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Explain: Teach Key Concepts
Lesson 1.2: What instruments measure pressure.
Remind Students: Air is a result of a column of air, which has mass, pushing down on an area.
Explain to Students: there are instruments that can measure air pressure. What instrument is
used to measure air pressure? A Barometer
 Barometer (A barometer measures atmospheric pressure, which is also called
barometric pressure.)
 What are the two common kinds of barometers? Mercury & Aneroid
o Mercury barometer - (Figure 2) - The open end of the tube rests in a dish of
mercury. The closed in is almost a vacuum.
 A space that is almost a vacuum is a space that contains almost no
air.
 Ask: How does a Mercury Barometer work? Air pushing down on
the surface of the mercury in the dish is equal to the pressure
exerted by the column of mercury in the tube that is inverted in the
dish. When the air pressure increases, it presses down more on the
surface in the dish.
 Ask: What does this increase in pressure do to mercury? The
increased pressure forces the column of mercury higher in the tube.
This rise of the mercury in the tube indicates what the air pressure
is at that time.
o Aneroid barometer – Figure 3
 Tell student to look at figure #3.
 Point out that the object shown in the red is the metal container
inside the barometer.
 Ask: What does the word aneroid mean? It means “without liquid.”
 Ask: How does the shape of the metal container shown in the two
images change? On the left, it is bulging. On the right , its walls are
pushed in.
 Ask: How does an aneroid barometer work?
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 The airtight metal chamber of an aneroid barometer is
sensitive to changes in air pressure.
 When air pressure increases, the thin chamber walls push in.
 When the pressure drops, the walls bulge out.
 The chamber wall is connected to a needle on a dial. As the
shape of the chamber changes, the needle on the dial moves,
indicating the air pressure.
Lead a Discussion: Units of Air Pressure
 Review the different units of air pressure that students are likely to hear on weather
reports. Students will most likely recognize “inches of mercury” as the unit of air
pressure.
 If the column of mercury in a mercury barometer measures 30 inches high, then the air
pressure is reported as 30 inches.
 (not in book ) Weather pressure units. In aviation and television weather reports,
pressure is given in inches of mercury ("Hg), while meteorologists use millibars (mb),
the unit of pressure found on weather maps. ...
 Meteorologists use a metric unit for pressure called a millibar and the average pressure
at sea level is 1013.25 millibars.
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Chapter 6 Air Pressure and Winds
Atmospheric Pressure
•Air Pressure – the mass of air above a given level–Go up in the atmosphere, there are fewer
molecules above us.
•Pressure always decreases with height!
•Standard Atmosphere–1013.25 hPa (mb) –29.92 in Hg–14.7 psi–The "normal" decrease in
pressure with increased altitude is provided by the standard atmosphere
Measuring Air Pressure
•Air pressure – mass of the atmosphere above any level or the force exerted by the air
molecules over a given area.
•Barometer– Instrument used to detect and measure pressure changes
•Millibar(mb) – unit of pressure most commonly found on surface weather maps. One
thousandth of a bar.
•Hectopascal(hPa) – new preferred unit of pressure on surface maps–1 mb = 1 hPa
•Inch of mercury (Hg) – English unit of measure for pressure. (Used on TV quite a bit)
Pressure Readings
•Station Pressure– barometer reading at a particular location and elevation (corrected)
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•Altitude corrections– must be made to compare a barometer reading taken at one elevation
with a reading taken at another site. –(Near the surface, pressure decreases on the average by
about 1 mb for every 10 m increase in elevation.
•Sea level pressure– adjusted barometer reading. –Size of correction depends on how high
station is above sea level.
Surface and Upper Air Charts
•Surface weather map (chart) – map that shows the distribution of sea level pressure with
isobars and weather phenomena. – Called a “Surface Chart”
Surface Charts
•Winds – arrows on a surface map indicate the direction fromwhich the wind is blowing
•Blue H’s – represent High pressure – or anticyclones
•Red L’s – represent Low pressure – or mid latitude cyclones- extratropical cyclones
•Isobars – lines of equal pressure [millibars (mb)]
Upper Air Charts
•Charts of equal pressure–“constant pressure chart”–the heights of these equal pressures
vary.
•Isoheights – contour lines –lines that connect points of equal elevation above sea level•H’s –
are high heights
•L’s – are low heights •Ridges – elongated highs- warm ridges