The Higher You Go...
Air Pressure and Density
Q1
• Air gets thinner with
altitude.
Unit Essential Question:
– Why it is harder to breath in
Denver, Colorado compared
to Southington, Connecticut.
What are the different features of the atmosphere that characterize our weather.
How does the atmosphere influence life and how does life influence the
atmosphere?
Presentation Objectives:
§Demonstrate the result of air pressure differences.
§ Explain how temperature and altitude affect density.
§ Explain how temperature and humidity affect air pressure.
§ Identify the instrument used to measure air pressure.
§ Define isobars and relate them to contour lines on a topographic map.
§ Describe the movement of air in an area of high pressure, low pressure, and between
Air density decreases with
increasing altitude!
the two.
Air Pressure
Q2
• Air pressure is the weight of the air above you
pressing down.
The farther down you are, the more “stuff” there is above.
In the atmosphere, as altitude increases, air pressure decreases!
Air pressure is the greatest in the Troposphere!
How Do We Measure It?
Q3
• Barometer!
– Units: millibar (mb), inches of mercury,
millimeters of mercury, pounds per square inch,
pascals (Pa), atmospheres.
Blowing Up A Balloon
Q4
• When you blow up a balloon, you put air
inside.
– Air inside the
balloon creates a
certain amount of
air pressure.
– Since you tie the
balloon closed, the
amount of pressure
remains constant!
The pressure inside the balloon and the atmospheric pressure
are equal because the balloon doesn’t change size.
A Difference In Pressure Matters!
Let’s see how this works using a
soda can!
Letting Go of The Balloon
Q5
• When balloons rise upward, the
inside pressure stays the same while
the outside pressure decreases.
• Since the pressure on the inside of the ballon is greater
than on the outside then the balloon expands.
It Eventually Pops!
Why Does Air Pressure Vary?
• Another reason pressure changes is humidity.
– A sample of humid air has some water vapor molecules
rather than the typical air molecules.
Dry Air
Nitrogen
28 units
300 units
Oxygen
32 units
Argon
40 units
Humid Air
240 units
Carbon Dioxide
44 units
Water Vapor
18 units
• Which type of air weighs more; Humid or dry? Dry!
• Which type of air is more dense; Humid or dry? Dry!
Humid!
• Which type of air creates LOW pressure?
Q6
Why Does Air Pressure Vary?
Q7
Highs and Lows
Q8-10
• Air is a high pressure area is cold and dry.
• Air pressure changes with temperature.
– Cold, dry air is more dense and therefore sinks.
• Air moves downward in a high pressure area.
• Air is a low pressure area is warm and humid.
– Warm, humid air is less dense and therefore rises.
Warm
Less Dense
Lower Pressure
• Air moves upward in a low pressure area.
Cold
More Dense
Higher Pressure
H
• Warm air is less dense than cold air.
– Warm air rises, which means it pushes down less.
– Cold air sinks, which means it pushes down more.
Sea and Land Breeze
L
H
Warm
Cold
Land
Sea
Cold
Land
H
Divergence
Air moves from HIGH to LOW!
Q11
How Do We Draw It?
Q12
• On a weather map, pressure is
represented by isobars.
– Lines of equal pressure
• Behave just like contour lines!!
• At the center of “hills” and “depressions,” we
identify areas of high pressure and low pressure.
H
L
Divergence Convergence
L
Warm
Sea
– When values steadily increase toward a central area, the
area with the largest pressure is labeled high pressure.
• A high pressure area is associated with nice, fair weather.
– When values steadily decrease toward a central area, the
area with the lowest pressure is labeled low pressure.
• A low pressure area is associated with bad, rainy weather.
Q13
Just Like Contour Lines
Calculating the Gradient
Q14
• How fast the air moves depends on the
pressure gradient.
– Determine the pressure gradient by looking at
distance between isobars.
• Steep pressure gradient is
when the isobars are
close together.
Pressure Gradient =
– Air moves faster!
• Gradual pressure gradient
is when the isobars are
farther apart.
– The distance is determined using the map scale.
Calculating the Gradient
Change in Pressure
Distance
Distance
• The change in pressure can be determined by
the isobars.
– Air moves slower!
Pressure Gradient =
Change in Pressure
=
Q15
28 mb
= 7 mb/km
4 km
From x —> y