and twenty
other gases making up the remaining 21%. These gaseous molecules are in
constant rapid motion (kinetic energy), colliding with each other to create what we
call air pressure.
Why does warm air rise?
As a parcel of air is heated near the earth’s surface, the air molecules move faster
(increased KE).
The molecules bump into each other more often, which spreads them apart (larger
volume).
As the volume of the parcel increases, the density of the air decreases and the
parcel of warm air rises.
Air Temperature and
Atmospheric Pressure
The role air pressure plays in influencing weather is one of the most important of any
of the key weather elements.
Warm air masses rise creating or leaving low-pressure at the surface.
Rising air also cools and condenses to form clouds. Low pressure is therefore
associated with cloudy skies and wet weather.
Once cooled, the heavier air sinks downwards creating high pressure at the earth’s
surface.
High pressure is associated with clear skies and sunny conditions. Since the
atmosphere seeks to create equilibrium, air moves from high to low pressure.
Temperature, Air Pressure and Wind
Air Temperature and
Atmospheric Pressure Summary
Warm Air rises leaving less molecules at the
earth surface creating low pressure. The warm
air rises, cools, condenses and forms clouds.
Low Pressure is associated with clouds.
Cools Air sinks creating more molecules at the
earth’s surface generating high pressure. High
Pressure is associated with clear skies.
Question: What does nature hate?
Answer: A vacuum
Nature hates an imbalance. When the pressure in
one location is different than the pressure in another
location, nature will attempt to balance the pressure
between these two locations.
In nature, things always move from high to low.
In nature, molecules will move from areas of high
concentration (high pressure) to areas of low
concentration (low pressure).
High to Low is the way to flow!
Changes in Pressure- Formation of Wind
The change in pressure from areas
of highs to lows which triggers the
movement of air is called the
pressure gradient or pressure
force.
The greater the difference between
the high and low pressure areas,
the greater the pressure gradient
force and the stronger the winds.
Air at the ground flows towards
lower pressure trying to equalize
itself.
Temperature and Pressure
Sample Questions
1. Pressure is always changing. One reason is the uneven
heating of the earth’s surface cause air masses to move.
2. When air is heated at the earth’s surface it volume
(increases/decreases) resulting in a(n) (increase/ decrease) in
pressure.
3. We call this (increase/decrease) in pressure (high/low)
pressure.
4. A second reason pressure is always changing is humidity.
Water Vapor weighs less than dry air and when water vapor is
present, the air is less massive meaning is exerts (more/less
pressure).
5. At sea level, atmospheric pressure has a value of ___76_
centimeters, or ___29.92__ inches or mercury. These values
are equivalent to ___1013_____ millibars of pressure.
For a parcel of rising air heated at the earth’s surface:
volume of the parcel is (increasing / decreasing)
temperature of the parcel is (increasing / decreasing)
density of the parcel is (increasing / decreasing)
pressure within the parcel is (increasing / decreasing)
As the parcel rises, what type of pressure is formed at the
earth’s surface? (low / high)
As the parcel rises, what type of pressure formed above it?
(low / high)
Pressure Maps
“Isobar Maps”
Isobar maps are essential for the identification of
areas of high and low pressure. These areas
are the result of uneven heating of the earth’s
surface.
Winds direction is determined by locations of
high and low pressure.
Wind Speed is determined by the differences in
pressure between a high and low and the
distance between the high and low.
We will use our understanding of
topographic maps to understand isobar
(pressure) maps
Introduction to Topographic maps
Differences in elevation are shown on
topographic maps by means of contour
lines, which are lines connecting points of
equal elevation.
A mountainous area will require many more
contours than a flat area of little relief.
Contour lines never cross or intersect one
another (but may merge to form a single contour
line where there is a vertical cliff).
The closer the contour lines are together, the
steeper the gradient.
Contour lines for most areas should be smooth,
curving lines.
Every contour line closes on itself.
Isobars
lines of constant pressure
A line drawn on a weather map connecting points
of equal pressure is called an isobar.
The isobars are measured in millibars.
The diagram below depicts a pair of isobars. At every
point along the top isobar, the pressure is 996 mb and
at every point along the bottom isobar the pressure is
1000 mb.
Any point in between these two isobars will have a
pressure somewhere between 996 mb and 1000 mb.
Point A, for example, has a pressure of 998 mb and
is therefore located between the 996 mb isobar and
the 1000 mb isobar.
Sea level pressure reports are available every
hour, which means that maps of isobars are
likewise available every hour. The solid blue
contours are isobars and the numbers along
particular contours indicate the pressure value of
the isobar.